The cell wall is secreted by the protoplasm. It was first observed by Robert Hooke in 1665 in a thin slice of cork.
The cell wall is chiefly composed of insoluble polysaccaride, cellulose, certain other compound such as hemicellulose, pectin, lignin, suberin, xylan and silicates of calcium and magnesium also occur in the cell wall.
Structure:- the cell wall is composed of four layers. a) middle lamella b) primary wall c) secondary wall and d) tertiary wall.
a) Middle lamella:- It is the first formed wall layer, formed at the time of cytokinesis ( division of cytoplasm is called cytokinesis ). It is made up of Ca and Mg pectate. It is common to all the cells, popularly known as the cementing layer.The softening of ripe fruits is caused by partial solubilisation of pectic compounds to produce jelly like consistency.
b) Primary wall:- It is laid down along the middle lamella. It is made up of cellulose and pectin and is about 1-3 micro meter in thickness. Meristematic ( those cells who can divide are called meristematic cells in plants ) and parenchyma cells have primary cell wall only.
c) Secondary wall:- It is thick and rigid and laid down next to the primary wall. It again consists of three sub-layers called S1, S2 and S3. It is made up of cellulose, lignin or suberin. It develops in those cells which cease to grow e.g. in tracheary elements and sclerenchyma.
d) Tertiary wall:- internal to secondary wall is another wall material. It is chiefly composed of xylans. this is found in the tracheids of gymnosperms.
Functions of cell wall:- i) It maintains the shape of the cell.
ii) It protects the cell from mechanical injury.
iii) It allows the material to pass in and out of the cell.
iv) Many enzymatic activities are known to occur in the cell wall.
v) Cutin and suberin check loss of water by transpiration.
vi) It provides mechanical support aganist gravity. It is due to the rigid cell wall that the aerial parts of the plants are able to keep erect and expose their leaves to sunlight.
vii) It counteracts osmotic pressure.
The cell wall is chiefly composed of insoluble polysaccaride, cellulose, certain other compound such as hemicellulose, pectin, lignin, suberin, xylan and silicates of calcium and magnesium also occur in the cell wall.
Structure:- the cell wall is composed of four layers. a) middle lamella b) primary wall c) secondary wall and d) tertiary wall.
a) Middle lamella:- It is the first formed wall layer, formed at the time of cytokinesis ( division of cytoplasm is called cytokinesis ). It is made up of Ca and Mg pectate. It is common to all the cells, popularly known as the cementing layer.The softening of ripe fruits is caused by partial solubilisation of pectic compounds to produce jelly like consistency.
b) Primary wall:- It is laid down along the middle lamella. It is made up of cellulose and pectin and is about 1-3 micro meter in thickness. Meristematic ( those cells who can divide are called meristematic cells in plants ) and parenchyma cells have primary cell wall only.
c) Secondary wall:- It is thick and rigid and laid down next to the primary wall. It again consists of three sub-layers called S1, S2 and S3. It is made up of cellulose, lignin or suberin. It develops in those cells which cease to grow e.g. in tracheary elements and sclerenchyma.
d) Tertiary wall:- internal to secondary wall is another wall material. It is chiefly composed of xylans. this is found in the tracheids of gymnosperms.
Functions of cell wall:- i) It maintains the shape of the cell.
ii) It protects the cell from mechanical injury.
iii) It allows the material to pass in and out of the cell.
iv) Many enzymatic activities are known to occur in the cell wall.
v) Cutin and suberin check loss of water by transpiration.
vi) It provides mechanical support aganist gravity. It is due to the rigid cell wall that the aerial parts of the plants are able to keep erect and expose their leaves to sunlight.
vii) It counteracts osmotic pressure.
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