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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Modifications or Chemical changes of the cell wall-digieduco

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 The various modifications or chemical changes undergone by the cell wall are as follows :


(A) LIGNIFICATION-Cellulose cell walls become lignified as a result of infiltration of the walls by lignin-a woody substance. The cell wall becomes thick and hard due to the deposition of such complex substance as lignin. After complete lignification process the cell loses protoplasm and becomes dead. Lignin is deposited on both primary walls of many kinds of cells, particularly those of xylem or wood fibres. In angiosperm wood middle lamella, primary walls and even the thin outermost layers of secondary walls are the most heavily lignifled parts of the cell wall. Lignifled walls are permeable to solutes and water. It gives strength, hardness and rigidity to the plant body. 


Tests :-(a) Phloroglucin and hydrochloric acid stain lignif1ed walls red. (b) On treatment of lignitied cells with chlor-zinc-iodine solution, the walls become yellow to yellowish brown. (c)Lignified cells turn golden yellow with aniline sulphate solution. 


(B) CUTINIZATION-It is the process by which cell walls become cutinised due to the infiltration of cutin-a fatty or waxy, elastic, and complex chemical substance. It is mainly deposited over the walls of the epidermal cells and to a lesser extent within the sub-epidermal cells and elsewhere. Cell walls impregnated with cutin are called cutinised Walls. Deposition of cutin on the outer cell walls of aerial plant parts is associated with the deposition of other waxy substances-as a result a “water proofing" layer, called cuticle is formed on the entire outer surface of walls of aerial parts. 


Cutin is impermeable to water. It gives firmness to the cell wall and helps greatly to the Control of water loss from underlying cells. 


Cutinised walls are smooth in the early stages but may be rough on maturity due to the development of cracks and ridges. Some fruits like pumpkin, plum etc retain water due to strongly cutinised epidermal walls. 


Tests:-Cutinised cell wall (a ) turn yellowish brown with iodine solution followed by sulphuric acid, (b) with caustic potash solution stains yellow, (c) with Sudan III, turns pinkish-red, (d) with Chlor-zinc-iodine solution stains yellow or brown colour. 


(C) SUBERIZATION- Cell walls become suberised due to the deposition of another type of waxy and fatty substance known as suberin. This process is called suberization. Suberin is almost identical with cutin ; it is chiefly deposited over the internal cork cells but also occurs less prominently in the cells of endodermis and exodermis. Suberization renders the walls impervious to water and gases. 


Tests :-Chlor-zinc-iodine solution turns suberised walls yellowish-brown, (b) Caustic potash solution turns suberised walls brown. 


(D) MUCILAGENOUS CHANGE-Cellulose cell wall undergoes change due to the deposition of viscous substance, called mucilage. This change in the cell wall is called mucilagenous change. Mucilage has a great power to absorb and retain sufficient amount of waterand thereby becomes viscous. 


It is present in the seed coats of Plantago ovata (Topmari). Ocimum fasilicum and Salvia coccinea seeds. The cell walls of the pericarps of fruits and flowers of Malvaceae are mucilagenous. 


The seeds of many plants are fixed to the soil due to the presence of mucilage. Aquatic plants are protected from insects by the presence of mucilage. 


Tests :-(a) Sulphuric acid and iodine solution turn mucilage into violet colour. (b) Methylene blue turns mucilage deep blue. 


(E) MINERAUZATION-Mineralization of cell walls is due to the deposits of silica and other mineral matters like oxalates of calcium and magnesium. Silica infiltrates heavily on the walls of outer tissues of leaves of some species of Equisetum, Saccharum and other grasses. Calcium oxalates are present on the walls of the species of Arum, Pistia, etc. in the form of needles. In the tunic of Allium cepa crystals like rods, prisms, cubes, etc. are present. Star-like calcium oxalate crystals, called sphaeraphides are present in Pistia sp. 


Calcium carbonate crystals are present in the epidermal cell walls of leaves of Ficus benghalensis, Ficus elastica, etc. 




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