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SELF INSTRUCTIONAL MATERIAL

M.Sc. PREVIOUS (BOTANY)

PAPER –I CELL & MOLECULAR BIOLOGY OF PLANTS

BLOCK -1

UNIT - I UNIT - II

MADHYA PRADESH BHOJ ( OPEN) UNIVERSITY, BHOPAL

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P 01BLOCK -1

CELL & MOLECULAR BIOLOGY OF PLANTS

UNIT -I : The Dynamic Cell : Structural Organization of Plant, Speacialized Cell Types, Bioenergetic , Cell Wall, Plasma membrane – Composition , Models , Function, Carriers, Channels, Pumps, Receptors, Plasmodesmeta, Structure, Function , Comparsion with Gap Junction

UNIT -II : Chloroplast : Structure , Genome Organization, RNA-Editing, Nucleochloroplastic Interaction, Mitochondria, Structure , Genome Organization, Biogenesis, Vacuoles, Tonoplast Membrane, Functions.

Editor : Dr. Renu MishraHOD – Botany & MicrobiologySri Satya Sai College for Women, Bhopal.

Writer : Dr. Ritu Thakur BaisAstt. Professor (M.Sc. PhD Botany)Sarojini Naidu Govt. Girls P.G. (Autonomous) College Shivaji Nagar,Bhopal.

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Unit – I THE DYNAMIC CELL

1.0 STRUCTURAL ORGANIZATION OF PLANT

1.1 INTRODUCTION

1.2 OBJECTIVE

1.1.1 MEMBRANE & WALLS

1.1.2 NUCLEUS & RIBOSOMES

1.1.3 MITOCHONDRIA & CHLOROPLASTS

1.1.4 CYTOSKELETAL FILAMENTS

1.1.5 LET US SUM UP

1.1.6 CHECK YOUR PROGRESS

1.1.7 ASSIGNMENT/ACTIVITIES

1.1.8 CHECK YOUR PROGRESS - KEY

1.1.9 REFERENCE/FURTHER READING

1.2.0 SPEACIALIZED CELL TYPES

1.2.1 CELLS OF GROUND TISSUE SYSTEM

1.2.2 CELLS OF DERMAL SYSTEM

1.2.3 CELLS OF VASCULAR SYSTEM

1.2.4 LET US SUM UP




1.3.0 BIOENERGETIC AND CHEMICAL FOUNDATION

1.3.1 ENERGY FLOW

1.3.2 LAW OF THERMODYANAMIC

1.3.3 ROLE OF ATP IN TRANSFERRING FREE ENERGY

1.3.4 LET US SUM UP





1.4.0 CELL WALL

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1.4.1 PRIMARY STRUCTURE

1.4.2 CHEMICAL COMPOSITION.

1.4.3 BIOGENESIS AND GROWTH.

1.4.4 LET US SUM UP





1.5.0 PLASMA MEMBRANE / CELL MEMBRANE

1.5.1 COMPOSITION OF PLASMA MEMBRANE

1.5.2 MODELS OF PLASMA MEMBRANE

1.5.3 FUNCTION

1.5.4 CARRIERS

1.5.5 CHANNELS

1.5.6 PUMPS (SITES FOR ATPase)

1.5.7 RECEPTORS

1.5.8 LET US SUM UP





1.6.0 PLASMODESMETA

1.6.1 INTERNAL STRUCTURE

1.6.2 FUNCTION OF PLASMODESMETA

1.6.3 COMPARSION WITH GAP JUNCTION

1.6.4 LETS SUM UP


1.6.6 ACTIVITES/ASSIGNMENTS

1.6.7 CHECK YOUR PROGRESS: THE KEY

1.6.8 REFRENCE/FURTHER READING

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1.1 INTRODUCTION

The cell is the fundamental unit of life, the building block from which all organisms are constructed.

The properties of cell exhibiting the characteristics of life, define both the potential capabilities & the

inherent limitations of all living organisms. The cell theory given in 1839 by German biologist

M.Schleiden & Theodor Schwann was originally based on the observation that different kinds of cells

resemble one another when observed microscopically &functionally. When the properties of many

different cell types was examined , it was found that cells share common characteristic functions carried

out by specialized subcellular structure known as organelles . (figure – 1)

The specialized found in plant, shoot, leaves, roots, flower and fruits are classified into three tissue

system – ground tissue system ,dermal tissue system and vascular tissue system .Each tissue system

carries different generalize function :the vascular tissue system transport water and solutes to long

distance in the plant, the dermal tissue system provide protection and perform exchange at the surface of

the plant, and the ground tissue system provide cells that carry out photosynthesis , storage and support.

Each tissue system has many specialized cells, a few cell types are found in more than one tissue system.

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The different types of specialized plant cell are distinguish by cell shape and by properties of the cell

wall and protoplast. Plant cell wall is one of the most important distinguishing feature of the different

kind of specialized cell. All plant have a thin and flexible primary wall, made of the polysaccharides

cellulose and other carbohydrates . Other cell types have addition to a primary wall, a thick, rigid

secondary wall in which cellulose is impregnated with lignin.

The internal atmosphere of cell differs from that of its external environment. This difference is maintain

throughout the life of the cell by thin surface membrane. The plasma membrane which controls the

entrance & exit of molecules & ions. The capacity of the plasma membrane to act as a selective barrier

between the cell & the medium is called permeability .Most plant cells, have a thick cellular wall that

cover & protects the plasma membrane . Some animal cells are surrounded by a cement like layer called

a cell coat, which generally plays no role in permeability but does have other important functions.

Cell requires chemical building blocks such as sugars, fatty acid, nucleotides & amino acid .Cells need

energy both to drive the chemical reaction involved in building cell components. Most of the chemical

reactions that take place in the cell would normally occur too slowly to maintain life as we know it. Cell

requires a set of information to carry out the reactions.

The extraneous coat of the plasma membrane of the plant cell is known as cell wall. The cell wall is a

rigid and protective layer around the plasma membrane which provides the mechanical support to the

cell. The surfaces of plant and bacterial cells exhibit many of these same properties, but they also exhibit

a few unique features that are not shared by the cells of animals. Plant cell walls provide a supporting

framework for intact plants. In addition to providing mechanical support & strength for the plant as a

whole, the cell wall protects individual cells from osmotic rupture and mechanical injury .The rigid cell

wall also plays a central role in determining the characteristic shapes of plant cells .Although cell walls

were once viewed as relatively inert secretions of the cell they surround, more recent studies have

revealed the wall to be a dynamic structure that carries out many activities. The cell wall also acts as a

permeability barrier and plays a role in certain types of secretary and metabolic events.

Although cell wall provides a thick encasement for the plant cell, this barrier does not seal the cell

completely from its surroundings. Plant cell wall usually contain small opening or plasmodesmeta,

through which adjacent cells maintains direct contact with one another. In electron micrograph

plasmodesmeta appears as a narrow channel in the cell wall that are linned by plasma membrane and

often transversed by a tubular of ER. Thus the plasma membrane, cytosol and ER are all in continuity

from one cell to the next. Plasmodesmeta tends to be concentrated in special areas of the cell wall called

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as PITS FEILDS, where the primary wall is thinner then the normal and secondary wall is absent.

The main focus of this topic is to learn about the connection between plant cells and how these

connection differs from the those found in the animal cells.

1.2 OBJECTIVE

The main objective of this unit is to understand the basic unit of life and Cell interior

The main focus of this sub topic is to get acquainted with each tissue system and their functions which is

carried out by different type of specialized plant cell.

The main focus of this sub topic is to know about the selective barrier of the cells and their molecular

architecture and the way in which they controls the transport.

The main emphasis of this sub topic is to know about the energy which cell needs to drive the chemical

reaction to maintain life.

The main aim of this sub topic is understand about the primary barrier of the cell along with its

chemical composition and arrangement pattern followed during the growth of the plant.

The main aim to study this topic is to know how cell communicates within plant Are similar structures

are found in animal cells , if yes than how these two can be compared.

1.1.1 MEMBRANE & WALLS

Cell maintains a selective barrier called the plasma membrane, a structure measuring 7-8 nm in

thickness that constitute the outer membrane boundary of all living cells .By regulating the passage of

material into & out of cell, the plasma membrane ensures that optimum conditions for living processes

prevail within the cell interior .Plasma membrane also plays an important role in cell -to-cell

communication, transmitting signals from the cell exterior to the cell interior. In plants a rigid cell wall

is found directly outside the plasma membrane .The rigidity and of the cell wall provides the shape of

the cell it encloses and protect the cell from adverse environmental condition . In addition to the

presence of plasma membrane at the outer cell surface, membranes are also employed to partition the

cell interior into multiple compartments which is exclusively found in animal and plant cells. An

elaborated system of interconnected membrane channels and vesicles known as Endoplasmic reticulum

(ER) plays an important role in transporting newly synthesised proteins to various destinations within

the cell. Closely associated with ER is the Golgi complex, a stack of membranes involved in processing

newly synthesised proteins and packaging them into membrane vesicles for storage and secretion.

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Several other membrane bound organelles also occurs in plants. Lysosomes are small membrane and

enclosed structure that serve a digestive functions to breaking down foreign materials & intracellular

constituents that are no longer needed by the cell. Peroxisomes carry out certain kind of oxidation

reactions and vacuoles, which are especially prominent in plant cells, a large vesicle that functions as

storage compartments & in maintaining water balance. In addition to the preceding membrane -enclosed

organelles, we will see shortly that membranes surround the genetic material and energy transforming

organelles of plant cell

1.1.2 NUCLEUS & RIBOSOME

Cells utilize genetic information to guide the synthesis of most of the cell's component. This genetic

information is stored in the nucleus surrounded by a double membrane envelope. The nucleus of plant

cell is largely occupied by a mass of intertwined chromatin fibres, which contains the DNA molecules

which stored most of cell genetic information .A small portion of nuclear DNA is localized in a small

spherical structure known as nucleolus, which contains DNA information involved in the formation of

ribosomes. A fluid like material called nucleoplasma fills the space around the chromatin fibres &

nucleoli condense into compact structure known as chromosomes. The outer boundary of the nucleus is

formed by two concentric membrane that together forms the nuclear envelope. Connections are observed

between the outer membrane of the nuclear envelope & ER. The most distinctive structural feature of

nuclear envelope is the presence of numerous nuclear pores .Nuclear pores help to regulate the flow of

the material between nucleus & cytoplasm. Genetic information and encoded in the cells DNA guides

the synthesis of specific protein molecule. This process of protein occurs on small cytoplasmic granules

known as Ribosomes. Prokaryotic ribosomes are slightly smaller than eukaryotic. Eukaryotic ribosomes

are found free in the cytoplasm and attached to the membrane of ER.

1.1.3 MITOCHONDRIA & CHLOROPLASTS

Metabolic reactions occurs virtually everywhere in the cell, but those that are most central to the flow of

energy are localised predominantly in the cytoplasm. Reactions involved in the initial breakdown of

energy rich nutrients occurs in the cytosol, which is the fluid like portion of the cytoplasm that surrounds

the cytoplasmic organelle. Once this initial process is complete, a second set of reactions occurs in

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which the most of the energy contained in the nutrients is released and used to drive the formation of

energy rich molecule ATP. Mitochondria are large and enclosed by two membranes. The inner of the

two membranes is folded into a series of CRISTAE that project into the internal cavity, or matrix, of the

mitochondria. They are the site of many chemical reactions, the matrix also contains DNA and

ribosomes. In addition to obtain energy from the breakdown of energy rich nutrients, plant cells are

capable of trapping energy from sunlight & converting into chemical energy. Photosynthesis takes place

in especialized organelle called CHLOROPLAST. Although chloroplast are usually longer than

mitochondria, the structure of the two organelles exhibits some fundamental similarities. Like

mitochondria, chloroplast are enclosed by two membranes surrounding an internal compartment, in this

place designated the stroma. Within stroma are the thylakoid membrane, which contain chlorophyll and

other light absorbing pigments. Like the matrix space of mitochondria, the stroma of chloroplast contain

both DNA and ribosomes.

1.1.4 CYTOSKELETAL FILAMENTS

Plant cells have developed an elaborate network of cytoplasmic filaments, such network of the filaments

are called cytoskeleton. It consist of three distinct components-(i)Microtubules- it is the largest

filaments, which is rigid hollow structures measuring 25nm in diameter and contain tubulin protein.

They are used in the construction of the mitotic spindle that moves chromosomes during cell division.

(ii) Actin filaments or Microfilaments- generates movements within the cytoplasm. They measures 6nm

in diameter and are constructed from actin. Actin filaments generate cytoplasmic streaming.

(iii) Intermediate filament- It is found between that of actin filaments and microtubules. Its protein

composition varies, depending on the cell type. The role of intermediate filaments in motility is yet too

established, but they play important role in structural support and in anchoring.

