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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GROSS STRUCTURE OF THE CELL WALL-DIGIEDUCO

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On the basis of development and structure, three parts are generally recognised in the cell wall, such as : 

GROSS STRUCTURE OF THE CELL WALL-DIGIEDUCO
Structure of cell wall


(i) Middle lamella-The middle lamella i.e. intercellular substance occurs between the primary walls of two contiguous cells. The middle lamella is amorphbus and Optically inactive i.e. isotropic. It is mainly composed of a pectic compound possibly combined with calcium. The middle lamella in woody tissues is commonly lignified (Esau, 1965). It can be dissolved by various substances including the enzyme pectinase. Kerr and Bailey (1934) used the term compound middle lamella when dealing with the wood tissue. This term is used to refer to the complexes of the more or less homogeneous lignifled layers. The compound middle lamella may be either three-layered or five-layered. When threelayered. it refers to the middle lamella proper and the adjoining primary walls. When five layered, it (i.e. compound middle lamella) refers to the middle lamella proper, the primary walls and the outer layer of the secondary walls of the adjoining cells. 


(ii) Primary cell wall- The primary cell wall is the first true cell wall to is formed by the developing cell. In many cells it is the only cell wall, as the middle lamela is regarded as intercellular substances and not a proper wall. This wall is formed on either side of the middle lamela by contiguous cells. It contains cellulose, hemicellulose and some pectin, it may be lignified. Owing to the presence of cellulose, the primary wall is optically active, thin very elastic and capable of great extension.

GROSS STRUCTURE OF THE CELL WALL-DIGIEDUCO
(A) Endosperm cells from the seed of pheonix dactylifera (B) Strychnos nux vomica showing thick cellulosic cell wall


All meristematic cells and many mature cells having living contents have primary walls, primary wall both grows in surface as well as in thickness. Changes of thickness of the wall during growth are considered to be reversible. The primary walls of the endosperm of pheonix dactylifera, Diospyiros varginiana etc are very thick and serve as a source of reserve carbohydrate. 


(iii) Secondary walls- Secondary walls are generally laid down after the primary wall ceases to increase in surface area, hence secondary walls do not extend to any considerable degree. Seconday wall is formed on the inner surface of the primary wall next to the cell lumen. It consists mainly of cellulose or of varying mixtures of cellulose and hemicelluloses ; lignin  and various other sustances may be deposited in the wall. Because of its high content of cellulose, the secondary wall is very strongly anisotropic i.e optically active. In many cases e.g. secondary walls of tracheids and fibres are three layered such as (a) the outer layer; (b) the middle layer and (c) the inner layer: Hence a cell wall may consists altogether of five layers viz. the middle lamella, the primary wall and a three-layered secondary wall. Of these layers the middle layer is the thickest. In some cells of the stem of Linum usitatissimum more than three layers may be present (Fahn, 1982). The inner layer of the secondary wall is also termed as tertiary wall or tertiary layer by some authors (Bailey, 1957 ; Meier, 1957). Frey-Wyssling (1976) suggests that an innermost layer with properties differing from those of the secondary wall may be present in additon to the inner layer of the secondary wall. According to him this layer should be termed tertiary lamella which may be differentiated into two strata viz. a nembranogenoic stratum and a warty stratum. 


Secondary walls are usually present in cells which are nonliving at maturity e.g. sclereids, fibres, tracheary elements etc. But cells with active living protoplasts e.g. xylem parenchyma and xylem ray cells, may have secondary walls. In the elongating elements of the protoxylem the secondary wall is not continuous but is formed in annular and helical bonds. The secondary cell wall is considered a supplementary wall whose main function  is mechanical.







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