CHAPTER – II

CELL AND CELL WALL

The cell is the basic unit of life. Plant cells (unlike animal cells) are surrounded by a

thick, rigid cell wall.

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Animal Cell Plant Cell

Shape: Round Rectangular

Glyoxysomes: NoSome plant cells have

glyoxysomes.

Centrioles: Always presentOnly present in lower

plant forms.

Lysosomes:Lysosomes occur in

cytoplasm.

Lysosomes usually not

evident.

Plasma Membrane:Yes; only cell

membrane

Yes; cell wall and a cell

membrane

Cell wall: None Yes

Chloroplast:Animal cells don't have

chloroplasts

Plant cells have

chloroplasts because

they make their own

food

Plastids: No Yes

Vacuole:One or more small

vacuoles (much smaller

than plant cells).

One, large central

vacuole taking up 90%

of cell volume.

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Table: :The following is a glossary of plant cell anatomy.Glossary of plant cell anatomy

No. Organelles Functions 1 Cell wall A thick, rigid membrane that surrounds a plant cell.

This layer of cellulose fiber gives the cell most of its support and structure. The cell wall also bonds with other cell walls to form the structure of the plant.

2 Cell membrane The thin layer of protein and fat that surrounds the cell, but is inside the cell wall. The cell membrane is semipermeable, allowing some substances to pass into the cell and blocking others.

3 Chloroplast An elongated or disc-shaped organelle containing chlorophyll. Photosynthesis (in which energy from sunlight is converted into chemical energy - food) takes place in the chloroplasts.

4 Chlorophyll Chlorophyll is a molecule that can use light energy from sunlight to turn water and carbon dioxide gas into sugar and oxygen (this process is called

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photosynthesis). Chlorophyll is magnesium based and is usually green.

5 Granum (plural grana

A stack of thylakoid disks within the chloroplast is called a granum.

6 Photosynthesis

A process in which plants convert sunlight, water, and carbon dioxide into food energy (sugars and starches), oxygen and water. Chlorophyll or closely-related pigments (substances that color the plant) are essential to the photosynthetic process.

7 Stroma part of the chloroplasts in plant cells, located within the inner membrane of chloroplasts, between the grana.

8 Thylakoid disk Thylakoid disks are disk-shaped membrane structures in chloroplasts that contain chlorophyll. Chloroplasts are made up of stacks of thylakoid disks; a stack of thylakoid disks is called a granum. Photosynthesis (the production of ATP molecules from sunlight) takes place on thylakoid disks.

9 ATP it is a high-energy molecule used for energy storage by organisms. In plant cells, ATP is produced in the cristae of mitochondria and chloroplasts.

10 Christae (Singular crista) the multiply-folded inner membrane of a cell's mitochondrion that are finger-like projections. The walls of the cristae are the site of the cell's energy production (it is where ATP is generated).

11 Amyloplasts An organelle in some plant cells that stores starch. Amyloplasts are found in starchy plants like tubers and fruits

12 Cytoplasm The jellylike material outside the cell nucleus in which the organelles are located.

13 Golgi body (Also called the golgi apparatus or golgi complex) a flattened, layered, sac-like organelle that looks like a stack of pancakes and is located near the nucleus. The golgi body packages proteins and

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carbohydrates into membrane-bound vesicles for "export" from the cell.

14 Nuclear membrane

The membrane that surrounds the nucleus.

15 Nucleolus an organelle within the nucleus - it is where ribosomal RNA is produced.

16 Nucleus Spherical body containing many organelles, including the nucleolus. The nucleus controls many of the functions of the cell (by controlling protein synthesis) and contains DNA (in chromosomes). The nucleus is surrounded by the nuclear membrane.

17 Centrosome (Also called the "microtubule organizing center") a small body located near the nucleus - it has a dense center and radiating tubules. The centrosomes is where microtubules are made. During cell division (mitosis), the centrosome divides and the two parts move to opposite sides of the dividing cell. Unlike the centrosomes in animal cells, plant cell centrosomes do not have centrioles.

18 Ribosome Small organelles composed of RNA-rich cytoplasmic granules that are sites of protein synthesis.

19 Rough Endoplasmic reticulum

A vast system of interconnected, membranous, infolded and convoluted sacks that are located in the cell's cytoplasm (the ER is continuous with the outer nuclear membrane). Rough ER is covered with ribosome that gives it a rough appearance. Rough ER transport materials through the cell and produces proteins in sacks called cisternae (which are sent to the Golgi body, or inserted into the cell membrane).

