Eukaryotic cells - Cell Wall || NEET BIOLOGY

STD-11     UNIT-3     CHA-8

CELL : STRUCTURE AND FUNCTIONS

Eukaryotic cells - Cell Wall

Cell Wall 

• It is the outer rigid protective supportive and semitransparent covering of plant cells, fungi and some protists.
• Cell wall was first seen in cork cells by Hooke in 1665. 
• Its thickness varies in different types of cells from 0.1 micrometer to 10 micrometer.
• Cell wall is a non - living extracellular secretion or matrix of the cell which is closely appressed to it.
• It is, however, metabolically active and is capable of growth.

Cell wall performs a number of functions:

1. Protects the protoplasm against mechanical injury 
2. Protects the cell from attack of pathogens
3. Provides rigidity and shape to the cell 
4. Counteracts osmotic pres sure 
5. Gives strength to the land plants to withstand gravitational forces
6. By its growth the wall helps in cell expansion 
7. Pits present in the wall help produce a protoplasmic continuum or symplast amongst cells 
8. Walls prevent bursting of plant cells by inhibiting excessive endosmosis.
9. Wall has some enzymatic activity connected with metabolism.
10. In many cases, wall takes part in offence and defence.
11. Cutin and suberin of the cell wall reduce the loss of water through transpiration.
12. Walls of sieve tubes, tracheids and vessels are specialised for long distance transport.
13. Some seeds store food in the form hemicellulose in cell wall.
14. Cell wall of plants consists of cellulose, hemicellulose, pectin and protein. Besides cel lulose, the algal cell walls contain galactans, mannans and calcium carbonate.

Chemical Composition of Cell Wall 

• Matrix - Water- 60 % . Hemicellulose- 5-15 %, Pectic Substances- 2–8 %, Lipids- 0.5-3.0 %,  Proteins - 1-2 % 2. 
• Microfibrils - Cellulose / fungus cellulose - 10-15 % . 
• Other Ingredients - Lignin , cutin , suberin , silica ( silicon dioxide ), minerals ( e.g. , iron , calcium , carbonate ) , waxes , tannins , resins , gum- variable, 

Structure of Cell Wall 

• A cell wall can have upto three parts 
• middle lamella 
• primary wall 
• secondary wall 

• Middle Lamella 
• It is a thin , amorphous and cementing layer between two adjacent cells . 
• Middle lamella is the first layer which is depos ited at the time of cytokinesis.
• It is just like brick work of the common wall be tween two adjacent rooms.
• Middle lamella is absent on the outer side of surface cells.
• It is made up of calcium and magnesium pectates.
• Primary Wall 
• It is the first formed wall of the cell which is produced inner to the middle lamella.
• The primary wall is com monly thin ( 0.1–3.0 um ) and capable of extension. 
• It grows by intussusception or addition of materials within the existing wall.
• Some cells possess only primary wall, e.g. leaf cells, fruit cells, cells of cortex and pith.
• The matrix of the wall consists of water, pectin, hemicellulose and glycoproteins.
• Pectin is the filler substance of the matrix.
• Proteins are structural and enzymatic.
• Secondary Wall 
• It is produced in some mature cells when the latter have stopped growth, e.g. , tracheids, vessel elements, fibres, collenchyma. 
• Secondary wall is laid inner to the primary wall by accretion or deposition of materials over the surface of existing structure.
• It is thick (3—10 um) and made up of at least three layers, sometimes more (e.g. latex tube of Euphorbia milli). 
• They are named as S , S2 , S. , Sy , etc. 
• The innermost layer of the secondary wall is sometimes distinct both chemically as well as in staining properties due to the presence of xylans . 
• It is then called tertiary wall , e.g. , tension wood in gymnosperms. 
• Secondary wall may be absent, irregularly deposited or formed uniformly in the cells.
• This results in differentiation of cells- parenchyma, collenchyma, sclerenchyma, tracheids and vessels.
• The composition of secondary wall is basically similar to the primary wall in having cellulose microfibrils embedded in a matrix of pectin and hemicellulose.
• Cellulose mi crofibrils of the secondary wall lie close, 
• parallel and at an angle to the longitudinal axis of the cell.
• Their orientation is different in the different layers of the secondary wall.
• A number of different materials may be deposited in the wall 
• The important ones are 
• Lignin-It reduces the water content of the wall matrix and increases its hardness.
• However, water permeability is not affected.
• The characteristic of lignification (and cutinisation) has evolved with the evolution of land plants.
• Suberin - The wall of cork and endodermal cells contains a special fatty substance called suberin.
• Suberin makes the walls impermeable.
• Cutin-The epidermal cells possess another fatty substance called cutin.
• Cutin is also laid as a distinct layer on the outside of the epidermal cell walls.
• It is known as cuticle.
• Cutin reduces the rate of epidermal or surface transpiration.
• Other substances which can be deposited in the cell wall are silica (e.g.grasses), minerals, waxes, tannins, resins, gums, etc.

Plasmodesmata

• Plasmodesmata are cytoplasmic bridges between adjacent plant cells which develop in the minute pores of their walls.

• They form a protoplasmic continuum called symplast.
• Cell wall and intercellular spaces form a non - living component of the plant body called apoplasm.
• A plasmoderma is 40–50 nm in diameter.
• It may be simple or branched.
• Plasmodesma is lined by plasma membrane . It encloses tubular extension of endoplasmic reticulum called desmotubule. 
• The space between desmotubule and plasma membrane contains 8-10 microchannels. 
• Plasmodesmata form channels for controlled passage of small sized particles between adjacent cells as well as transfer of some specific signals.

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