1.1.5 LETS SUM UP

o All organism are composed of cells that performs the same basic functions.

o All cells are enclosed by plasma membrane, in plant and bacterial cell rigid cell wall is present.

o In Eukaryotic cells intra cellular membrane form a series of cytoplasmic compartments known as

endoplasmic reticulum, Golgi complex, lysosome, peroxisomes and vacuoles.

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LETS CHECK YOUR PROGRESS-

1. The cell structures without the cell wall is termed as-

a. Organelle

b. Cytosol

c. Protoplast

d. Protosol

2. The organelle which helps in flow of genetic information are

a Nucleus and ribosomes

b. endoplasmic reticulum and ribosomes

c. Plasma membrane and ribosomes

d. Mitochondria and ribosomes

3. The elaborate system of interconnecting membrane channels and vesicles are known as-

a Mitochondria

b . Cell wall

c Plasma membrane

. d Endoplasmic reticulum

4. Eukaryotic cells which stores main genetic information is-

a Nucleus

b Ribosomes

c. Plasma membrane

d. Mitochondria

5. A small membrane enclosed structures that serves a digestive function is-

a. Vacuoles

b. Ribosomes

c. Lysosomes

d. Protosol

o In eukaryotic cell genetic information is stored in a membrane enclosed nucleus which is larger and

have a complex internal membrane system.

o In eukaryotic cells mitochondria are involved in brackdown of nutrients and converting their energy

into a useful chemical form.

o The chloroplast of plant cells carry out photosynthesis which traps energy from the sun light.

1.1.6 LETS CHECK YOUR PROGRESS

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1.1.7 ACTIVITIES/ASSIGNMENTS-

Collect different plant material and prepare slides to observe the cell .

Prepare the model of typical plant cell.

1.1.8 LETS CHECK YOUR PROGRESS: KEY

1. c

2. a

3. d

4. a

5. c

1.1.9 FURTHER READINGS

Principles of Cell and Molecular Biology Second Ed. Lewis J.Kleinsmith, Valerie M.Kish.

1.2.0 SPEACIALIZED CELL TYPES

1.2.1 CELLS OF BASIC GROUND TISSUE SYSTEM

Parenchyma cell are the generalized multipurpose cell in plant .Most parenchyma cell have thin primary

wall those varies from spherical to barrel like in shape .parenchyma cell store food reserves as a potato

tuber (starch storing parenchyma).the parenchyma cell of green leaves a specialized for photosynthesis

these cell contain numerous large chlorophylls and are called chlorenchyma. Other parenchyma cell are

specialized for the transport of solute through the cell membrane .these cell have the greatly enlarged

area due to the highly convoluted inner surface of the cell. Transfer cell are found in the 11ectarines

where the extensive membrane houses transport channel that’s secrete sugar and other nectare

component to the exterior of the cell. Collenchyma cell function in the support of the growing tissue.

Individual collenchyma cell are long and narrow and have unevenly thickened primary wall.

Cholenchyma cell for cable of thousand of cell that together can provide membrane support while stem

or leaves elongates. Collenchyma in the veins of the leaves form the strings of celery stages.

Sclerenchyma cell function in the support of tissue that are expanding. Sclerenchyma cell are long

narrow with a thick, hard, rigid secondary wall .unlike parenchyma and collenchyma cells that are living

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cells sclerenchyma cells are dead at maturity. The cells function in mechanical support is carried out by

the strong cell wall. Sclerenchyma fibres make natural woddy tissue and also form long strands in the

leaves and stem of many plant . Natural fiber ropes such as those from hemp or the sisal plant are made

up of thousands of sclerenchyma cell. Some sclerenchyma cell called sclereids much shorter than fibers,

these form the hard layers of shells and peach pits and small cluster of sclereids form in pear fruits

(Fig I)

PARENCHYMA CELL COLLENCHYMA CELL

SCLERENCHYMA CELL

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1.2.2 CELLS OF THE DERMAL SYSTEM

Epidermal cell form the surface layer of the plant ,The typical epidermal cell are flat and form a

continuous layer with no spaces between the cell .Each epidermal cell secretes hydrophobic polymer

cutin on the surface , which reduce the amount of water lost by evaporation .Most cell also secrete

waxes on the surface of cutin which reduce transpiration as well as wettability of the leaves epidermal

cell of green leaves contain choloroplast allowing light to penetrate for the photosynthesis. Epidermal

cell of petals contain anthocyanin pigment within the vacuoles or oranges carries pigment within the

plastids, giving rise to bright colours. Guard cells are specialized epidermal cell that functions as a small

pores in the plant surface, allowing the carbon dioxide needed for photosynthesis to diffuse from the

external atmosphere into the chlorenchyma tissue .Guard cell are crescent shape , contain green

choloroplast ,and are rapidly change there shape in response to changes in wide status. As guard cells

take up water ,the pore opens and as water evaporates the pore closes. The two guard cells and a pore are

called stomata. Trichomes are hair like cells that project from the surface of plant. They function to

reduce water loss by evaporation and trapping water vapour near the plant surface .In some plant

trichome are glandular and secrete sticky or toxic substance that repel insect herbivores .

(FigII)

STOMATALCELL

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TRICHOME CELL (DERMAL SYSTEM)

1.2.3 CELLS OF VASCULAR TISSUE SYSTEM

The vascular tissue system is composed of both xylem and phloem tissue. Xylem functions to carry

water and mineral nutrient absorbed at the root tip throughout the plant, stem and leaves. Vessels

element are the major tissue involved in the transport of water and the solutes. Vessel element are

elongated cells with thick secondary wall and the rigid wall keep the vessel element from collapsing.

Like sclerenchyma vessel element are dead at maturity ,so that each cell are empty tubes .Before vessel

element die the cell protoplast releases enzyme that degrade the cell wall ends of the cell forming a

perforation as a continuous pipe. Other xylem cells called tracheids also function in transport of water

and solute but are efficient because they lack perforations. Xylem tissue also contains sclerenchyma cell

function in support and parenchyma cells that function as transfer cell. Phloem tissue function to

transport the product of photosynthesis from green tissue to parts of the plant that are energy rich

carbohydrates required for storage by a process called translocation. Sieve element are the cell of

phloem that are elongated with primary wall. Phloem sap travel under a positive pressure and the

thick ,elastic cell walls allow the cells to adjust fluctuation in pressure over a day night cycle. Sieve

element have large conspicuous pores on the end wall, forming plates. The sieve plate pore allow the

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phloem sap to travel cell to cell along the file of cell called a sieve tube. Each phloem element is living,

with an intact plasma membrane, and the differential permeability of the membrane prevents solute

leaking out of the sieve tube. Sieve tube element lack some other component of the cytoplasm this

feature functions to keep the pore unplugged. Companion cell parenchyma are associated with sieve

elements. The nucleus of the companion cell must direct the metabolite to the companion cell itself and

of its sister sieve elements.

(Fig III)

(

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1.2.4 LET US SUM UP

Specialized cell are found in plants every part.

These cells are classied into three system.

Each tissue system carries different functions.

Each tissue system has many specialized cells which has distinct shapes and cell wall properties.

Parenchyma cells are the generalized, multipurpose cell of the ground system.

In the dermal system Guard cells are the specialized epidermal cells that performs the function of

exchange of gases.

In vascular system xylem and phloem are the specialized cells that carries water and mineral

nutrients absorbed by the roots to the parts of the plants.

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1. Cells which helps in transport of photosynthetic product is-

a. Epidermal cell.

b. Trichome.

c. Vessels.

d. Phleom.

2. The tissue system which provides protection and exchange at the surface of the plant is-

a. Ground tissue system.

b. Vascular tissue system.

c. Dermal tissue system

d All of the above.

3. Transfer cells are found in-

a. Nectaries.

b. Epidernal cells.

c. Guard cells.

d All of the above.

4. The cell which dies at maturity is-

a. Sclerenchyma

b. Parechyma

c. Collenchyma

d None of the above.

5. Natural ropes are made up of thousands of –

a. Sclerenchyma

b. Parechyma

c. Collenchyma

d None of the above.

1.2.5 LETS CHECK YOUR PROGRESS-

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1.2.6 ACTIVITES/ASSIGNMENTS-

Prepare charts of each tissue system and descried their cell structure and function.

With the help of mascuration techniques visualize different vessel cells of collected

angiospermic material.

Collect different type of plant material and visualize guard cells and trichome and prepare

record of it.

1.2.7 LETS CHECK YOUR PROGRESS : KEY

1. d

2. c

3. a

4. a

5 a.


Burgess , Jeremy, An Introduction to Plant Cell Develop. York: Cambridge University Press.

Gunning , Brian E.S. , and Martin W.Steer. Plant Cell Structure and Function.

1.3.0 BIOENERGETIC AND CHEMICAL FOUNDATION

1.3.1 ENERGY FLOW

To perform various task cell require energy to grow, move, synthesis & transportation. In scientific

concept energy means capacity to perform work or ability to cause some kind of change to occur. For

cell to form function energy must flow into cell from its surrounding. Energy occurs in different form,

cells obtain energy either in the form of organic molecules that contain chemical energy or energy from

photosynthetic organisms as photons of light. This energy is of two types.

Kinetic energy deals with motion of molecules, thermal radiation etc. Potential energy is the energy

stored in bonds connecting atoms.

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1.3.2 LAWS OF THERMODYANAMICS

Thermodynamics principle helps us to understand about these energy law & their transformation:

First Law – Energy can never be created nor be destroyed however it can be transform from one form

to another .Energy of universe is constant, either change occurs in energy form or it's physical location.

Transformation takes place between various forms of energy such as heat, light, electricity, mechanical

energy & chemical energy. In thermodynamics particular location in which energy changes occurs is

reffered as system & rest of the universe is surrounding. Energy content of an individual system can be

changed, but total energy content of system & surrounding remains constant. In cell 1st law is applicable

in the sense that energy is transformed from surroundings & is transformed into those forms which is

used by cell, for example in autotrophs, light energy is converted into chemical energy which is

essentially used by heterotrophs.

Second Law- 1st law does not tells us about the probability that any such process is actually occuring.

As a biologist one must like to know the direction of reaction & whether energy is released or

absorbed.This law states that energy event in the universe occurs in the direction that cause the system &

surrounding to exhibit a net increase in randomness i.e Entropy. According to second law the Entropy of

system & surrounding always increases but this is not true that entropy of individual system always

increase. It may increase or decrease or remain same .Free energy or G (Gibbs). Describes the

thermodynamic factors that allow us to apply the second law of thermodynamics to a individual

chemical reaction without requiring us to measure the entropy change. It represents the energy that can

be harnessed to do useful work. For living organisms where pressure and volume remains constant, the

change in free energy that accompanies any biological process is determined by two parameters;

Δ E =total internal energy/ enthalpy

S=change in entropy

G= Δ E – TΔS

ΔS; where G = freeen ergy, T=absolute temperature

If Entropy increases then free energy decrease ;

ie, (ΔS= positive) (ΔG=negative)

(ΔE<0; ΔG<0 ; ΔS>0)

& reactions proceeds in the directions that causes decrease in the free energy of the system i.e Exergenic

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reactions , i.e they realize free energy (products contains less bond energy ) .It indicate the direction of

the reactions i.e breakdown of complex organic molecule into simpler one is exergenic reactions .

If Entropy decreases then free energy increases ;

i.e (ΔS= negative) (ΔG=positive) (ΔE>0);

Such type of chemical reactions are called as Endergenic reactions.

Photosynthetic organism are huge, complex, Endergenic reactions centers in which ΔG can be made

favourable by coupling them to external energy supply i.e light . ΔG indicate the direction of reactions.

Most biological reaction differ from standard condition, particularly in the concentrations of the

reactants. We can estimate free energy changes for different temperature by using the equation;

ΔG'=ΔG0' +2.303RT log(product/reactant)

where, R=gas constant, T= absolute temperature, ΔG0'=standard free energy, ΔG'= is measure of actual

change in free energy, that occurs with a particular mixture of reactants & products at given

concentration, the value of ΔG' thus varies, depending on the conditions involved.

ΔG0' is constant under standard conditions. It can be calculated under conditions of equilibrium.