20 Smooth Endoplasmic reticulum

A vast system of interconnected, membranous, infolded and convoluted tubes that are located in the cell's cytoplasm (the ER is continuous with the outer nuclear membrane). The space within the ER is called the ER lumen. Smooth ER transport materials through the cell. It contains enzymes and produces

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and digests lipids (fats) and membrane proteins; smooth ER buds off from

rough ER, moving the newly-made proteins and lipids to the Golgi body and membranes.

21 Vacuole A large, membrane-bound space within a plant cell that is filled with fluid. Most plant cells have a single vacuole that takes up much of the cell. It helps maintain the shape of the cell.

22 Mitochondriya Spherical to rod-shaped organelles with a double membrane. The inner membrane is infolded many times, forming a series of projections (called cristae). The mitochondrion converts the energy stored in glucose into ATP (adenosine triphosphate) for the cell.

Cell wall

A cell wall is a tough, flexible and sometimes fairly rigid layer offering

protection against mechanical stress. It is located outside the cell membrane and

provides these cells with structural support and protection. A major function of the

cell wall, in multicellular organisms, is to act as a pressure vessel, preventing over-

expansion when water enters the cell. In other word the creation of a stable osmotic

environment by preventing osmotic lysis and helping to retain water. It permits the

organism to build and hold its shape (morphogenesis). The cell wall also limits the

entry of large molecules that may be toxic to the cell. They are found in plants,

bacteria, fungi, algae, and some archaea. Animals and protozoa do not have cell

walls.

The materials in a cell wall vary between species, and in plants and fungi also

differ between cell types and developmental stages. In plants, the strongest

component of the complex cell wall is a carbohydrate called cellulose, which is a

polymer of glucose. In bacteria, peptidoglycan forms the cell wall. Archaean cell

walls have various compositions, and may be formed of glycoprotein S-layers,

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pseudopeptidoglycan, or polysaccharides. Fungi possess cell walls made of the

glucosamine polymer chitin, and algae typically possess walls made of glycoproteins

and polysaccharides. Unusually, diatoms have a cell wall composed of silicic acid.

Often, other accessory molecules are found anchored to the cell wall.

Plant cell wall Composition

The major carbohydrates making up the primary (growing) plant cell wall are

cellulose, hemicellulose and pectin. The cellulose microfibrils are linked via

hemicellulosic tethers(rope/chain) to form the cellulose-hemicellulose network, which

is embedded in the pectin matrix.

Up to three strata or layers may be found in plant cell walls:

The middle lamella, a layer rich in pectins. This outermost layer forming the

interface between adjacent plant cells and glues them together.

The primary cell wall, generally a thin, flexible and extensible layer formed

while the cell is growing.

The secondary cell wall, a thick layer formed inside the primary cell wall

after the cell is fully grown. It is not found in all cell types. In some cells, such

as found xylem, the secondary wall contains lignin, which strengthens and

waterpoofs the wall.

Cell walls in some plant tissues also function as storage depots for

carbohydrates that can be broken down and resorbed to supply the metabolic and

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growth needs of the plant. For example, endosperm cell walls in the seeds of cereal

grasses, nasturtium, and other species, are rich in glucans and other

polysaccharides that are readily digested by enzymes during seed germination to

form simple sugars that nourish the growing embryo. Cellulose microfibrils are not

readily digested by plants, however.

The most common hemicellulose in the primary cell wall is xyloglucan. The

outer part of the primary cell wall of the plant epidermis is usually impregnated with

cutin and wax, forming a permeability barrier known as the plant cuticle.

Primary wall composition and architecture:

Primary walls isolated form higher plant tissues and cells are composed

predominantly of polysaccharides together with lesser amounts of structural

glycoproteins (hydroxyproline-rich extensins) , phenolic esters (ferulic and coumaric

acids), ionically and covalently bound minerals (e.g. calcium and boron), and

enzymes. In addition walls contain proteins (expansins) that are believed to have a

role in regulating wall expansion. Lignin, a macromolecule composed of highly cross-

linked phenolic molecules, is a major component of secondary walls.

The major polysaccharides in the primary wall are :

Cellulose: a polysaccharide composed of 1,4-linked β-D-glucose residues

Hemicellulose : branched polysaccharides that are structurally homolgous to

cellulose because they have a backbone composed of 1,4-linked β-D-hexosyl

residues. The predominant hemicellulose in many primary walls is xyloglucan. Other

hemicelluloses found in primary and secondary walls include glucuronoxylan,

arabinoxylan, glucomannan, and galactomannan.

Pectin : a family of complex polysaccharides that all contain 1,4-linked α-D-

galacturonic acid. To date three classes of pectic polysaccharides have been

characterized: Homogalacturonans, rhamnogalacturonans, and substituted

galacturonans.

