Plant Cell to Tissue

8/7/2019 Plant Cell to Tissue

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From Plant Cell to Tissue


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Introduction

Plants are unique among the eukaryotes,

organisms whose cells have membrane-

enclosed nuclei and organelles,

because they can manufacture their own

food - Chlorophyll, which gives plants their

green color, enables them to use sunlight

to convert water and carbon dioxide into

sugars and carbohydrates


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The basic plant cell- similar with the

typical eukaryote cell, but does not

have:

Centrioles

lysosomes,

intermediate filaments,

cilia, or flagella, as does the animal cell.


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Plant cells have a number of specialized

structures, including a rigid cell wall,

central vacuole,

plasmodesmata, and chloroplasts.

Also have

Nucleus mitochondria


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http://micro.magnet.fsu.edu/cells/plants/plantmodel.html


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Cell wall

plant cells have a rigid wall surrounding the

plasma membrane.

It is a far more complex structure, however, and

serves a variety of functions, from protecting thecell to regulating the life cycle of the plant

organism.

The rigid cell wall of plants is made of fibrils of

cellulose embedded in a matrix of several other

kinds of polymers such as pectin and lignin


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There are two types of cell wall:

Primary cell walls

The cell walls of parenchyma and meristems areuniform in thickness. Although each cell appearsencased within a box, in fact primary cell walls areperforated permitting plasmodesmata to connectadjacent cells.

Secondary cell walls

The cells of

sclerenchyma

collenchyma

Xylem

have secondary deposits of lignified cellulose whichprovide mechanical strength to the tissue.


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Functioning of cell wall

Provide rigidity to cell for structural and

mechanical support.

Maintaining cell shape Prevent expansions when water enters the

cell.

Protect against pathogen and theenvironment


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Vacuoles

Each plant cell has a large, single vacuole that storescompounds, helps in plant growth, and plays animportant structural role for the plant.

Vacuoles are bounded by a single membrane. Young plant cells contain many small vacuoles, but in

the cells mature, these unite to form a large centralvacuole.

Plant cells avoid bursting in hypotonic surroundings bytheir strong cell walls.


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functions

Removing unwanted structural debris

Containment of waste products

Maintaining an acidic internal pH

Storing small molecules Enabling the cell to elongated rapidly

maintaining turgor in the cell

storing foods (e.g., proteins in seeds)

Aid in destruction of invading bacteria or misfoldedprotein


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The central vacuole in plant cells (see Figure) is

enclosed by a membrane termed the tonoplast,an highly integrated component of the plant

internal membrane network (endomembrane)

system.

The central vacuole also contains plant wastes

that taste bitter to insects and animals, while

developing seed cells use the central vacuole as

a repository for protein storage.


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Chloroplast

Chloroplasts are the most familiar plastids.

They are usually disk-shaped and about 5-8 m in diameter and 2-4m thick.

Surrounded by lipid-bilayer membrane

A typical plant cell has 20-40 of them.

The chloroplast is made up of 2 membrane:

A smooth outer membrane which is freely permeable tomolecules.

A smooth inner membrane which contains many transporters:

integral membrane proteins that regulate the passage in anout of the chloroplast of

small molecules like sugars

proteins synthesized in the cytoplasm of the cell but usedwithin the chloroplast


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Chloroplasts are plant cell organelles that are involved in energystorage and the synthesis of metabolic materials- conductphotosynthesis.

enclosed in a double membrane and the area between the two layersthat make up the membrane is called the intermembrane space.

The outer layer of the double membrane is much more permeablethan the inner layer,

Enclosed by the chloroplast membrane is the stroma, a semi-fluidmaterial that contains dissolved enzymes and comprises most of thechloroplast's volume.

chloroplasts possess their own genomes (DNA), the stroma containschloroplast DNA and special ribosomes and RNAs as well.

In higher plants, lamellae, internal membranes with stacks (eachtermed a granum) of closed hollow disks called thylakoids, are alsousually dispersed throughout the stroma.

