With the exception of reproductive cells and some unicellular lower plants, all plants cells are bounded on the outer side by a rigid wall. The cell wall is the non-living portion of the cell. Cell wall provides mechanical strength to the cell and acts as a protective layer around plasma membrane. Cell walls forming materials are actually secreted by protoplasm.
The Structure and the function of the cell wall and cytoplasm are given below:
Cell wall is made of cellulose. other chemical substances such as hemicellulose, pectin, lignin, cutin, and chitin may also be present in various proportion. Besides, calcium and magnesium in the form of carbonates and silicates may also be present. Cellulose contains 90 percent water and is composed of fibers, called cellulose fiber. The hemicellulose is composed of monosaccharide units such as arabinose, xylose, mammose and galactose. The pectin is composed of gluconic and galactonic acid. The lignin is composed of coniferyl alcohol. The cutin is composed of fatty acids. The cutin is composed of fatty acids. The chitin is composed of a polymer of glucosamine.
Structure of Cell Wall
The cell wall is differentiated into three layers. Under the electron microscope, the entire structure resembles wood fibers. The three layers of the cell walls are differentiated into:
(i) Primary cell walls. It forms first. It is the outermost layer of the cell. In the immature meristematic cell, it is the only cell wall. It is thin and permeable. The epidermis may possess cutin. suberin. waxes etc. to make the cell walls impermeable.
(ii) Secondary cell walls. It is formed next to primary cell wall and near to plasmamembrane or tertiary cell wall if formed later. It is thick and permeable.
(iii) Tertiary cell walls. In some plant cell a tertiary cell wall beneath the secondary cell wall may form which is composed of cellulose and xylan.
(iv) Middle lamella. It is the base of the cell wall. It is a cementing layer composed of pectin, lignin,and some proteins. Over this base layer primary cell wall develops. During cell division first middle lamella is formed between the two daughter cells and then on both side of the middle lamella cell walls gradually develop.
Plasmodesmata. During the formation of middle lamella and cell walls several pores are left on them.These pores are very minute and are called plasmodesmata.
primary cell walls : The first layer of deposition over the middle lamella forms the prlmary cell wall.
Secondary cell walls : The deposition over the primary cell wall forms the secondary cell walls.
Middle lamella: A thin layer of intercellular deposition forming a cell plate that divides a cell into two during cell division. Over this cell plate primary and secondary cell walls form.
Plasmodesmata: Cytoplasmic bridge between adjacent cell.
Plasmamembrane : Lipoprotein membrane surrounding the cytoplasm of the cell.
Cell wall in other organisms:
(a) In fungi. In most fungi the cell wall is made of chitin. In some lower fungi cellulose may be found in the cell walls.
(b) In bacteria. In prokaryotes the cell walls are made of peptidoglycan (a non-cellulose polysaccharide and amino acids). In some, again cell wall is completely absent (e.g. slime mould, mycoplasma).
Functions of cell wall:
(i) Protects cell contents from external influence, injury and infection.
(ii) Provides rigidity to the cell and thus helps maintaining shape and size of the plant parts, as well as keep the leaves and branches erect.
(iii) Helps root hair to absorb water and minerals.
(iv) Suberin cutin etc. of cell walls prevent evaporation through surface and also act as protective layers.
The cells of protists and animals have no wall around the cell membrane. In such cells a filamentous covering is found which is made of oligosaccharides. This extra layer protects the plasmamembrane. This is termed as cell-coat. The cell-coat also has some other functions. It is believed that this filamentous coat helps in recognising other cells during tissue formation. In certain walls the cell-coat is strengthened by calcium salt and silicon deposit.
Cytoplasm occupies the major part of the cell. It is semi-transparent, granular, jelly-like, viscous substance. Various cell organelles such as nucleus, mitochondria, ribosomes, Golgi bodies etc. remain suspended in it. Various enzymes, vitamins. t-RNA, minerals etc. are also found in the cytoplasm. Food materials also remain stored in the cytoplasm. Distribution of food, minerals, enzymes etc. takes place through the cytoplasm.
The cytoplasm without cell organelles that fills up the space of the entire cell is called hyaloplasm or cytosol or cytoplasmic matrix. The peripheral layer of a viscous, relatively non-granular cytoplasmic matrix is called endoplasm. The inner matrix around the nucleus which is more granular and less viscous is called endoplasm. Within the cell some non-protoplasmic fluid-filled cavities occur. These are called Vacuoles. The cytoplasm surrounding the vacuole acts as vacuole membrane called tonoplasm.
Functions of cytoplasm:
1. Cytoplasm is the seat of biosynthesis of fatty acid, proteins and nucleotide. It is also the site of glycolysis.
2. Cytoplasm helps distribution of nutrients, enzymes and other metabolites in the cell.
3. It Is the storehouse of raw materials needed for metabolic functions.
4. Cytoplasm maintains the shape of the cell.
5. Cell organelles remain suspended in the cytoplasm. Cytoplasm provides the ideal environment for the function of cell organelles.