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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Cell wall- It's definition, nature, properties and functions-digieduco

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Definition-ln plants almost all cells (except the reproductive cells like gametes,  zoospores etc.) are provided with a rigid wall, called cell wall. The protoplast is  enclosed within the cell wall ; presence of cell wall is the characteristic feature of all plant cells. 

The cell wall was discovered in the 17th century before the presence of protoplast was recognised, and since then the protoplast became the main object of study.


Nature-The cell wall, at first, is very thin and delicate, and is modified in various ways with the maturing up of cells. As the cells mature, some changes take place- these are increase in structure and in extent, in chemical composition, in the modification of gross physical structure owing to the absorption of end walls etc. 


The cell wall is the secretory product of the protoplast and is a non-living boundary wall. For a long time, the cell wall was regarded as non-living excretion of the living cell matter (protoplasm)-hence, according to that concept, the cell wall does not form a living system. However, the cell wall is by no means independent of the protoplasm (Cutter, 1978). Now-a-days much evidence has been found that in plant cells, specially in young stage, “organic unity exists between the protoplast and the wall” and that “the two together form a single biological unit” (Fahn, 1982). The cell wall grows when comes in contact with the protoplast but outside ofit. Cytoplasm is present in the wall of mature living cells in the form of plasmodesmata. It is still a question whether during growth of the cell the relation between cytoplasm and the wall is closer than at maturity. According to some workers the cytoplasm penetrates the growing wall, but electron microscope studies of meristematic cells indicate the presence of ectoplast which delimits the cytoplasm from the cell wall. 


Properties-Properties of cell wall are affected by the cell's environment, nutrition and stage of differentiation. Cell walls exhibit different degrees of plasticity, elasticity and tinsel strength in relation to their chemical composition and their sub-microscopic and microscopic structure. Plasticity is the property of becoming permanently deformed when subjected to changes in shape and size while elasticity is the property of recovery of the original size and shape after deformation. Plasticity of walls is observed clearly by their permanent extension in some stages of growth of cells by volume (Heyne, 1940) ; similarly elasticity property of walls may be illustrated by the reversible changes in volume due to changes in turgor pressure (Frey-Wyssling, I959). Tinsel strength is the remarkable characteristic of mechanical cells, specially of the extra xylary fibres of monocotyledons and dicotyledons. 


Differences in optical and other physical properties of walls are interrelated with the orientation of microfibrils, e.g. walls having the microfibrils oriented parallel to the long axis of the cell do not show their anisotropy in transverse section and do not contract longitudinally ; while walls in which the microfibrils are oriented at right arigles to the iong axis of the cell are strongly birefringent in transverse sections and contract longitudinally on drying (Bailey, I954). 


Cellulose, which occurs abundantly in cell walls, has a great influence upon their properties : other substances e.g. lignin, suberin etc. add their properties or modify those imparted by cellulose. Tinsel strength is the remarkable characteristic of cellulose, whilc lignin increases the resistance of walls to pressure and protects the cellulose fibrils from becoming creased.


Functions-(i) Cell wall protects the living protoplasm from external injury. (ii) It also gives definite shape to the cell and texture to the tissue. (iii) Cell walls have supportive function i.e. they provide mechanical strength to the cell. (iv) Being permeable, the cell wall allows water and mineral salts to pass through it. (v) Cell walls play an important role in some Physiological activities such as absorption, transpiration, translocation, secrction etc.(Frey-Wyssling ,1959), (vi) Cell walls also connect the living protoplats of adjacent cells through Plasmodesmata. 




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