The numerous thylakoids in each stack are thought to be connectedvia their lumens (internal spaces).


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Plasma Membrane

All living cells, prokaryotic and eukaryotic, have a plasmamembrane that encloses their contents and serves as asemi-porous barrier to the outside environment.

The membrane acts as a boundary, holding the cellconstituents together and keeping other substances from

entering. The plasma membrane is permeable to specific

molecules, however, and allows nutrients and otheressential elements to enter the cell and waste materialsto leave the cell.

Small molecules, such as oxygen, carbon dioxide, andwater, are able to pass freely across the membrane, butthe passage of larger molecules, such as amino acidsand sugars, is carefully regulated.


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the plasma membrane is composed of a double layer(bilayer) of lipids, oily substances found in all cells (see

Figure 1). Most of the lipids in the bilayer can be more precisely

described as phospholipids, that is, lipids that feature aphosphate group at one end of each molecule.

Phospholipids are characteristically hydrophilic ("water-

loving") at their phosphate ends and hydrophobic("water-fearing") along their lipid tail regions.

In each layer of a plasma membrane, the hydrophobiclipid tails are oriented inwards and the hydrophilicphosphate groups are aligned so they face outwards,

either toward the aqueous cytosol of the cell or theoutside environment.

Phospholipids tend to spontaneously aggregate by thismechanism whenever they are exposed to water.


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Plasma membrane proteins function in

several different ways: Many of the proteins play a role in the

selective transport of certain substancesacross the phospholipid bilayer

as channels or active transport molecules.

as receptors, which bind information-providingmolecules, such as hormones, and transmit

corresponding signals based on the obtainedinformation to the interior of the cell.

exhibit enzymatic activity, catalyzing variousreactions related to the plasma membrane.


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Microfilaments

Microfilaments are solid rods made of globular

proteins called actin.

These filaments are primarily structural in

function and are an important component of thecytoskeleton.

Microfilaments range from 5 to 9

nanometers in diameter and are designedto bear large amounts of tension.


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Microfilaments are solid rods made of a protein knownas actin. When it is first produced by the cell, actin

appears in a globular form. In microfilaments, however, which are also often referred

to as actin filaments, long polymerized chains of themolecules are intertwined in a helix, creating afilamentous form of the protein (F-actin).

All of the subunits that compose a microfilament areconnected in such a way that they have the sameorientation.

each microfilament exhibits polarity, the two ends of thefilament being distinctly different.

This polarity affects the growth rate of microfilaments,one end (termed the plus end) typically assembling anddisassembling faster than the other (the minus end).


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Microtubules These straight, hollow cylinders are found throughout the

cytoplasm of all eukaryotic cells and carry out a variety offunctions, ranging from transport to structural support.

Microtubules, which are about 25 nanometers in diameter,form part of the cytoskeleton that gives structure andshape to a cell, and also serve as conveyor belts moving

other organelles throughout the cytoplasm. microtubules are the major components of cilia and

flagella, and participate in the formation of spindle fibersduring cell division (mitosis).

The length of microtubules in the cell varies between 200

nanometers and 25 micrometers, depending upon the taskof a particular microtubule and the state of the cell's lifecycle.


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Microtubules are biopolymers that arecomposed of subunits made from an abundant

globular cytoplasmic protein known as tubulin. Each subunit of the microtubule is made of two

slightly different but closely related simpler unitscalled alpha-tubulin and beta-tubulin that are

bound very tightly together to formheterodimers.

In a microtubule, the subunits are organized insuch a way that they all point the same directionto form 13 parallel protofilaments.

By adding or removing globular tubulin proteins,the length of polymeric microtubules can beincreased or decreased.


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Mitochondria

Mitochondria are oblong shaped

organelles found in the cytoplasm of all

eukaryotic cells.

In plant cells, they break down

carbohydrate and sugar molecules to

provide energy, particularly when light isn't

available for the chloroplasts to produceenergy.


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Mitochondria are membrane-enclosed organellesdistributed through the cytosol of most eukaryotic cells.