If 0is substituted in eq (a)for ΔG'

0=ΔG0'+2.303 RT log(product(eq)) ........... (a)

( reactant(eq))

ΔG0'= -2.303 RT log(product(eq).......... (b)

(reactant(eq)

Equilibrium constant Keq= (product(eq)) ........... (c)

(reactant(eq))

Keq can be substituted in eq (b)

ΔG0'= -2.303 RT log Keq ...........(d)

(Equilibrium constant at pH=7)

If ΔG0'= positive, direction of reaction , reactant -----> product,

reaction is exergenic i.e Keq>1 & (product)> (reactant);

If ΔG0'= positive, direction of reaction , reactant <----- product,

reaction is endergenic i.e Keq<1 & (product)< (reactant);

1.3.3 ROLE OF ATP IN TRANSFERRING FREE ENERGY

One widely occurring pattern involves the use of high energy phosphorylated compounds that release

energy when their phosphate groups are removed. Such compounds play a key role in transferring

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energy from thermodynamically favorable to thermodynamically unfavorable processes. The most

common example of such high-energy compound is adenosine triphosphate (ATP) .The structure of

ATP and the reactions involved in the removal of its phosphate groups is hydrolysis reaction i.e

exergonic, giving adenosine diphosphate (ADP) & a free phosphate group. The terminal phosphate of

ADP can also be removed in another

1.3.4 LETS SUM UP –

The use of energy by cell is governed by the law of thermodynamics

The first law states that the total amount of energy in the universe is constant which means that a

cell`s energy requirements can be met only by taking in energy from the surroundings and

transforming it into useful forms.

The direction in which the chemical reaction proceeds is governed by the second law of

thermodynamics, which states that the events proceeds in the direction that causes the entropy of

the universe to increase. All reactions proceeds in the direction that cause the free energy of the

system to decrease.

ATP plays a central role in coupled reactions , trapping energy released by the oxidation .


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1.3.6ACTIVITES/ASSIGNMENTS-

Enlist various examples which fulfills the law of thermodynamics.

1.3.7 LETS CHECK YOUR PROGRESS- THE KEY

Answer of the above question is already provided in notes. Review your work accordingly.

1.3.8 FURTHER READING


1.4.0 CELL WALL

1.4.1 PRIMARY STRUCTURE

Cell wall is complex in nature .It remains differentiated in the following layers:

(i) Middle Lamella

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Write within the space provided

a) Explain Entropy and free energy

……………………………………………………………………………………………………

……………………………………………………………………………………………………

……………………………………………………………………………………………………

……………………………………………………………………………………………………

……………………………………………………………………………………………………

b) Compare exergonic and endergonic reactions

…………………………………………………………………………………………………

……………………………………………………………………………………………………

……………………………………………………………………………………………………

…………………………………………………………………………………………………

……………………………………………………………………………………………………

(ii)Primary cell wall

(iii)Secondary cell wall

(iv)Tertiary cell wall

(i) Middle Lamella-The cells of plant tissue generally remain cemented together by intercellular matrix

known as middle lamella. It is composed of pectin, lignin & some other proteins. They are dissolved in

strong acids.

(ii) Primary Cell Wall- It first formed cell wall. Its thickness is about 100-200nm.It is the outer most

layer of the cell & it forms the only cell wall in the immature meristamatic & parenchymatous cells. It is

comparatively thin & permeable. It is composed of loosely organised network of cellulose microfibrils

associated with hemicellulose pectins & glycoproteins. Pectins are important in imparting flexibility &

makes primary cell wall to expand during cell growth. The cellulose synthesizing enzyme that

synthesize cellulose microfibrils are localized within plasma membrane. These are called rosettes; add

glucose molecules to growing microfibrils. It is composed of polysaccharide cellulose but other

substance may be incorporated in it. Lignin or suberin may also present in it & epidermal cells of leaves

& stem also posses cutin & cuter waxes & are impermeable to water. Primary cell wall of yeast and

fungi is composed of chitn.

(iii) Secondary Cell Wall- It lies after primary cell wall. It is thick, permeable & lies close to tertiary

cell wall or lies close to plasma membrane .It consist of bulk of many layers and at maturity cell wall

consist It gives shape & mechanical strength to the cell .It is chemically composed of compactly

arranged cellulose & lignins.

(iv) Tertiary Cell Wall- It is found beneath the secondary cell wall. It differs form primary and

secondary cell wall, and contains xylans. (FIG I)

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1.4.2 CHEMICAL NATURE

Plant's cell wall are composed of carbohydrates known as cellulose. Besides cellulose various chemical

substances as hemicelluloses, pectin, lignin, cutin & chitn. The cell wall also contains certain minerals

like calcium & magnesium in the form of carbonates & silicates. The cellulose is a polysaccharide & it

is the most abundantly occurring chemical substance of most plant cell. Chemically cellulose contains

long chains of glucose molecules. Glucose is the structural unit of cellulose & about 3000 glucose

molecule are linked together to form long chain of cellulose molecule. A bundle of 100 chain molecules

of cellulose forces the elementary fibril known as micelle. The 20 micelle when get arranged parallel

form the fibrils of 25nm thick known as microfibrils (5-15nm in diametre).These microfibrils form large

sized bundles 0.5micrometre thick of cellulose fibre to form the macrofibrils. The macrofibrils

consisting of many cellulose fibrils. The hemicellulose is composed of monosaccharide units such as

arabinose, xylose, mannos& galactose. It occurs sometimes in between the macrofibrils of cellulose.

(FIG II)

HEMICELLULOSE

They are heterogeneous groups of carbohydrates polymers constructed from various 5&6 carbons sugars

including xylose, arabinose, mannose & galactose. The hemicellulose xylan which utilizes the pentose

xylose as its main building block, accounts as much as 50% of the cell wall in woody tissues.

Hemicellulose molecule binds to the cellulose microfibrils &to each other creating a coating that helps

to bond these microfibrils together into a rigid interconnected network of cellulose & hemicellulose.

(FIG II)

PECTINS

These are the polymers of carbohydrates D-galactouronic acid & D-glucouronic acid units .Like the

glycosoaminoglycane of animal cell, they readily form hydrated gels. This property of pectin molecules is

responsible for gelatin that occurs during the process of making jams and jellies .Pectin molecules are

24

involved in binding adjacent cell wall together & in forming the matrix in which cellulose microfibrils are

embedded. (FIG II)

LIGNINS

They are group of polymerized aromatic phenols. They occurs mainly in woody tissues, as cross linked

network that contributes to the hardening of the cell wall. Lignin molecules are localized between the

cellulose fibrils, where they function to rest compression forces . (FIG II)

GLYCOPROTEINS

They are important constituent of plant cell wall, accounting as much as 10% of the total mass. Among

the wall glycoprotein a group of common glycoprotein are called extensins. Extensins & other

glycoprotein forms cross linked networks with each other as well as with cellulose microfibrils (FIG II)

OTHER COMPONENT

In addition to the preceding components a small percent of the total mass of the cell wall is accounted

for the lipids, including waxes & other complex polymers .Minerals such as Ca & K also occurs in the

plant cell wall in the form of inorganic salts. (FIG II)

1.4.3 BIOGENISIS & GROWTH

The cell wall originates in the developing cell plate. During telophase of the mitotic cell cycle, the

phragmosome, a flattened membranous vesicle containing cell wall components,froms across the cell

within a cytoskeletal array called the phragmoplast. The noncellulosic cell wall polysaccharides

synthesized in the golgi apparatus and packaged in the vesicles fuse with the growing cell plate. The

plate grows outward until the edges of the membranous vesicle fuse with the plasma membrane, creating

two cells. Finally the new cell wall fuses with the existing primary cell wall, The plant golgi apparatus is

a factory for the synthesis, processing and targeting of glycoproteins. Thus except for cellulose the

polysaccharides, the structural proteins, and a broad spectrum of enzymes are coordinately secreted in

golgi derived vesicles and targeted to the cell wall.

Cell enlargement depends on the activities of endoglycosidase, endotransglycosylase or expansin or

some combination of these. But cell shape is largely governed by the pattern of cellulose deposition.

25

Termination of cell growth is accompanied by cross linking reaction involving proteins and aromatic

substances.

1.4.4 LET US SUM UP –

The plant cell surface is covered by a cell wall that provides enormous strength and mechanical

support to the whole plant.

The plant cell is composed of various composition of cellulose, hemicellulose, pectin, lignin,and

glycoproteins.

The cell wall is synthesized in a stepway fashion, sterting with a thin zone called the middle

lamella, follwed by the relatively thin and flexible primary cell wall and finally the much thicker

and the stronger secondary wall.

Cell wall developes from the cell plate during telophase.

26


1.4.6 ACTIVITES/ASSIGNMENTS

Prepare charts of cell wall arrangement pattern.

With the help of staining techniques visualize different cell wall component.

27

1. Calcium pectate is found in-

a. Hemicellulose.

b. Middle lamella.

c. Primary cell wall.

d. Secondary cell wall

2. The molecule that binds to the cellulose creating a coating of rigid interconnecting network is-

a. Hemicellulose.

b. Pectin

c. Lignin.

d. All of the above.

3. Cellulose microfibrils are twisted in ropelike fashion is termed as-

a. Hemicellulose.

b. Fibrils.

c. Lignin.

d. Macrofibrils.

4. The second most abundant organic compound on earth is-

a. Hemicellulose.

b. Pectin

c. Lignin.


5. The molecule responsible for gelation is-

a. Hemicellulose.

b. Pectin

c. Lignin.

d. cellulose.

1.4.7 CHECK YOUR PROGRESS : THE KEY

1. b

2. a

3. d

4. c

5. c

1.4.8 FURTHER READINGS


Biochemistry and Molecular Biology Of Plants, Buchanan Gruissem Jones

1.5.0 PLASMA MEMBRANE

1.5.1 COMPOSITION OF PLASMA MEMBRANE

Plasma membrane or biological membranes are composed of lipids, proteins & small amounts of

carbohydrates. The ratio of proteins to lipid varies considerably among different membranes.

Phospholipids are present in almost all the membranes. Cholesterol is common in the membrane of

mamalian cells. Cardiolipin is found only in the inner mictochondrial membrane .The plant plasma

membrane has a high sterol to phospholipid molar ratio. Carbohydrates are bound to the membrane in

the form of glycoproteins when attached to lipids. Carbohydrates are not Langmuir present in the

chloroplast lamellae, mictochondrial membrane s and other membranes of cell organelles. The major

component of the plant plasma membrane is carbohydrates in the form of glycolipids, glycoproteins &

various cell wall polysaccharides. The plant cell membrane has to perform some other functions than in

animal cell, particularly in meditating the transport of solutes into & out of the cell.

1.5.2 MODELS OF PLASMA MEMBRANE

(i) Lipid Monolayer Model Of Langmuir-The first scientific attempt to know the membrane

was made by who suggested that membrane was composed of phospholipid of one molecule

thick. It was shown by an experiment in which the phospholipid was spread on water. This

form layer of one molecules thick on water surface. Phospholipid are known as amphiphatic

molecules which contain both hydrophilic & hydrophobic regions .Langmuir interpreted his

28

model that hydrophilic or head groups of the lipid molecules remain attach to the water

surface and the hydrophobic tails remains free towards the air.(Fig I)

.

(ii) Lipid Bilayer Model Of Gorter & Grendel- Gorter & Grendel proposed a lipid bilayer model of

membranes structure from the experiments of RBC's. When lipid extracted from RBC's where

spread on the water surface, it was found that lipids were also spread as one layer on water. But it

covers twice the area on the water surface than that the area of the cell from which lipid is extracted.

The model of Gorter & Grendel gives a new impetus to membrane research as they first tried to

describe the structure of membrane at the molecular level. .(Fig II)

(iii) The Danielli-Davson Model-They concluded that biological membrane could not be of lipid alone.

Danielli & Davson proposed a molecular model of the membrane in which hydrophilic had groups

of the lipid molecules is covered on both of the side by a protein layer .The proteins are attached to

the hydrophilic head groups by lipid bilayer by ionic bonds . But in this model , the distance between

ends of the fatty acid chains (hydrophobic tails) is not specified . .(Fig III)

(iv) Robertson's Model Or Unit Membrane Hypothesis:-The presence of common structure in all

biological membrane led to postulate Unit membrane hypothesis .For detailed study of the

membrane structure & its molecular organisation , Robertson selected mylein as its experimental

29

sample .He selcted mylein rather than typical membrane because in case of mylein , multiple layers

of membrane are present which forms quasi-crystalline structure .He carried out investigations on

electron microscope using different stains for lipid & proteins . He found that both lipid & proteins

are present in the membrane .Lipid are present in two layers covered by proteins with lipid head

groups projecting outward towards both membrane surfaces .Robertson's observation corroborates

the structure proposed by Danielli & Davson .The electron microscopic observations & X-rays

diffraction data confirmed the Danielli & Davson model of membrane structure . .(Fig IV)

(v) Fluid Mosaic Model- In this model the main component is the lipid bilayer with hydrophilic groups

oriented towards outside & the hydrophilic groups towards inside of the layer . The basic

requirement for the basic requirements of the molecular organisation of the membrane is free energy.

