Cell membrane function and difference-digieduco

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  Functions of cell membrane : (a) The proteins and enzymes present in the cell membrane helps in the transport of certain substances like sugar, sodium and other ions etc. across the cell membrane. (b) Cell membrane protects the internal structures of the cell and different organelles of the cytoplasm. (c) It maintains the shape of the cell. (d) It acts as a selective permeable membrane because it allows to pass certain substances while others are not, hence it helps in the transport of selective materials from and to the cells. (e) The membranes on the cytoplasmic organelles and nucleus most probably formed from the plasma membrane. (f) Other important functions of cell membrane of the cells are endocytosis and exocytosis . Endocytosis is the process by which materials are transported into the cells by the formation of vesicles. It includes two processes-phagocytosis (cell eating) and pinocytosis (cell drinking). The reverse process of endocytosis is exocytosis. During this p
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CHEMICAL COMPOSITION OF THE CELL WALL -DIGIEDUCO

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Carbohydrate cellulose is the basic and most common compound in plant cell walls. Cellulose is associated with various substances e.g. with other compound carbohydrates and with lignin, specially in the walls of woody tissues. The common carbohydrate constituents of the walls other than cellulose are hemicellulose and pectic compounds. The fatty compounds, suberin, waxes etc. occur in varying proportions in the walls of many types of cells. Various other organic compounds and mineral substances may also be present. Water is another most important and most variable component of cell walls-part of this water occurs in microcapillaries and is relatively free, the rest is associated with hydrophilic substances. A group of proteins  containing hydroxyproline may be present in the primary walls of various tissues. The various chemical composition of cell wall are as follows

Cellulose-It is a relatively hydrophillic crystalline compound having the general empirical formula (C6H10O5)n. Cellulose is closely related to starch as a polysaccharide hexosan ; the molecules of cellulose are chainor ribbon like structures with 1,000 or more of the glucose residues connected together by oxygen bridges with beta-1, 4 glucosidic bonds. The length of each chain varies greatly and may be upto 4รบ. (Frey-Wyssling, 1959). 


Hemicelluloses-These are like cellulose ; they are built up not of glucose molecules, but of those of other sugars. Practically hemicelluloses are a group of polysaccharides of certain solubilities. Xylans, mannans, galactans, glucans etc. are examples of some of the individual members of hemicelluloses. 


Pectic substances-These substances occur in cell walls is three forms such as protopectic, pectin and pectic acid. Pectic substances belong to the polyuronids i.e. polymers composed mainly of uronic acid. They are related to hemicellulose but have different solubilities. Pectic compounds are amorphous, colloidal, plastic and highly hydrophilic. Pectic compounds not only constitute the intercellular substance in the middle lamella, but also occur associated with cellulose in the primary wall. 


Gums and mucilages are also regarded as compound carbohydrates of the cell walls.These substances are related to pectic compounds owing to same property of swelling to water. The mucilages occur in some mucilaginous and gelatinous types of cell walls-such walls are found in the outer cell layers of many aquatic plant bodies and in seed coats. 


Lignin is another most important composition of the cell wall. The structure of lignin is not fully understood. It is not a carbohydrate, but a polymer made up of units of Phenylpropane derivatives. Physically lignin is rigid. It is an end product of metabolism and after formation it functions mainly as a structural component of the cell wall. Lignin may be present in the middle lamella, the primary wall and the secondary wall. 


Mineral substances like silica, calcium carbonate etc. and various organic compounds like resins, tannins, fatty substances, volatile oils, acids, crystalline pigments etc. may impregnate walls. 


Cutin, suberin and waxes are most important fatty substances present in the walls. Suberin and cutin are highly polymerised compounds composed of fatty acids, they are closely related, not meltable and insoluble in fat solvents. Waxes are meltable and soluble in fat-solvents. Cutin foms a continuous layer termed cuticle on the surface of epidemis of aerial plant parts, it also occurs with cellulose in the outer walls of epidermis. Suberin occurs with cellulose in cork cells of the periderm. Waxes may be associated with suberin and cutin and are present in the surface of cuticle in various forms. 




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