The number of mitochondria present in a cell dependsupon the metabolic requirements of that cell, and mayrange from a single large mitochondrion to thousands ofthe organelles

Their main function is the conversion of the potential

energy of food molecules into ATP. Mitochondria have: an outer membrane that encloses the entire structure

an inner membrane that encloses a fluid-filled matrix

between the two is the intermembrane space

the inner membrane is elaborately folded with shelf like cristaeprojecting into the matrix.

a small number (some 510) circular molecules ofDNA

The number of mitochondria in a cell can increase by their fission


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The inner mitochondrial membrane contains

proteins with four types of functions:

Those that carry out the oxidation reactions of

the respiratory chain.

ATP synthase which makes ATP in the matrix.

Specific transport proteins that regulate the

passage of metabolite into and out of the matrix. Protein import machinery.

Inner membrane


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Outer membrane

contains numerous intergral proteinscalled porins, which contain a relativelylarge internal channel (about 2-3 nm) that

is permeable to all molecules of 5000daltons or less.

contains enzymes involved in such diverseactivities as the elongation of fatty acids,oxidation of epinephrine (adrenaline), andthe degradation of tryptophan.


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Nucleus

The nucleus is a highly specialized organelle

that serves as the information processing and

administrative center of the cell.

This organelle has two major functions: it storesthe cell's hereditary material, or DNA, and it

coordinates the cell's activities, which include

growth, intermediary metabolism, protein

synthesis, and reproduction (cell division).


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a) Nuclear envelope

The nuclear envelope is a double-layered membrane

that encloses the contents of the nucleus during most ofthe cell's lifecycle.

The space between the layers is called the perinuclearspace and appears to connect with the roughendoplasmic reticulum.

The envelope is perforated with tiny holes called nuclearpores. These pores regulate the passage of moleculesbetween the nucleus and cytoplasm, permitting some topass through the membrane, but not others.

The inner surface has a protein lining called the nuclear

lamina, which binds to chromatin and other nuclearcomponents.

During mitosis, or cell division, the nuclear envelopedisintegrates, but reforms as the two cells complete theirformation and the chromatin begins to unravel and

disperse


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Endoplasmic reticulum

The endoplasmic reticulum is a network of sacs

that manufactures, processes, and transports

chemical compounds for use inside and outside

of the cell. It is connected to the double-layered nuclear

envelope, providing a pipeline between the

nucleus and the cytoplasm.

In plants, the endoplasmic reticulum also

connects between cells via the plasmodesmata.


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The endoplasmic reticulum membrane allows

molecules to be selectively transferred between

the lumen and the cytoplasm.


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There are two basic kinds of endoplasmic reticulummorphologies:

rough and smooth. The surface of rough endoplasmic reticulum is

covered with ribosomes, This type of endoplasmicreticulum is involved mainly with the production andprocessing of proteins that will be exported, orsecreted, from the cell.

The ribosomes assemble amino acids into protein units,which are transported into the rough endoplasmicreticulum for further processing.

Chemicals, such as carbohydrates or sugars, are added,then the endoplasmic reticulum either transports thecompleted proteins to areas of the cell where they areneeded, or they are sent to the Golgi apparatus forfurther processing and modification.


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Rough ER Contains protein-manufacturing ribosomes

Transports proteins destined for membranesand secretion

Smooth ER Metabolic processes

Synthesis of lipids, carbohydrates

Connected to the nuclear envelope Consist of tubules and vesicles


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also called Golgi body or Golgi complex

and found in both plant and animal cells, is typically comprised of a series of five to

eight cup-shaped, membrane-covered

sacs called cisternae Similarly, the number of Golgi bodies in a

cell varies according to its function.


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The Golgi apparatus is the distribution and shipping

department for the cell's chemical products. it modifies proteins and fats built in the endoplasmic

reticulum and prepares them for export as outside of the

cell.

Golgi stack has two distinct ends (cis), or faces.