The term fluid is given because the lipid layer is present in the fluid state. The transition of fluid

layer from non fluid (gel) conditions to a liquid crystalline (fluid) state depends on the temperature

of the cell. According to this model, proposed by SJ Singer & Jhon Nicholson, the principle of

membrane organisation is as follows:

1. Lipids are present in two layers.

2. Proteins are arranged in two ways:

a) Some are embedded in lipid layer, called integral proteins &

b) Some are present on surface of the lipid bilayer, called the peripheral proteins.

3. The lipid layer is usually in liquid crystal line, i.e., fluid state. .(Fig V)

1.5.3 FUNCTION

30

It forms protective covering over cytoplasmic organelles.

It is selective permeable in nature which allows only selectable molecules to pass through it.

Substance that pass through it by simple diffusion,facilitated diffusion and by active transport

method.

Simple diffusion deives the net movement of dissolved solutes as well as water molecules and

the process is termed as osmosis.

Faciliated diffusion refers to the assisted movement of a substance down its electrochemical

gradient.

Active transport is carried out by membrane transport proteins.

1.5.4 CARRIERS

Carriers do not catalyze ATP-hydrolysis. In other words, the transport process do not involve

chemical modification of any of the compound bound to the carrier. Rather carrier catalyze the

movement of inorganic ions and simple organic solutes across the membrane. One defining feature

of the carrier is that they display saturability when the kinetics of transport are expressed to the

substrate concentration. The array of ions and solutes translocated by carrier is vast. The principal

inorganic nutrients including NH4+/ NO3-,Pi,K+ and SO4- are all transported into the cell by

plasma membrane carrier. Carriers are also responsible for taking up ions that play less roles in

metabolism like Cl- ions. The organic solutes translocated into the cell by carrier are the

fundamental building blocks of polymers: sugars, amino acid, puriens and pyrimidines. .(Fig I).

Active transport involves membrane carrier that moves substance against an electrochemical

gradienti.e. an energy requiring process.The energy releasing activity includes the hydrolysis of high

energy molecules such as ATP, the absorption of photons of light, the transfer of elrctrons and the

movement of other substance down their respective electrochemical gradients.The active transport of

sodium and potassium ions is the classic example of an energy requiring transport system powered

by the hydrolysis of ATP. In most cells the concentration of potassium is higher than the

concentration of the sodium.Maintaining this unequal distribution of sodium and potassium is

important because potassium is needed inside the cell for the activity of certain enzymes and the

sodium is required for the active transport of sugar and amino acids.

31

1.5.5 CHANNELS-

Channels mediates the facilitated diffusion of small ions. As diffusion through ion channels does not

require complex conformational changes it is significantly faster than transport mediated by protein

carrier. Channels are simply open holes in the membrane. The two important points that distinguish

these channels from other openings are- they are capable of discriminating between different ions;

thus separate channels exist for transporting important cellular ions such as sodium. Calcium

potassium and chloride. In addition channels have gates that can be regulated in response to

appropriate stimuli.The ability of channels to regulate the diffusion of specific ions across cellular

membrane plays a significant role in many types of cellular communication.The outward rectifier

KCO1 is a member of the “two-pore” K+ channel family and is sensitive to the concentrations

of cytosolic Ca2+. An outward rectifier, KCO1 has been identified at the molecular level from an

Arabidopsis EST, based on the highly conserved P-domain motif TxGYGD. This channel is a

member of the “two-pore” K+ channel family, so-called because there are two P-domains in each

subunit. In contrast to Shaker-type channels, KCO1 has only four transmembrane spans in each

subunit. Two Ca2+-binding motifs called EF hands reside toward the C terminus of the protein. The

functional attributes of KCO1 as an outward rectifier have been confirmed by heterologous

expression. Analysis of these currents and the underlying channels demonstrates that although these

are cation-selective, they do not select as effectively for K+ as the inward rectifiers do. Such channels

probably constitute a major pathway for Na+ uptake into plant cells and could have important

implications for salinity tolerance. These channels are partially blocked by external Ca2+. An even

less selective channel has been observed in the plasma membrane of xylem parenchyma cells. This

channel is almost as permeable to anion as to cations; such low selectivity is unusual in plasma

membrane ion channels. Perhaps this channel could provide a pathway for release of salts from the

symplasm into the xylem.

Monovalent cation channels at the vacuolar membrane are Ca2+-sensitive and mediate

vacuolar K+ mobilization. Both types ofchannels activate instantaneously in response to an

imposed voltage. The fast vacuolar (FV) channel, which exhibits little selectivity among monovalent

cations, in inhibited when cytosolic Ca2+ concentrations exceed 1 pM and is activated when cytosolic

pH increases. Conversely the vacuolar K+ (VK) channels, which is highly selective for K+ over other

32

monovalent cations, is activated by cytosolic Ca2+ in the nanomolar to low micromolar concentration

range and is inhibited by increasing cytosolic pH. A wide range of metabolic and developmental

signals in plants trigger an increase in cytosolic free Ca2+. Thus, the increase in the concentration of

free Ca2+ activates downstream targets that further transducer the initial signal into the end response.

Voltage-gated Ca2+-permeable channels reside in the plasma membranes of a range of cell types and

have been characterized by both patch clamp and planar lipid approaches. The channels exhibit

various degrees of selectivity for Ca2+ over K+-from about 2:1 to 20:1. The channels are activated by

membrane depolarization.

Channels can interact in two fundamentally different ways, First passage of an ion (usually Ca2+)

through one channel can result in concentration changes that gate a different channel. Second,

channel opening can change Vm, leading to activation of voltage-gated channels. Both aspects of

channel interaction are combined in membrane-based signaling pathways that transduce and amplify

incoming signals. .(Fig I)

Voltage-dependent K+ channels at the plasma membrane stabilize and allow controlled K+

uptake and loss. Molecular evidence demonstrates definitively that the time-dependent inward and

outward currents are carried by separate classes of ion channel. The channels carrying these currents

are said to rectify. Like values, rectifying channels carry current in one direction but not the other.

For this reason, the channels are known as K+ inward rectifiers and K+ outward rectifiers. Both are

inhibited by millimolar concentrations of tetraethylammonium, a diagnostic blocker of K+ channels.

Plant cell inward rectifiers are members of the Shaker family of voltage-gated channels. Plant

inward rectifier subunits are products of a multigene family, members of which exhibit tissue-

specific expression. One member, KAT1 is expressed selectivity in guard cells another, AKT1 is

found in roots and hyadothodes. The AKT1 gene of Arabidopsis has been disrupted by using T-DNA

insertional mutagenesis. The resulting mutant, referred to as AKT1-1, has dramatically reduced K+

uptake less growth in media with low K+ concentrations. The analysis of K+ uptake in ATK1-1

mutants indicates that AKT1 encodes a high-affinity K+ channel.

Several structural features define plant inward rectifying channels as embers of the Shaker family, a

super family

1.5.6 PUMPS (SITES FOR ATPase)

33

ATP- powered pumps transport ions and various small molecules against the concentration

gradients. All ATP powered pumps are transmembrane proteins with one or more binding sites

located on the cystolic face of the membrane. These proteins are commonly called ATPases, they

normally do not hydrolyse ATP into ADP and Pi unless ions or other molecules are simultaneously

transported. The general structure of the four classes of ATP-powered pumps are shown in Fig-1.

The membrane of three classes P, F, and V transport ions only whereas members of the ABC super

family transport small molecules. All P-class ion pumps posses two identical catalytic alpha subunits

that contains two smaller beta subunits that have regulatory functions. During the transport process

at least one of he alpha subunit is phosphorylated, hence named P-class. This class includes the

Na+/K+ ATPase in the plasma membrane which maintains the low cytosolic Na+ and high K+

concentration typical of animal cell. The H+ pumps that generates and maintains the membrane

electric potential in the plants, fungal and the bacterial cells also belongs to this class. The structure

of the F-class and V-class are similar to one another and more complicated than P-class pump. F and

V class contains several different transmembrane and cystolic subunits. V-class pumps generally

function to maintain the low pH of the plant vacuoles and of the lysosomes by pumping protons

from the cytosolic to the exoplasmic face of the membrane against a protin electrochemical gradient.

F-class pumps are found in the bacterial plasma membrane and in the chloroplast and mitochondria.

In contrast to V- class pumps, F-class pump generally function to power the synthesis of ATP from

ADP and Pi by the movement of proton from the exoplasmic to the cytosolic face of the membrane

down the electrochemical gradient. The ABC super family includes several hundred different

transport proteins. Each ABC protein is specific for a single substrate or group of related substrate,

which may be ions, amino acid, sugars, phospholipids, peptides, polysaccharides, or even proteins.

All ABC trasport proteins shares. A structural organization consisting of four “core” domains: two

transmembrane domains forming the passageway through which transported molecules cross the

membrane and the two cytosolic ATP binding domains. In some ABC proteins mostly in the bacteria

the core domains are present in four separate polypeptides. .(Fig I)

34

1.5.7RECEPTORS

Signals can be perceived by protein receptors or through changes in membrane potential. To

initiate transduction, a receptor, Most known receptors are present in the plasma membrane,

although some are located in the cytosol or other cellular compartments. At least three different

classes of cell surface receptors have been detected in animals, but whether all three exist in plants is

still uncertain. Most identified receptors have turned out to be proteins. Protein receptors are not

easily identified - for example, the breaking of the dormancy of some buds or imbibed seeds by such

chemicals as ethanol, ether, azide, or cyanide. Either these chemicals are able to occupy established

cellular receptors or, more likely, many of them modify the membrane potential, the voltage across

the plasma membrane. The membrane potential can act as a receptor. The plasma membrane uses

pups and proteinaceous pores, called channels, to control the flux of ions into and out of the cell. The

sequences of many receptors have been determined. Many receptors have seven hydrophobic

domains placed strategically throughout the molecule. These hydrophobic domains are thought to

represent regions of the receptor that span the plasma membrane. In some transmembrane protein

receptors, the C-terminal region is phosphorylated by protein kinases. Two possible families of

protein kinases are distinguished on the basis of the amino acids they phosphyorylate on their

substrate proteins. Another class of receptors is the so called receptor-like protein kinases. The RLKs

of plants typically consist of a large extracytoplasmic domain, a single membrane-spanning segment,

and a cytoplasmic domain containing the active site of a protein kinase. Numerous RLKs have been

identified in plant cells, including protein kinases with seven membrane-spanning domains. Such

RLKs have benn detected in the male reproductive tissues of plants, where they are implicated in

incompatibility reactions that prevent fertilization.

Intracellular receptors can act as ion channels. Other receptors are located in intracellular

membranes and can act as Ca2+ channels. The most well-known receptor in this class binds the

second messenger inositor 1, 4, 5-triphosphate. Channels for another second messenger, cyclic ADP-

ribose (cADPR), have been reported recently. Occupation of the receptor (which may be composed

of four subunits) leads to the opening of Ca2+ channels and an influx of Ca2+ intot he cytoplasm from

the vacuole and the ER, each of which contains Ca2+ many orders of magnitude greater than the

cytosolic concentrations. In contrast to plasma membrane-bound receptors, these protein subunits

each have four membrane-spanning domains. Other membrane-spanning proteins also may have

35

important functions in signal transduction.

Not all tissues or cell types are able to respond to all signals. For example, fruit tissues become

sensitive to ethylene at a certain stage of ripening, whereas guard cells are totally insensitive to high

concentrations of the gaseous hormone. Different responses by different tissues to the same signal

can in part be explained by families of receptors. Auxin, for example can induce perieyel cells to

form adventitious or lateral rots, but in coleoptile cells it promotes elongation. Different receptors

are probably involved in each response. However, divergent downstream elements of the signal

transduction pathway may also distinguish the developmental responses to auxin exhibited by

different cell types. Tissue-specific signals transduction pathways are thus defined not only by the

presence or absence of receptors but also by the presence or absence of downstream apparatus

required to transduce the responses. Tissues can adapt or desensitize themselves to continuous

signals, and receptor concentrations can change during development. For example, when etiolated

seedlings are exposed to red or white light, the cellular concentrations of phytochromes decrease

rapidly.

The gas ethylene regulates ripening, germination, elongation, senescence, and pathogen responses.

Several ethylene receptors have been cloned through isolation of ethylene insensitive mutants and

subsequent use of molecular technology to identify the mutant gene. ETR1, a 79-kDa protein with a

transmembrane domain, was the first receptor cloned from Arabidopsis. The C terminus of ETR1 is

homologous to a bacterial two component system hybrid kinase. ETR1 exists as a dimer in the

plasma membrane. Ethylene joins the two monomers together and permits intermolecular

phosphorylation. Mutations in ETR1 (designated efr1) lead to loss of physiological sensitivity to

ethylene. ETRI has also been expressed in yeast to demonstrate that the protein binds ethylene with

high affinity. Competitive ethylene antagonists inhibit this binding. Expression of efr1 in yeast leads

to loss of ethylene binding, confirming that ETR1 is thus a true ethylene receptor. Genes encoding

other ethylene receptors have also been identified, including ERS (ethylene response sensor), Nr

(never ripe, .1 developmentally regulated gene from toma to; and LeTAE1 (a tomato ETR1 homolog

expressed during flower and fruit senescence.)

o Many auxin-binding proteins have been detected, but whether they represent receptors for

different auxin-mediated processes is still uncertain. Indole 3-acetic acid (IAA, reterred to here as

36

auxin) is a growth regulator with a wide variety of functions in cell division and expansion. Auxin

has been studied intensively for the past 50 years and, not surprisingly, receptors for the auxin signal

have been actively sought. Conventioanl pharmacological techniques have uncovered one well-

characterized auxin-binding protein (ABP1). The possible receptor function of this protein was

controversial for many years but has recently been established.