The organization and interactions of wall components is not known with

certainty and there is still considerable debate about how wall organization is

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modified to allow cells to expand and grow. Several models have been proposed to

account for the mechanical properties of the wall.

Some of the functions of the primary wall:

Structural and mechanical support.

maintain and determine cell shape.

resist internal turgor pressure of cell.

control rate and direction of growth.

ultimately responsible for plant architecture and form.

regulate diffusion of material through the apoplast.

carbohydrate storage - walls of seeds may be metabolized.

protect against pathogens, dehydration, and other environmental factors.

source of biologically active signalling molecules.

cell-cell interactions.

Secondary cell walls:

Plants form two types of cell wall that differ in function and in composition.

Primary walls surround growing and dividing plant cells. These walls provide

mechanical strength but must also expand to allow the cell to grow and divide. The

much thicker and stronger secondary wall (see figure on right), which accounts for

most of the carbohydrate in biomass, is deposited once the cell has ceased to grow.

The secondary walls of xylem fibers, tracheids, and sclereids are further

strengthened by the incorporation of lignin.

The evolution of conducting tissues with rigid secondary cell walls was a

critical adaptive event in the history of land plants, as it facilitated the transport of

water and nutrients and allowed extensive upright growth. Secondary walls also

have a major impact on human life, as they are a major component of wood and are

a source of nutrition for livestock. In addition, secondary walls may help to reduce

our dependence on petroleum, as they account for the bulk of renewable biomass

that can be converted to fuel. Nevertheless, numerous technical challenges must be

overcome to enable the efficient utilization of secondary walls for energy production

and for agriculture.

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Secondary cell walls contain cellulose (35 to 50%), xylan, a type of

hemicellulose, (20 to 35%) and a complex phenolic polymer called lignin (10 to

25%). Lignin penetrates the spaces in the cell wall between cellulose, hemicellulose

and pectin components, driving out water and strengthening the wall. The walls of

cork cells in the bark of trees are impregnated with suberin, and suberin also forms

the permeability barrier in primary roots known as the Casparian strip. Secondary

walls - especially in grasses - may also contain microscopic silica crystals, which

may strengthen the wall and protect it from herbivores.

Plant cells walls also contain numerous enzymes, such as hydrolases,

esterases, peroxidases, and transglycosylases, that cut, trim and cross link wall

polymers. Small amounts (1-5%) of structural proteins are found in most plant cell

walls; they are classified as hydroxyproline-rich glycoproteins (HRGP),

arabinogalactan proteins (AGP), glycine-rich proteins (GRPs), and proline-rich

proteins (PRPs). Each class of glycoprotein is defined by a characteristic, highly

repetitive protein sequence. Most are glycosylated, contain hydroxyproline (Hyp) and

become cross-linked in the cell wall.

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Properties.

The composition, properties, and form of the cell wall may change during the cell

cycle and depend on growth conditions.

1. Rigidity

The rigidity of cell walls is often over-estimated. In most cells, the cell wall is

flexible, and has considerable tensile strength. The apparent rigidity of primary plant

tissues is a function of hydraulic turgor pressure of the cells and not due to rigid cell

walls. This flexibility is seen when plants wilt, so that the stems and leaves begin to

droop, or in seaweeds that bend in water currents. The rigidity of healthy plants

results from a combination of the wall construction and turgor pressure.

In plants, a secondary cell wall is a thicker additional layer of cellulose which

increases wall rigidity. Additional layers may be formed containing lignin in xylem cell

walls, or containing suberin in cork cell walls. These compounds are rigid and

waterproof, making the secondary wall stiff. Both wood and bark cells of trees have

secondary walls. Other parts of plants such as the leaf stalk may acquire similar

reinforcement to resist the strain of physical forces.

Certain single-cell protists and algae also produce a rigid wall. Diatoms build a

frustule from silica extracted from the surrounding water; radiolarians also produce a

test from minerals. Many green algae, such as the Dasycladales encase their cells in

a secreted skeleton of calcium carbonate. In each case, the wall is rigid and

essentially inorganic.

2. Permeability

The primary cell wall of most plant cells is semi-permeable and permit the

passage of small molecules and small proteins, with size exclusion estimated to be

30-60 kDa. Key nutrients, especially water and carbon dioxide, are distributed

throughout the plant from cell wall to cell wall in apoplastic flow.

Algal cell walls : like plants, algae have cell walls. Algal cell walls contain cellulose

and a variety of glycoproteins. The inclusion of additional polysaccharides in algal

cells walls is used as a feature for algal taxonomy.