- The cis face of a Golgi stack is the end of the

organelle where substances enter from the

endoplasmic reticulum for processing,

- the trans face is where they exit in the form of

smaller detached vesicles.


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Proteins, carbohydrates, phospholipids, and othermolecules formed in the endoplasmic reticulum aretransported to the Golgi apparatus to be biochemicallymodified during their transition from the cis to the transpoles of the complex.

Enzymes present in the Golgi lumen modify thecarbohydrate (or sugar) portion of glycoproteins byadding or subtracting individual sugar monomers.

In addition, the Golgi apparatus manufactures a variety

of macromolecules on its own, including a variety ofpolysaccharides.


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Ribosomes Ribosomes are mainly found bound to the endoplasmic

reticulum and the nuclear envelope, as well as freelyscattered throughout the cytoplasm,

The organelles serve as the protein productionmachinery for the cell and are consequently most

abundant in cells that are active in protein synthesis,such as pancreas and brain cells.

Some of the proteins synthesized by ribosomes are forthe cell's own internal use, especially those that areproduced by free ribosomes.

Many of the proteins produced by bound ribosomes,however, are transported outside of the cell.


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b) Chromatin

Packed inside the nucleus of every humancell is nearly 6 feet of DNA, which isdivided into 46 individual molecules, one

for each chromosome and each about 1.5inches long.

Instead, it is combined with proteins andorganized into a precise, compactstructure, a dense string-like fiber calledchromatin.


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The nucleus contains the chromosomes of thecell.

Each chromosome consists of a single moleculeof DNA complexed with an equal mass ofproteins.

the DNA of the nucleus with its associated

proteins is called chromatin. Most of the proteinconsists of multiple copies of 5 kinds ofhistones.

Chromatin also contains small amounts of awide variety ofnonhistone proteins. Most of

these are transcription factors (e.g., the steroidreceptors) and their association with the DNA ismore transient.


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c) Nucleolus

The nucleolus is a membrane-less organelle

within the nucleus that manufactures ribosomes,

the cell's protein-producing structures.

A nucleus may contain up to four nucleoli, but

within each species the number of nucleoli isfixed.

After a cell divides, a nucleolus is formed when

chromosomes are brought together into

nucleolar organizing regions.

During cell division, the nucleolus disappears.


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d) Nuclear pores

The nuclear envelope is perforated with holescalled nuclear pores.

These pores regulate the passage of moleculesbetween the nucleus and cytoplasm, permittingsome to pass through the membrane, but notothers.

Building blocks for building DNA and RNA areallowed into the nucleus as well as moleculesthat provide the energy for constructing geneticmaterial.


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Nuclear pores are fully permeable to small

molecules up to the size of the smallest proteins,but form a barrier keeping most large moleculesout of the nucleus.

The nuclear pore complex is comprised of

several subunits. Surrounding the inside of the pore is a

nonmembranous material.

The actual pore wall is predominantly comprised

of columnar subunits, and lumenal subunits, withthe help of transmembrane proteins, anchor theentire pore complex into the nuclear envelope


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Plasmodesmata

Plasmodesmata (singular, plasmodesma) are

small channels that directly connect the

cytoplasm of neighboring plant cells to each

other, establishing living bridges between cells. Similar to the gap junctions found in animal cells,

the plasmodesmata, which penetrate both the

primary and secondary cell walls (see Figure)

- allow certain molecules to pass directlyfrom one cell to another and are important in

cellular communication.


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The plasmodesmata is structured in an entirely different

configuration than the animal cell gap junction becauseof the thick cell wall.

plant cells can be considered to form a synctium, ormultinucleate mass with cytoplasmic continuity.

Somewhat cylindrical in shape, plasmodesmata are linedwith the plasma membrane so all connected cells areunited through essentially one continuous cellmembrane.

A majority of plasmodesmata also contain a narrow tube-

like structure called the desmotubule, which is derivedfrom the smooth endoplasmic reticulum of the connectedcells


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THE END

Plant Cell to Tissue

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