Phytochorome, a clearly identified receptor for red light, has protein kinase activity in

cyanobacteria. Phytochromes form a family of 120-kDa proteins. The photoreactive moiety

(chromophore) of these proteins is an open-chain tetrapyrrole. Two forms of phytochrome, A and B,

can each form dimers in solution and physiological evidence suggests that both may dimerize in

vivo. . (Fig I)

1.5.8 LET US SUM UP

Plasma membrane is a selective barrier of the cell.

Plasma membrane is constructed from phospholipids and proteins.

Membrane phospholipids forms a bilayer in which their hydrophobic tails are buried in the

membrane interior and their hydrophilic head groups are exposed at the membrane surface.

Substance that pass through it by simple diffusion,facilitated diffusion and by active transport

method.

Simple diffusion drives the net movement of dissolved solutes as well as water molecules and

the process is termed as osmosis.

Faciliated diffusion refers to the assisted movement of a substance down its electrochemical

gradient.

Active transport is carried out by membrane transport proteins that are capable of moving

substance against the electrochemical gradient. The energy required for active transport can be

provided by the hydrolysis of energy rich compounds.

37

1. The unit membrane model was proposed by-

a Daniel and Davson.

b. Robertson

c Singer and Nicholson

d. All of the above

2. Lipid bilayer concept was propounded by-

a Daniel and Davson.

b. Robertson

c Singer and Nicholson


3. The main function of the plasma membrane is -

a Protection.

b. Active transport

c Passive transport


4. Faciliated diffusion of small ions is carried out by-

a Pumps

b. Receptors

c Carriers

d. Channels.

5. RLKs are the example of the-

a Pumps

b. Receptors

c Carriers

d. Channels.

Membrane transport proteins that carries out facilitated diffusion are divided into carrier and

channel proteins.

Channel proteins forms small channels through which specific ions can pass.

1.5.9LETS CHECK YOUR PROGRESS-

38

1.5.10ACTIVITIES/ASSIGNMENTS-

Prepare the different models of plasma membrane

Prepare models of ATPase and enlist their examples.

Design small experiment like patoto osmoscope to explain the function of plasma membrane

1.5.11 CHECK YOUR PROGRESS: KEY

1 b

2. a

3. d

4. d

5. d



Gennis, R.D . Biomembranes: Molecular Structure and Function, Springer-Verlag New York.

M.K.Jain Introduction to Biological Membranes 2nd Edition Wiley New York.

1.6.0 PLASMODESMETA

1.6.1 INTERNAL STRUCTURE

In vascular plants the basic plasmodesmal structure is a tube of plasma membrane surrounding a strand

of modified ER, with particulate material between them. Fig I interprets the ultra structure of vascular

plants in a cross and longitudinal sections. Essental features of plasmodesmeta is a cell to cell tubule of

the plasma membrane that surrounds a cylindrical strand of tightly furled ER, the desmotubule. A thin

39

darkly stained central rod occupies the center of the desmotubule. A cytoplasmic sleeve or the

cytoplasmic annulus lies between the desmotubule and plasma membrane is considered as possible

pathway for the cell-to-cell water and solute movement. The desmotubule is essentially a solid strand of

lipid, the central rod is composed of the lipid polar groups and a few proteins that can physically occupy

the inner core of the tightly furled lipid bilayer. Much of the cytoplasmic annulus is occupied by

proteinaceous material. These particles are associated with both the outer surface of the desmotubules

and the inner surface of plasma membrane. In cross section, 7-to-9 gaps occurs between the particles.

The distance across the gaps, is about 2-3nm as comparable with the 4nm channel diameter. These gaps

are the physical basis of cell-to-cell transport. A wide range of plasmodesmal morphologies have been

observed, within the same plants. These include differences in length, branching and size of the central

cavity. The functional significance of the variations is largely unknown.

(FIG I)

1.6.2 FUNCTIONS

40

It plays important role in cell to cell communication. To investigate this function microelectrodes have

been employed to monitored the flow of electric current from cell to cell in plant tissue (as in case of

animals gap junctions.). Such studies reveals that electric current passes between plant cells linked by

plasmodesmeta more readily than it does between the cells that are not linked by plasmodesmeta. The

magnitude of the current flow is directly related to the number of plasmodesmeta present. This suggest

that it plays role in cell-to-cell communication, comparable to that played by gap junctions in animals.

1.6.3 COMPARISON WITH GAP JUNCTION

In plants junctions are termed as plasmodesmeta, whereas in animals it is termed as cell junctions.

In plants plasmodesmeta's are found in pit-feilds where primary cell wall is thin and secondary cell

wall is absent. In animals 3-types of junctions are found-

(i) Tight junction- which creates the permeability barriers across the layers of cell.

(ii) Plaque bearing junctions- which stablizes the cells against the mechanial stress.

(iii) Gap junctions- which prevents the movement across the cells.

Gap junctions in electron micrograph appears as regions in which the plasma membrane of the two

adjacent cells are aligned in parallel and separated by a tiny gap of the 3nm.Plasmodesmeta electron

micrograph reveals that these are narrow channels in the cell wall that are linend by plasma

membrane and transverse by the tubules of ER.

Connexin proteins are observed on the membrane surface o the gap junctions, which are clusters of 6

connexin molecules surounded by aquoues channels. No such structures are seen in plasmodesmeta.

Permeability of the gap junctions can be altered (closed and opened) by the changes in the

membrane potential, intracellular pH and cyclic AMP levels. Plasmodesmeta permeability is

subjected to regulation by the internal conc. of Ca ions, but its mechanism is not well understood.

In addition to their role in electrical coupling, gap junctions may have developmental and metabolic

functions. No such functions are performed by plasmodesmeta.

1.6.4 LETS SUM UP-

Channels in the cell wall known as plasmodesmeta permits the direct cytoplasmic connection for

communication between the adjacent cell.

41

They are present on the specialized areas of the cell wall called pit fields

In the area around the pit field primary wall is thinner and secondary wall is absent.

Unlike plasmodesmeta in animal cells gap junction permits small ions and molecules to pass

directly between adjacent cells.

The permeability of plasmodesmeta is subject to regulation by the internal concentration of

calcium ions.


42

.

1.6.6ACTIVITIES/ASSIGNMENTS-

Prepare the model of plasmodesmeta.

43

WRITE WITHIN THE SPACE PROVIDED

1. Explain the structure of plasmodesmeta in the given space.

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

2. Compare plasmodesmeta with gap junction of animal cell

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

……………………………………………………………………………………..

1.6.7 LETS CHECK YOUR PROGRESS- THE KEY

Answer of the above question is already provided in notes. Review your work accordingly.

1.6.8 REFRENCE/FURTHER READING


Morgan , N.G . Cell Signalling Gulford Press New York.

Hardy, D. G. Biochemical Messengers, Chapman and Hall , London.

UNIT-II CHLOROPLAST2.1 INTRODUCTION

2.2 OBJECTIVES

2.1.1 STRUCTURAL ORGANIZATION-

2.1.1.1 ISOLATION OF CHLOROPLAST COMPONENTS

44

2.1.1.2 FUNCTION

2.1.2 GENOME ORGANIZATION-

2.1.3 CHLOROPLAST REPLICATION AND PROTRIN SYNTHESIS

2.1.4 GROWTH AND DVISION OF CHLOROPLAST

2.1.5 RNA-EDITING

2.1.6 NUCLEOCHLOROPLASTIC INTERACTION.

2.1.7 LET`S SUM UP


2.1.9 ACTIVITIES/ASSIGNMENTS.


2.1.11 REFERENCE/FURTHER READINGS.

2.2 MITOCHONDRIA

2.2.1 STRUCTURE ORGANIZATION:

2.2.1.1 ISOLATION FRACTION

2.2.1.2 FUNCTIONS

2.2.2 GENOME ORGANIZATION-

2.2.2.1 MITOCHONDRIAL DNA REPLICATION

2.2.2.2 TRANSFER RNA`S

2.2.2.3 PROTEIN SYNTHETSIS FACTORS

2.2.2.4 ALTERED GENETIC CODE

2.2.3 BIOGENESIS.

2.2.4 LET`S SUM UP


2.2.6 ACTIVITIES/ASSGINMENTS

2.2.7 CHECK YOUR PROGRESS : KEY

2.2.8 REFRENCE/ FURTHER READINGS

2.3.0 VACUOLES

2.3.1 TONOPLAST MEMBRANE

2.3.2 FUNCTIONS-

2.3.2.1 DIGESTION

2.3.2.2 pH AND IONIC HOMEOSTASIS

2.3.2.3 DEFENCE AGAINST MICROBIAL PATHOGENS AND HERBIVORES

45

2.3.2.4 SEQUESTRATION OF TOXIC COMPOUNDS

2.3.2.5 PIGMENTATION.

2.3.3 LET`S SUM UP


2.3.5 ACTIVITIES/ASSGINMENTS


2.3.7 REFRENCE/ FURTHER READINGS

2.1 INTRODUCTION

A Leeuwenhoek observed the presence of chloroplasts in algae and, by the year 1800, chloroplasts had

been identified in the leaves of a wide variety of plant cells. Shortly thereafter it was discovered that

chloroplasts rupture when placed in a hypotonic solution, suggesting that they are bounded by a

semipermeable membrane.

The first insight into the biological function of chloroplasts was provided in 1894 by the German plant

biologist, Thomas Engelmann.In the algae Spirogyra chloroplast winds from one end of the cell to the

other. When the algal cells were illuminated with a finely focused beam of light and examined by light

microscopy. the oxygen-seeking bacteria were seen to congregate near those regions of the cell where

the chloroplast had been illuminated. It was therefore concluded that the light-absorbing and oxygen-

producing steps of photosynthesis are housed within the chloroplast.

Mitochondria means ‘Thread like granules’ found in all types of cells except prokaryotic cells and RBC

of mammals. first systematic studies was initiated in 1850 by Rudolph Kölliker who studied them in

cytoplasm of muscle cells. He isolated these particles from shredded muscle tissue and discovered that

they swell when placed in heater leading to conclusion that these particles are surrounded by

semipermeable membrane. In 1990 Leonor Michaelis discovered dye Janes green, which stains

mitochondria, but as cell consume oxygen the colour gradually disappears. Such changes indicate a

change in oxidation-reduction state suggesting its function in oxidation-reduction reaction. In 1948

functionally active mitochondria were finally isolated by George Hogeboom, Schneider and George

Palade. Lehninger demonstrated that isolated mitochondria are capable of carrying out the Kreb’s cycle,

electron transport and oxidative phosphorylation.

Vacuoles are fluid-filled compartments. They usually occupy more than 30% of the cell volume. Apical

meristem cells typically contain numerous small vacuoles which coalesce into one or a few large

46

vacuoles as the cell matures and expands in large mature cells the space occupied by vacuolar

compartments can approach 90% of the cell volume with most of the cytoplasm confined to a thin

peripheral layer connected to the nuclear region by tranvacuolar strands of cytoplasm.

2.2 OBJECTIVES-

To get acquainted with the double membrane organelle performing the function of photosynthesis

To understand the characteristic feature of the eukaryotic organelle frequently termed as

“POWERHOUSE” of the cell

To learn and understand the plant vacuoles as multifunctional compartments.