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Manosyl form microfibrils in the cell walls of a number of marine green algae

including those from the genera, Codium, Dasycladus, and Acetabularia as

well as in the walls of some red algae, like Porphyra and Bangia.

Xylanes

Alginic acid is a common polysaccharide in the cell walls of brown algae

Sulfonated polysaccharides occur in the cell walls of most algae; those

common in red algae include agarose, carrageenan, porphyran, furcelleran

and funoran.

Other compounds that may accumulate in algal cell walls include sporopollenin and

calcium ions.

The group of algae known as the diatoms synthesize their cell walls (also

known as frustules or valves) from silicic acid (specifically orthosilicic acid, H4SiO4).

The acid is polymerised intra-cellularly, then the wall is extruded to protect the cell.

Significantly, relative to the organic cell walls produced by other groups, silica

frustules require less energy to synthesize (approximately 8%), potentially a major

saving on the overall cell energy budget and possibly an explanation for higher

growth rates in diatoms.

Fungal cell walls : Chemical structure of a unit from a chitin polymer chain.There

are several groups of organisms that may be called "fungi". Some of these groups

have been transferred out of the Kingdom Fungi, in part because of fundamental

biochemical differences in the composition of the cell wall. Most true fungi have a cell

wall consisting largely of chitin and other polysaccharides. True fungi do not have

cellulose in their cell walls, but some fungus-like organisms do.

True fungi : Not all species of fungi have cell walls but in those that do, the plasma membrane is followed by three layers of cell wall material. From inside out these

are:

a chitin layer (polymer consisting mainly of unbranched chains of N-acetyl-D-

glucosamine)

a layer of β-1,3-glucan

a layer of mannoproteins (mannose-containing glycoproteins) which are

heavily glycosylated at the outside of the cell.

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Fungus-like protists :The group Oomycetes, also known as water molds, are

saprotrophic plant pathogens like fungi. Until recently they were widely believed to

be fungi, but structural and molecular evidence has led to their reclassification as

heterokonts, related to autotrophic brown algae and diatoms. Unlike fungi,

oomycetes typically possess cell walls of cellulose and glucans rather than chitin,

although some genera (such as Achlya and Saprolegnia) do have chitin in their

walls. The fraction of cellulose in the walls is no more than 4 to 20%, far less than

the fraction comprised by glucans. Oomycete cell walls also contain the amino acid hydroxyproline, which is not found in fungal cell walls.

The dictyostelids are another group formerly classified among the fungi. They

are slime moulds that feed as unicellular amoebae, but aggregate into a reproductive

stalk and sporangium under certain conditions. Cells of the reproductive stalk, as

well as the spores formed at the apex, possess a cellulose wall. The spore wall has

been shown to possess three layers, the middle of which is composed primarily of

cellulose, and the innermost is sensitive to cellulase and pronase.

Prokaryotic cell walls:

Bacterial cell walls

Diagram of a typical gram-negative bacterium, with the thin cell wall

sandwiched between the red outer membrane and the thin green plasma membrane.

Schematic of typical gram-positive cell wall showing arrangement of N-

Acetylglucosamine and N-Acetlymuramic acid

Further information: Cell envelope

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Around the outside of the cell membrane is the bacterial cell wall. Bacterial

cell walls are made of peptidoglycan (also called murein), which is made from

polysaccharide chains cross-linked by unusual peptides containing D-amino acids.

Bacterial cell walls are different from the cell walls of plants and fungi which are

made of cellulose and chitin, respectively. The cell wall of bacteria is also distinct

from that of Archaea, which do not contain peptidoglycan. The cell wall is essential to

the survival of many bacteria, although L-form bacteria can be produced in the

laboratory that lack a cell wall. The antibiotic penicillin is able to kill bacteria by

preventing the cross-linking of peptidoglycan and this causes the cell wall to weaken

and lyse.

There are broadly speaking two different types of cell wall in bacteria, called

Gram-positive and Gram-negative. The names originate from the reaction of cells to

the Gram stain, a test long-employed for the classification of bacterial species.

Gram-positive bacteria possess a thick cell wall containing many layers of

peptidoglycan and teichoic acids. In contrast, Gram-negative bacteria have a

relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a

second lipid membrane containing lipopolysaccharides and lipoproteins. Most

bacteria have the Gram-negative cell wall and only the Firmicutes and Actinobacteria

(previously known as the low G+C and high G+C Gram-positive bacteria,

respectively) have the alternative Gram-positive arrangement. These differences in

structure can produce differences in antibiotic susceptibility, for instance vancomycin

can kill only Gram-positive bacteria and is ineffective against Gram-negative

pathogens, such as Haemophilus influenzae or Pseudomonas aeruginosa. 21

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