Short essay on Plant and Animal Cells

Both plant and animals posses eucaryotic cells. A plant cell is characterised by a cell wall, central vacuole, plastids and anastral spindle (centrioles are generally absent). Golgi apparatus consists of separate units called dictyosomes. Lysosomes are rare. Glyoxysomes and crystals may occur.

Animal cells are bathed by tissue fluid. They are covered by cell membranes only. Vacuoles are small. Plastids are absent. Spindle is amphiastral (centrioles present). Due to absence of wall, animal cells will burst if placed in hypotonic solution, Glyoxysomes and crystal are absent. Lysosomes are present.

A typical plant cell consists of cell wall and protoplast. Cell wall is absent in animal cells. Protoplast (Hanstein, 1880) is made of a plasmalemma, cytoplasm, nucleus and vacuoles.

Cell-wall:

It is the outer rigid protective covering of plant cells and some procaryotes. Cell wall has a matrix of hemicellulose, pectin, glycoprotein and water in which are embedded microfibrils. Microfibrils are made up of cellulose in plant cells, chitin or fungus cellulose in fungi and peptidoglycan (mucopeptide) in bacteria.

Plant cell wall may have lignin for strength (e.g. woody tissue), silica for stiffness and protection (e.g. epidermal cells), cutin for preventing loss of water (e.g. epidermal cells), suberin for impermeability (e.g. cork cells, endodermal cells), etc.

The cell wall gives a definite shape and provides protection to the protoplasm. It is nonliving in nature and is permeable. Adjacent cells in the plant body remain interconnected by plasmodesmata. A cell wall may have upto three parts-middle lamella, primary wall, secondary wall.

(i) Middle lamella:

It is the outermost region which functions as a cementing layer between two cells. It is absent on the outer free surface. It ruptures to create intercellular spaces. It is formed of calcium and magnesium pectate. Pectic acid is long polygalacturonic acid compound in which a-D-galacturonic acid units are joined together by glycosidic linkages in (1 : 4). It is hydrophilic in nature.

(ii) Primary wall:

It is thin (0.1 – 3.0 (im), elastic, first wall layer. It contains microfibrils oriented variously, commonly forming a loose network. Cellulose content (5% dry weight basis), hemicellulose (50% dry weight basis), lipid content (5-10% dry basis) are high. An association of about 100 cellulose chains is termed as a micelle, 20 micelles constitutes a microfibril while an aggregation of 250 microfibrils is called fibril. Lignin, hemicellulose, some pectic substances and proteins present altogether form amorphous matrix.

(iii) Secondary wall:

It is laid inner to primary wall. It is less hydrated i.e. less hydrophilic in nature (30-40%) as compared to primary wall. Cellulose content is high with long closely arranged straight and parallel microfibrils. Hemicellulose content is 25%. Protein is very little while lipid is nearly absent. Lignin deposit is quite common. Secondary wall is often made of three layers-S₁, S₂, & S_3. In each layers, microfibrils lie straight, parallel to one another but at an angle to longitudinal axis. Orientation is different in different layers. It is more pronounced in dead cells such as tracheids and sclerenchyma. In gymnosperms tension wood, the innermost layer contains xylan and is called ‘tertiary wall’. Filler substance of wall matrix is pectin. It has methylated and polymerised galacturans, glucouronic acid and neutral sugars.

Hemicellulose is made of polymerised arabino- galactans, xylans, mannans, etc. It binds microfibrils with matrix.

Lignin is formed from coniferyl and coumaryl alcohols and aldehydes through dehydrogenation and polymerisation. Lignin reduces hydration and increases hardness of wall.

Cell wall possesses minute cytoplasmic bridges between adjacent cells. They are called ‘plasmodesmata’ (Strassburger, 1901). Plasmodesmata produce a continuum of living material called symplasm cell walls and intercellular spaces constitute a nonliving component of plant body known as apoplasm. Certain thick cell walls possess unthickened areas called pits.

In some animal cells, a cell coat is present outside the cell membrane, made up of ‘glycocalyx’ alongwith deposition of silica & calcium salts.