2.1.1 STRUCTURAL ORGANIZATION-

When viewed with the light microscope, chloroplasts typically appear as oval or spherical structures

exhibiting a prominent green pigmentation. They occur in the leaves and other green tissues of higher

plants, as well as in algae. The chloroplasts of leaf cells typically measure about 2-4 11min diameter and

up to 10 11min length, which is roughly twice the size of a typical mitochondrial profile. The

chloroplasts of algae may even be larger, and often assume unusual shapes such as spirals, cups, and

circular bands that wind around the cell. In. algae as few as one or two large chloroplasts are present per

cell, but plant cells typically contain several dozen or more. These multiple chloroplasts appear to be

largely independent of each other rather than forming an interconnected network, as often occurs in

mitochondria. Relatively little internal structure can be seen when chloroplasts are viewed with the light

microscope, although early light microscope did detect tiny green granules within the chloroplast, which

they named grana. Electron micrographs revealed that the organelle is surrounded by a membrane

envelope. This chloroplast envelope consists of two closely apposed membranes, termed the outer and

inner membranes, which are separated from each other by an intermembrane space. The chloroplast

envelope encloses the interior space, or stroma, of the chloroplast, which is analogous to the

mitochondrial matrix. Within the stroma is another set of membranes, called thylakoid membranes,

which are organized as a system of flattened sacs. The internal space bounded by the thylakoid

membranes is called the thylakoid lumen. Thylakoid membranes are organized in two different ways,

referred to as the stacked and unstacked configurations. In stacked or grana thylakoids, thylakoid sacs

are stacked upon each other like a pile of coins, generating large membrane masses that correspond to

the grana observed by light microscopy. Grana stacks are connected to one another by membrane-bound

47

channels referred to as unstacked thylakoids. Each unstacked thylakoid is a large flattened sheet that

makes connections to many or all of the individual thylakoids of a given granum stack. In this way, the

individual thylakoid lumens of 0each granum stack become interconnected 'both with one another and

with the thylakoid lumcns of other granum stacks. Hence the entire thylakoid membrane system of the

chloroplast defines a single, enormously complex, membrane enclosed compartment.

Negative staining has revealed that the outer surfaces of thylakoid membranes are studded with

spherical CFt particles, which resemble the Fl particles observed on mitochondria envelope, while

mitochondrial cristae are continuous with the inner membrane surrounding mitochondrion. As a result,

chloroplasts consist of three separate compartments (intermembrane space, stroma, and thylakoid

lumen), whereas mitochondria have only two compartments (intermembrane space and matrix).(Fig I)

48

2.1.1.1. ISOLATION OF CHLOROPLAST COMPONENTS

The development of techniques for isolating chloroplasts and their various components has played an

important role in advancing our understanding of the functional organization of this organelle.

Chloroplasts can be isolated from plant cells in one of several different ways. Some procedures utilize

harsh homogenization techniques, such as grinding cells with sand using a mortar and pestle, to break

the cell wall and release the chloroplasts into' solution. Following removal of nuclei by centrifugation at

low speed, chloroplasts are collected by centrifuging at low speed, chloroplasts are collected by

centrifuging at higher speeds. A more recent approach to chloroplast isolation employs the enzymes

cellulase and pectinase to break down the cell wall. The advantage of this approach is that the resulting

wall-less cells, or protoplasts, can be disrupted by gentler techniques prior to isolating chloroplasts by

centrifugation. Chloroplasts isolated under harsh conditions are usually capable of carrying out the light-

induced production of oxygen, ATP, and NADPH, but not C02 fixation. Electron micrographs of such

defective class II chloroplasts reveal the presence of little or no stroma, and broken or missing outer

envelopes. In contrast gentle disruption techniques yield completely intact class I chloroplasts that are

capable of carrying out the complete pathway of photosynthesis, including C02 fixation. Class I and II

chloroplast preparations have both been useful as starting material for isolating the various membranes

and compartments that make up the chloroplast. In one commonly used procedure, class I chloroplasts

are suspended in a hypotonic solution to rupture the chloroplast envelope, followed by isodensity

centrifugation to separate the stroma, outer envelope, and. thylakoids from one another. Alternatively,

class II chloroplasts lacking the outer envelope and stroma can be used as starting material for separating

the thylakoid membranes into stacked and unstacked thylakoids. In this approach chloroplasts are

disrupted in a special apparatus, called a French pressure cell,that shears the thylakoid membranes by

forcing them through a small orifice following rapid decompression. This shearing process breaks the

connections between the stacked and unstacked thylakoids, thereby allowing the two membrane

fractions to be separated from one another by differential centrifugation. Biochemical studies involving

components isolated in these ways have provided a great deal of information concerning the functional

organization of the chloroplast

(Fig II)

49

2.1.1.2 FUNCTIONS

The most important function of chloroplasts is to carry out the process of photosynthesis. The

process includes the light reaction and dark reactions.

The light reaction takes place in the grana of the thylakoid membrane. In light reaction NADPH

and ATP are produced.

Dark reaction occurs in stroma of the chloroplast.

In this reaction reducing power of NADPH is used to fix co2 into carbohydrates.

50

2.1.2. GENOME ORGANIZATION

Chloroplast DNA is also circular and present in multiple copies. Early studies of chloroplast DNA

utilized isodensity centrifugation as the main tool for its isolation. Although this approach works

well with unicellular plants in higher plants most of the isolated DNA components initially believed

to be chloroplast DNA turned out not to be chloroplast DNA at all. These early experiments, in

which DNA prepared from isolated chloroplast was analyzed by cesium chloride isodensity

centrifugation, revealed the presence of three DNA components: a major component with a density

of about 1.696g/cm that was thought to represent contaminating nuclear DNA, and two minor DNA

components denser than nuclear DNA that were thought to represent chloroplast DNA.

But in 1971 the chloroplast DNA of the alga Euglena was isolated as a single large circle,

suggesting that the linear typical plant leaf cell contains about 10,000 chloroplast DNA circles

distributed among 50 to 100 chloroplasts, giving each chloroplast between 100 to 200 DNA

molecules. Depending on the organism. chloroplast DNA contains anywhere from 70,000 to more

than 5,00,000 base pairs, with an average of 1,50,000 base pairs being typical for the chloroplasts of

higher plants the presence of DNA in chloroplasts is not indisputable evidence that this DNA

contains genes governing chloroplast traits. Independent support for the existence of circular DNA in

chloroplasts has come from the genetic studies of Ruth Sagar, who employed the antibiotic

streptomycin to induce mutations in chloroplast genes of the green alga Chlamydomonas.

51

2.1.3 REPLICATION AND PROTEIN SYNTHESIS- Chloroplast DNA replicate by a semi

conservative mechanism involving the formation of D Loops.(Fig A)

Chloroplast contains 70S ribosomes.

Chloroplast ribosomes resembles with

prokaryotes in following features-in use of N-formylmethionine as initiating codon and in basic

structures of their RNAs.

Chloroplast synthesize polypeptides whose

genes reside in chloroplast DNA.

Chloroplast protein synthesis has been

studied the first product of chloroplast protein synthesis to be identified was the large subunit of

rubisco, the CO2-fixing enzyme of the calvin cycle that consists of a large catalytic subunit and a

smaller regulatory subunit. synthesis of the small rubisco subunits is selectively inhibited in the

presence of cycloheximide, whereas formation of the large subunit is blocked by chloromphenicol.

52

Thus the small subunit in the chloroplast. Verification of this conclusion has come from experiments

showing that isolated chloroplasts synthesize the large but not the small rubisco subunit. Moreover

the gene coding for the large rubisco subunit has been identified in chloroplast DNA, thereby

providing further support for the idea that this polypeptide is synthesized in chloroplasts. although

rubisco is a soluble protein located in the chloroplast stroma, many polypeptides synthesized by

chloroplasts have turned out to be thylakoid membrane proteins. Among them are subunits of protein

complexes involved in photosynthetic electron transfer (photosystem II, the cytochrome b0f complex

and photosystem I) and subunits of the ATP-synthesizing CF0-CF1 complex.

Chloroplasts also synthesize some of the

polypeptides found in chloroplast ribosomes and chloroplast RNA polymerase. As in the case of

mitochondria, all the polypeptides synthesized by chloroplasts are encoded by chloroplast DNA.(Fig

III)

2.1.4 GRWOTH AND DIVISION OF CHLOROPLASTS OR PROPLASTIDE

The microscopic evidence suggests that chloroplasts usually arise by growth and division of

existing chloroplasts. In algae, which contain only one or two large chloroplasts per cell, the fate of

individual chloroplasts is easy to watch light microscopists discovered that each chloroplast in cells of

the green alga spirogyra divides in half prior to cell division. chloroplasts isolated from higher plant

tissues have also been reported to divide when placed in an artificial growth medium. Not all plant cells

contain chloroplasts. The various cell types found in higher plants all arise from a rapidly dividing,

undifferentiated tissue called meristem. Meriste1n cells lack chloroplasts, but they have small organelles

called proplastids that contain chloroplast DNA. Proplastids measure 1 μm or less in diameter and

consist of an' undifferentiated stroma surrounded by a double-membrane envelope. Depending on where

they occur in the plant and how much light they receive, proplastids develop into different kinds of

plastids designed to serve different functions. Some proplastids differentiate into amyloplasts, which are

starch-filled particles that predominate in starchy vegetables such as potatoes. Other proplastids acquire

red, orange or yellow pigments, forming chromoplasts that give flowers and fruits thyeir distinctive

colours. Although proplastids can develop into a variety of organelles, their main fate in photosynthetic

tissues is to evolve into chloroplasts. The conversion of proplastids into chloroplasts requires light,

which triggers an enlargement of the proplastid and the formation of membrane tubules that project from

53

the inner membrane into the stroma. The tubules then spread into flat sheets that line up in parallel to

form thylakoids. At the same time, chlorophyll and other components of the photosynthetic electron

transfer chain are produced and incorporated into the thylakoids. If developing plant seedlings are

allowed to grow in the dark, a different sequence of events takes place. The proplastids still enlarge and

membrane sheets invaginate from the inner membrane, but these sheets condense into a set of

interconnected membrane tubules known as a prolamellar body. The membranes of a typical prolamellar

body arc arranged as highly ordered hexagonal arrays of membrane tubules. At this stage the plastid is

called an etioplast. l.ksides developing from proplastids in dark-grown seedlings, etioplasts also arise

when green plants containing mature chloroplasts are transferred from light to darkness. Under such

conditions, the thylakoid membranes break up into tubules that fuse together to produce a typical

prolamellar body converting the chloroplast into an etioplast. They act as chloroplast precursors that can

develop into functional chloroplasts when exposed to light. Within a few hours after an etioplast is

illuminated, the prolamellar body becomes transformed into an irregular mass of tubules; membrane

sheets then grow out from the tubules and develop into thylakoid membranes. This conversion of an

etioplast into a chloroplast is accompanied by the formation of components of the photosynthetic

electron transfer chain, such as chlorophyll. Etioplasts contain high concentrations of

protochlorophyllide, which is a metabolic precursor of chlorophyll. When etioplasts are illuminated, the

absorption of light by protochlorophyllide triggers its rapid conversion to chlorophyll.

2.1.5 RNA Editing Because rRNAs and tRNAs are non-coding chemical modification to their nucleotides

affect only the structural feature and possibly catalytic activities of the molecules, With mRNAs the

situation is very different: chemical modification has the potential of change the coding properties of

the transcript, resulting in an equivalent alteration in the amino acid sequence of the protein that is

specified. A notable example of RNA editing occurs with the human mRNA for apolipoprotein B.

The gene for this protein codes for a 456-amino-acid polypeptide, called apolipoprotein B100, which

is synthesize: d in liver cells and secreted into the bloodstream where it transports lipids around the

body. A related protein, apolipoprotein B48, is made by intestinal cells. This protein is only 2153

amino acids in length and is synthesized from an edited version of the mRNA for the full-length

protein. In intestinal cells this mRNA is modified by deamination of a cytosine, converting this into

n uracil. This changes a CAA codon, specifying glutamine, into a UAA codon, which causes

54

translation to stop, resulting in the truncated protein. The deamination is carried out by an l{NA-

binding enzyme which, in conjunction with a set of auxiliary protein factors, binds to a sequence

immediately downstream of the modification position within the mRNA (Smith and Sowden, 1996).

Although not common, RNA editing occurs in a number of different organisms and includes a

variety of different nucleotide changes. Some editing events have a significant impact on the

organism: in humans, editing is partly responsible for the generation of antibody diversity

(Neuberger and Scott, 2000; Section 12.2.1) and has also been implicated in control of the HIV-1

infection cycle (Bourara et aI" 2000). One particularly interesting type of editing is the deamination

of adenosine to inosine, which is carried out by enzymes called adenosine deaminases acting on

RNA (AGARs) (Reenan, 2001). Some of the target mRNAs for these enzymes are selectively edited

at a limited number of positions, These positions are apparently specified by double-stranded

segments of the pre-mRNA, formed by base-pairing between the modification site and sequences

from adjacent introns. This type of editing occurs for example, during processing of the mRNAs for

mammalian glutamate receptors (Scott, 1997). Selective editing contrasts with the second type of

modification carried out by ADARs, in which the target molecules become extensively deaminated,

over 50% of the adenosines in the RNA becoming converted to inosines. Hyperediting has so far

been observed mainly, but not exclusively, with viral RNAs and is thought to occur by chance, these

RNAs adopting base-paired structures that fortuitously act ns substrates for ADAR. It may, however

physiological importance in the etiology of disease caused by the edited viruses. This possibility is

raised by the discovery that viral RNAs associated with persistent measles infections (as opposed to

the more usual transient version of the disease) are hyperedited (Bass, 1997)

2.1.6 NUCLEO CHLOROPLASTIC INTERACTION

Nuclear Genes Cooperate with

Chloroplast Genes in Making Chloroplast proteins chloroplasts carry less then 10 percent of the

genetic information required to assemble chloroplast. Most of the polypeptides needed by

chloroplasts are encoded by nuclear genes, synthesized on free cytoplasmic ribosomes, released into

the cytosol, and imported into chloroplasts using the targeting mechanisms. Polypeptides destined

for uptake by chloroplasts contain signal sequences that bind to receptor proteins located in the outer

membrane of chloroplast. After binding, the polypeptide chain is inserted into or transferred across

55

the chloroplast membranes, depending on its ultimate location in the organelle. The signal

sequences are then cleaved by proteases to generate mature protein molecules.

Since some genes coding for chloroplast

polypeptides occur in the nucleus while others reside within the chloroplast the question arises as to

how the expression of the genes located in the organelles is coordinated with those residing in the

nucleus. This issue is especially important for proteins that require polypeptides encoded by genes

located in two different compartments for example rubisco requires one polypeptide encoded by

chloroplast DNA and one polypeptide encoded by nuclear DNA. The mechanism that coordinates

the synthesis of polypeptides made in separate locations but destined for assembly into the same

protein is not well understood. In chloroplasts, some insights have emerged from studies involving

the synthesis of the large and small subunits of rubisco. If an inhibitor of cytoplasmic protein

synthesis is added to intact cells to block formation of the small rubisco subunit by cytoplasmic

ribosomes, synthesis of the large rubisco subunit by chloroplasts eventually slows down as well,

even though the inhibitor has no direct effect on protein synthesized within the organelles are

manufactured in the cytoplasm and then imported? The answer to this question may lie in the

evolutionary history of chloroplasts.

Chloroplast though to have evolved from

bacteria that were ingested by eukaryotic cells a billion or more years ago. As time progressed and

the ingested bacteria evolved into chloroplasts, most of the bacterial genes were either lost or

transferred to the nucleus. But why weren’t all the chloroplast genes relocated to the nucleus? The

answer may be related to the fact that many polypeptides encoded by chloroplast DNA are

hydrophobic molecules that reside in the thylakoid membrane. Because it is thermodynamically

unfavourable to place a hydrophobic molecule in an aqueous environment, it would be

disadvantageous to synthesize hydrophobic polypetides on free cytoplasmic ribosomes and then

release them into the cytosol prior to transport into the chloroplast. Such polypeptides are therefore

synthesized on ribosomes attached to the thylakoid membrane; this arrangement allows the

polypeptides to be inserted directly into the membrane as they are being synthesized, just as

ribosomes bound to the ER insert their newly synthesized polypeptides directly into the ER

membrane. In most cases the gene that codes for a given chloroplast polypeptide occurs either in the

organelle or in the nucleus, but not both. However, exceptions do occur. Most of these nuclear

sequences contain fragments of different chloroplastic genes that are fused together, making them

genetically inactive. Thus when similar sequences are present in both nuclear and chloroplast DNA,

56

in some instances the nuclear sequences are functional and in other cases the chloroplast sequences

are functional.

2.1.7 LET`S SUM UP-

The chloroplast is the site of photosynthesis in eukaryotic cells.

Chloroplast is enclosed by a double membrane envelope consisting of an outer membrane and inner

membrane

Inside the chloroplast contains a mixture of stacked and unstacked thylakoid membrane that enclose

a compartments known as the thylakoid lumen.

The compartment between the thylakoid and the chloroplast envelope is the stroma.

The light reactions of photosynthesis occur in the thylakoid membrane while the dark reaction

occurs in the stroma.

Chloroplast contains their DNA molecules which are transcribed into ribosomal, transfer and

messenger RNA`s.

Chloroplast DNA`S codes for120 or more products including 4rRNA`S 30 or more tRNA`s 20

ribosomal proteins, 4 subunit of chloroplast RNA polymerase, the largest subunit of rubisco, and

several dozens polypeptide components of the cytochrome b6f complex , photosystem I and II and

chloroplast ATP synthase.

Genetic information encoded in chloroplast

DNA`S is transcribed and translated using RNA`s polymerase , ribosomes , and tRNA`s that are

different from those used in the

transcripition and translation of information

encoded in nuclear genes.

New chloroplast arise by growth and

division of either existing chloroplast or precursor organelles known as proplastides and etioplast

Most of the chloroplast polypeptides are

encoded by nuclear genes , realeased into cytosol, and imported into chloroplast.

57

2.1.8 LET’S CHECK YOUR PROGRESS

2.1.9 ACTIVITES/ASSIGNMENTS-

59

1. Chloroplast contains- a. carotenoids b. chlorophyll c. leucoplast d. All of the above2. The flattened membrane sac of chloroplast is called- a. crista b. F1 factor c. stroma d. thylakoid3. The chloroplast of many algae contains- a. Grana b. thylakoid c. pyrenoid d stroma4. Protoplast differentiates into a. Grana b. thylakoid c. plastids d stroma5. Chloroplast arises from – a De-nova synthesis

b Growth and division

c Division

d All of the above

Prepare a model of chloroplast reflecting its internal details.

Isolate chloroplast from various tissues and prepare slides .

2.1.10 CHECK YOUR PROGRESS : THE KEY 1 b 2 d 3 c 4 c 5 b

2.1.11 REFRENCES- CELL AND MOLECULAR BIOLOGY-2nd EDITION P.K.GUPTA.

MOLECULAR CELL BIOLOGY 2nd EDITION DARNELL,LODISH;BALTIMORE

BIOCHEMISTRY AND MOLECULAR BIOLOGY OF PLANTS BUCHANAN. GRUISSEM.

JONNES.

2.2 MITOCHONDRIA

2.2.1 STRUCTURE ORGANIZATION

George Palade published the first high resolution electron micrographs of Mitochondria. The

Mitochondria, as an organelle exhibits a complex internal architecture. It is encompassed by too

membranes-an outer mitochondrial membrane and inner mitochondrial membrane. Outer member define

the external boundaries of the organelle and inner mitochondrial membrane exhibits numerous folds

known as aristae that projects into the mitochondrial interior.

The detailed structure of mitochondria is shown in fig.1

Inner mitochondrial membrane is convoluted, the folds consisting of double membranes and called

cristae(animals) & tubule or microvilli in plants. The fine structure of mitochondria can change in

60

different cells of a tissue, at different stages of development the aristae may be simple or branched

forming a complex network.

Attached to M-face [matrix facing] of Inner membrane are repeated units of stalked particles called

elementary particles or inner membrane subunits or oxyosome. These particles are 8.5mm (85A) in

diameter and are regularly spaced at intervals of 10nm on the inner surface of these membrane.

The outer and inner mitochondrial membrane provider the mitochondria into two district compartments

the inter membrane spaces located between outer and inner membrane and mitochondrial matrix. When

is enclosed by the inner membrane space.

2.2.1.1 ISOLATION OF MITOCHONDRIAL COMPONENTS

The first successful technique for separating inner and outer membrane developed by Donald

Parson. In this procedure mitochondria are placed in a hypotonic solutions exposed to the detergent

digitonion until the outer membrane ruptures, releasing the contents of the inter membrane space into

solution. The inner membrane, outer membranes and components of inter membrane space are then

separated by centrifugation. Isolated inner and outer membrane can be readily distinguished from each

other by electron microscopy. Outer membrane book like empty sacs and inner membranes from

vesicles called mitoplast containing matrix material within. Mitoplast can be further fractioned into inner

membrane and matrix component by treating with detergent Lubrol. Lubrol distrups inner membrane

into small inner membrane vesicles containing F1 particles. Biochemical studies of isolated fraction.

provides important information regarding the location of various metabolic activities of the

mitochondria.

2.2.1.2 Function

Outer membrane of Mitochondria is permeable and contains enzymes involved in lipid metabolism.

61

It contains monoamine oxidase enzyme Fatty acyl Co-A synthetase glycerolphosphatase acyl

transfrase & Porin.

Inner Mitochondrial is the main site of Mitochondrial ATP formation and it is semi permeable in

matrix .

Mitochondrial matrix contains all of the enzymes of Kerb’s cycle.

Inner Mitochondrial membrane is also the site of several enzymatic pathways that are not directly

related to energy metabolism e.g. involves in the synthesis of steroid harmones.

2.2.2..GENOME ORGANIZATION

The genetic evidences indicate that mitochondria contain genes distinct from those located in the

nucleus. Mitochondrial DNA’s was identified on the basis of its unique density. The amount of

mitochondrial DNA found in eukaryotic cells varies. In animal mitochondrial DNA-typically accounts

for 0.1 to 1.0 percent of a cells total DNA contents in Yeast its value is high as 15%. The DNA located

in mitochondria exhibits several features that distinguished it from nuclear DNA. One major difference

is that mt DNA lacks histone and hence not packaged into nucleosomes.

Mt DNA is typically Circular. The mitochondrial DNA circles of animals cells are the smallest about

15,000-20,000 bp in size. Mt DNA of fungi and protists are larger i.e. 20,000-1,00,000bp and plant mt

DNA are largest ranging from 2,00,000bp to million bp in length. One to the tendency of large circular

DNA’s to break during isolation many reports suggest that plant mt DNA is linear rather than circular.

Few unicellular prokaryotes such as Paramecium have linear mol of mt DNA’s.

In animals, fungi and protists mitochondrial contains few dozens to several 100’s mol of DNA, all of

which are identical.

The first direct evidence for an association between cytoplasmic inherited tracts and mt DNA was

produced by Piotr Solnimski et.al. they demonstrated that mt DNA obtained from yeast bearing petite

mutation has a different density than the mt DNA of normal yeast Petites is induced by treatment with

ethidium bromide as a model system. These studies revealed that ethidium bromide stimulates the

breakdown of mt DNA and inhibits mt DNA replication. Long term exposure to ethidium bromide

causes the complete destruction of mt DNA, but if treatment is stopped before all the DNA has been

degraded, the remaining DNA resumes replication until the total context of mt DNA reaches normal

levels. But the new mt DNA is grossly abnormal containing few of its sequence present in the original

62

mt DNA. The presence of abnormal mt DNA in petite yeast strongly argues that this DNA is

responsible for the abnormalities in mitochondrial structure and respiratory activity observed in petites

mutants.

2.2.2.1 Mitochondrial DNA-replication

The replication of mt DNA is carried out by DNA p-ax that operates independently of nuclear DNA p-

ase, as mt DNA synthesis can take place anytime during the cell cycle i.e. they are not restricted to S-

phase when nuclear DNA replicates. In 1971 Jerome Vinogard and Piet Borst independently discovered

that mt DNA mols exhibit a small 100p.displaced from the main circle and mode of replication is semi

conservative. Denaturation of DNA mols causes a small fragment of S.S.DNA to be released, suggesting

that loop is caused by displacement of one of the two strand of double helix, therefore it is termed as ‘D’

loop (Displacement loop)

mt DNA codes for ribosomal RNA’s, t-RNA’s and small mitochondrial polypeptides

Several evidence suppose the conclusion that mt DNA serves as a template for transcription of mt

RNA’s. 2nd-mitocondria contains their own RNA-p-ase that can be distinguished from nuclear. RNA’s

by differing susceptibility to inhibitors. 3rd ethidium bromide inhibits mt DNA synthesis and not nuclear

DNA synthesis supports the conclusion that independent RNA synthetic pathways exist in nuclear and

mitochondria.

63

mitochondria synthesis proteins using a unique sets of ribosomes, tRNA’s and protein factors.

The mRNA’s transcribed by mt DNA are translated into polypeptides within mitochondria using.

Protein synthesizing machinery that differs from the machinery present in the cytorol.

Ribosomes: Mitochondrial ribosomes differs from cytoplasmic ribosomes in size & chemical

contraction (Table – I)

I – sizes of various types of ribosomes

Source IntactRibosomes

RibosomalSub units

RibosomalRNA’s

Mitochondria

(mammals)

55-605 Large – 455

Small - 355

165

125

Mitochondria

(yeast)

755 Large – 535

Small - 355

215

145

Mitochondria

(Plants)

785 Large – 605

Small – 405

265

185, 55

Cytoplasm

Eukaryotes

805 Large – 605

Small - 405

285

185, 5.85, 55

2.2.2.2. Transfer RNA’s

Mitochondria contains t-RNA’s and amino-acyl t-RNA synthetase. The amino-acyl t-RNA synthetase

are coded by nuclear DNA by t-RNA’s are transcribed from mitochondria differs in sequence from the

corresponding t-RNA’s found in cytosol. Mammalian mt DNA codes for only 22t RNA’s following

wobble rules Another unusual features of mitochondrial t-RNA is presence of the t-RNA for

formylmethionine. This type of t-RNA is once thought to be found in bacterial only. Its presence in

mitochondria suggest that protein synthesis in the two organelle inhibits some similarities to the protein

synthesis of bacterial.

2.2.2.3 Protein synthesis factors

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Protein synthesis factors of mitochondria are structurally distinct from its counterpart in the cytosol.

Several protein synthesis factors present in mitochondria are functionally inter changeable with those

obtained from bacterial cells.

2.2.2.4 Altered Genetic code

Examining base sequence of mitochondrial DNA’s has revealed that mRNA is translated into

polypeptide chair using a genetic code that is slightly different from universal code system difference in

codon usage can occur even among the mitochondria of different organism (Table – II)

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II – Examples of Altered Genetic Code Usage in Mitochondria

S.N. Codon

Universal code system

Mammals Yeast

1 AUA Ile Met Met

2 UGA Stop Trp Trp

3 AGA,

AGG Arg Stop Arg

4 CUU

CUC

CUA

CUG

Leu Leu Thr

Mitochondria synthesize polypeptides whose genes resides in mt-DNA.

Nuclear gene co-operates with mitochondrial gene in making mitochondrial protein.

2.2.3. BIOGENESIS

New mitochondria arises by growth and division of existing mitochondria or pro mitochondria

Microscopic evidences support this idea. Mitochondria in the process of dividing have been observed

under phase contrast microscope and membrane partition dividing mitochondria in half can be detected

by electron microscope in green algae. Micromonas which contain single mitochondria but the

microscopic observations do not rule out the possibilities that mitochondria also arise by other

mechanism. In 1960’s David Luck designed experiment on Neurospora, that focused whether such

alternative mechanism exist. 1st Neurospora colonies were incubated in 3H-choline (a phospholipid

precursor), so it could be incorporated within mitochondrial membranes. Cultures were then transferred

to non-radioactive medium and allow to grow for varying periods of time and finally examined by

microscopic auto radiography and silver grains were counted to determine the distribution of

radioactivity one of the 3 patters would be emerged:

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1. If new mitochondria are formed by growth & division of existing mitochondria then each

doubling in the number of mitochondria, after the 3H choline is removed it should be

accompanied by 50 percent reduction in the no of auto radiographic grains/ mitochondria.

2. If de-novo synthesis occurs then new mitochondria formed after abnormal of 3H-choline should

be non-radioactive while the radioactivity in the pre-existing mitochondria would remain the

same.

3. If mitochondria develops from other membrane then the pre-existing radioactive mitochondria

should retain their radioactivity and new mitochondria should at 1st be radioactivity because the

membrane from which it develop would be radioactive. But as the time parse, the absence of 3H-

choline from the medium would decrease in the amount of radioactive in the cellular membranes

from which mitochondria develops and there form the average amount of radioactive/labeled

mitochondria would decrease Data obtained in the studies followed 1st pattern suggesting that

new mitochondria aries from growth and division of existing mitochondria.

In some cases mitochondria develop in the cells that appears to have no pre-existing

mitochondria e.g. yeast which can survive either aerobic or aerobically. When switched to

anaerobic conditions, the cells meet all their energy requirement through glycolysis and so

mitochondria gradually disappears. When some cells are exposed to oxygen, mitochondria re-

appears. Previously such conditions was thought to be de-novo formation. But electron

microscopic examination revealed the presence of small double-membrane enclosed vesicular

about 1mm in diameter that lacks costae and cytochrome but contains mt DNA. When

anaerobically grown Yeast are exposed to-Oxygen the inner membrane forms crested that

synthesize new cytochrome and other respiratory chain components. Because of their ability to

differentiate into mitochondria the small double-membrane vesicles are called pro mitochondria.

2.2.4. LET`S SUM UP-

Mitochondria functions as ATP generating organelles.

It is found in nearly all eukaryotic cells.

It is bounded by two membrane with the inner membrane folded into cristae, that are responsible

for energy generation by the tricarboxylic acid cycle, electron transport and oxidative

phosporylation.

Within the region enclosed by the inner membrane, the matrix , they have DNA,ribosomes, and

other molecules necessary for protein synthesis

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1. The word Mitochondria was coined by-

a Kolliker

b Altman

c Bensley

d C. Benda

2. In plants the inner mitochondrial foldings are called-

a matrix

b stroma

c mitochondrial space

d microvilli

3. Mitochondrial DNA replicates by-

a Rolling circle model

b Semi-Conservative model

c Conservative model

d All of the above

4. Biogenesis of mitochondria takes place through-

a De-nova synthesis

b Growth and division

c Division

d All of the above

5. Mitochondrial DNA codes for-

a ribosomal RNA

b transfer RNA

c small mitochondria polypeptides

d All of the above.

Mitochondrial ribosomes are 70S type and posses altered genetic code system.

2.2.5. CHECK YOUR PROGRESS

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2.2.6 ACTIVITIES/ASSIGNMENTS- Prepare a model of mitochondria reflecting it`s internal details.

Isolate mitochondria from various tissues and prepare slides .

Compare and list out the difference between plant and animal mitochondria

2.2.7 LET`S CHECK YOUR PROGRESS: THE KEY

1. c

2. d

3. b

4. b

5. d

2.2.8 REFRENCES-

CELL AND MOLECULAR BIOLOGY-2nd EDITION P.K.GUPTA.

MOLECULAR CELL BIOLOGY 2nd EDITION DARNELL,LODISH;BALTIMORE

2.3.0 VACUOLES

2.3.1 TONOPLAST MEMBRANE

Vacuoles is surrounded by vacuole membrane known as tonoplast or vacuolar membrane. It resembles

the plasmalemma, but it differs in function and is often thinner. The plasmalemma controls the entry and

exist of solutes into the cytoplasm, the tonoplast transports solutes into and of the vacuoles and thus

controls the water potential of the cell. This is especially important for example, in the guard cells of the

stomatal apparatus. Potassium and other ions are pumped in or out of guard cells to swell or shrink,

closing or opening of stomates. The tonoplast is derived from the ER, but recent evidences suggest that

this may be via Golgi apparatus, as in case with the plasmalemma. In some cases ER may small (dilate)

directly to form vacuoles.

2.3.2 FUNCTIONS OF VACUOLES

69

vacuoles plays several metabolic roles-

1. Storage and accumulation: vacuoles store a large varieties of molecules, including inorganic

ions, organic acids, sugars, enzymes, storage proteins and many types of secondary metabolites.

Proteins in the tonoplast transport all of these molecules except for storage proteins into the

vacuoles. The resulting accumulation of solutes in the vacuole drives the osmotic uptake of

water, producing the turgor pressure needed for cell enlargement. Some vacuoles have high

concentration of pigments, which produce the colour of many flowers or the red or red maple

leaves. In maple leaves red colour is so concentrated in epidermis that they mask the green

colour of the chloroplast many primary metabolites can be retrieved from vacuoles.

2. Plant cell expansion is driven by a combination of osmotic uptake of water into the vacuoles and

altered. Cell wall extensibility the water taken into the vacuoles generates turgor pressure which

not only expands the primary cell wall but also creates stiff load bearing structures in

conjunction with the wall. Wilting and the associated softening of plant organs is caused by loss

of water from the vacuoles and surrounding cytosol.

To maintain the turgor pressure of continuously expanding cells solutes must be actively

transported into the growing vacuoles to maintain its osmolarity. An electro chemical gradient

across the tonoplast provides the driving force for this uptake solutes. The gradient is produced

and maintained by two electrogenic protons pump. V-type H+ ATpase and vacuolar H+

pyrophosphates (H-Ppase). The principal solutes in vacuoles include the ions K+, Na+, ca2+, Mg2+,

Cl–, So42–, Po4

3– and No3– and primary metabolites, such as amino acids, organic acids and sugar.

The moment of water across the tonoplast is mediated by aquaporin channels consisting of

tonoplast intrinsic proteins (TIPs).

2.3.2.1 Digestion (vacuoles as lysosomes)

The enzymes in vacuoles digest vacuous materials absorbed in them. This probably happens when the

protoplast of wood cells break down and die vacuole behaves like lysosomes, a cellular organelle

common in animal and some fungal and protest cells. Lysosomes contain hydrolytic enzymes that break

down materials they absorb or these enzyme digest mucho of the protoplasm following cell death and

breakdown of lysosome membranes. The importance of this role for vacuole is still begin investigated

because not all enzymes that breakdown protein in cells are present in vacuoles. In Yeast 90% of these

enzymes are located in vacuoles.

70

2.3.2.2 pH and ionic homeostasis

Homeostasis is the tendency for various physiological parameters to be maintained at some

relatively constant level. A good example in plant is relatively constant concentration of various

substance in the cytosol. The concentration of H+(pH) provides an example and the vacuole plays an

important role in the maintenance of constant cytosolic pH provides an example and the vacuole plays

an important role in the maintenance of constant cytosolic pH. Excess of hydrogen ions in the cytorol

may be pumped into the vacuoles. Plant vacuoles have a pH between 5.0 and 5.5 but the range extend

from 25 or so (lemon fruit vacuoles) to greater than 7.0 in inactivated protein storage vacuoles. By

controlling the release of protons and other ions into the cytorol cells can regulate not only cytosolic pH

but also the actively of enzymes, the assembly of cytoskeletal structures and membrane fusion events.

Ca+ and phosphate ions would be toxic to the cytosol if their concentration became too

high. The vacuole absorbs these ions and keeps their concentration in the cytorol within limits.

Phosphate and nitrate provides examples of essential ions stored in vacuoles. If cytosolic phosphate

or nitrate level drops too low, these lows can move from vacuoles into the cytorol. The same is for

sugar, amino acids and many other stored materials. Therefore vacuoles may be a dumping ground,

but it is a warehouse.

2.3.2.3Defense against microbial pathogens and herbivores

Plant cells accumulates an amazing varieties of toxic compound in their vacuoles, both to

reduce feeding by herbivores and to destroy microbial pathogens these compounds include the

following:

1. Phenolic compounds alkaloids cyanogenic glycosides and protease inhibitor to discourage insect

and animal behaviors.

2. Cell wall degrading enzymes, such as chitinase and glucanase and defense molecule such as

saponins to destroy pathogenic fungi and bacteria.

3. Latexes would clogging emulsions of hydrophobic polymers that posses insecticidal and

fungicidal properties which serves as antiherbivory agents.

2.3.2.4 Sequestration of toxic compounds

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Plants cannot escape from toxic sites nor they can eliminate toxic materials like heavy materials like

oxalate by excretion. Instead, plants sequester these compounds into vacuoles. For example to

remove oxalate specific cells develops vacuoles containing an organic matrix within which oxalate is

allowed to react with Ca to form Ca-oxalate crystals. In other plant cell types membrane of the ABC

family of transporters are used to transport xenobiotics from the cytoplasm into the vacuoles.

Accumulation of toxic compounds in leaf vacuoles is one of the reasons leaves are shed on a regular

basis.

2.3.2.5 Pigmentation

Vacuoles that contain anthocyanin pigments are found in many types of plant cells.

Pigmented flowers, petals and fruits are used to attract pollinators and seed disperse. Some leaf

pigments screen outs uv are visible light, preventing photo oxidation damage to the photosynthetic

apparatus. This screening appears to be essential for the survival of the leaves of evergreens that

grow in freezing conditions during hunters prevents the absorbed light energy being used in

photosynthesis.

2.3.3 LET`S SUM UP

Vacuoles are fluid-filled compartments enclosed by a membrane called tonoplast.

They enables plant cells to grow rapidly with expenditure of minimal amount of energy .

They perform a multitude of functions including storage, digestion, ionic homeostasis, defence

against plant pathogens etc.

72


73

1. The development of vacuoles can be seen in-

a mature cell

b elongated cells

c meristematic cells

d All of the above

2. Vacuoles are filled with cell-sap which constitutes-

a 60% water & soluble salts

b 90% water & soluble salts

c 50% water & soluble salts

d 80% water & soluble salts.

3. Colouring of leaves in autumn is imparted by

a. organic acid & pigmemts

b. colloidal solutions & pigments

c carotenoid pigments & tannins

d anthocyanin & carotenoid pigment

4. Anthocyanin pigment is formed by-

a. sugars

b organic acid

c soluble salts

d sugar & direct sunlight

2.3.5 ASSIGNMENTS/ACTIVITIES-

Prepare list of various inorganic components of vacuoles

Prepare model or charts of vacuoles.

List out the difference in respect to functions present in plant and animal vacuoles.


1 c 2 b

3 d

4 d

2.3.7 REFRENCES

BIOCHEMISTRY AND MOLECULAR BIOLOGY OF PLANTS BUCHANAN. GRUISSEM.

JONNES.

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