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 ..read more

  In young condition (e.g. the cells of meristematic tissue), the cells are compactly arranged without showing any space between them. As cells mature, the contact between the adjacently situated cells is gradually broken down, so that some spaces called intercellular spaces develop between cells. Intercellular spaces may vary in shape and size-this is due to the difference in shape and arrangement of the surrounding cells. Large intercellular spaces are called chambers while much elongated ones are called canals. Functionally intercellular spaces generally form aerating or conducting sys ..read more

 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 sec ..read more

As the cell grows, the primary walls of the young cells stretch and increase in the surface area and thickness. These stretched primary walls are generally not of uniform thickness but have conspicuous thin depressions in them at intervals, these are called primary pit fields. When pit-fields are numerous and deeply sunken, the wall in which they occur appears to be beaded in transverse section. One of the characteristic features of the primary pit-fields of living cells is the presence of plasmodesmata, although they occur also in other regions of the wall.  Secondary walls also have ca ..read more

The thickening of the cell wall takes place due to the deposition of secondary cell wall materials like cellulose, lignin, cutin, suberin, etc over the primary cell wall and finally the cell attains its full size and the wall becomes thick. The depostion of the secondary cell wall materials is localised in many cases, specially in case of cells and vessels of xylem tissue-here the secondary cell wall materials are not uniformly deposited on the primary cell wall, rather they are localised to certain regions on the primary cell wall. Thereby some portion of the cell wall remains thin and other ..read more

Cell wall grows in surface as well as in thickness. Due to the stretching of thin, soft and elastic primary cell wall the cell increases in size. In a young cell, the wall is thin and delicate ; as the cell grows and becomes mature, the growth in thickness of the cell wall begins. there are two classical theories regarding the growth of the cell wall in thickness, such as : (a) that growth by intussusception method, where the new cell wall materials i.e. new microbfibrils are held to be laid down between the existing microfibrils of the expanding cell wall, and (b) that of growth by apposition ..read more

 Formation of secondary wall materials on the primary wall does not take place uniformly, instead some thin areas are left out-those thin areas are called pit-fields. Thin and delicate strands or fibrils of cytoplasm, called plasmodesmata (singular : plasmodesma), pass through such pit-fields of the cell wall at intervals, thus connecting the living protoplasts of adjacent cells. Plasmodesmata usually occur in groups but they may be evenly distributed over the entire wall. When they are grouped they are localised In the primary pit-fields.  Most of the plasmodesmata are found to be ..read more

The principal component of cell wall is cellulose, the ultra structure of cell walls is therefore based on cellulose. Work with electron microscope shows that cellulose in cell walls occurs in the form of long-chain molecules (Roelofsen, 1959; Albersheim, 1975; Frey-Wyssling, 1969 -76; Frey-Wyssling and Muhlethaler, 1965). These chain like molecules may be arranged randomly or in a more or less regular fashion. Each such cellulose molecule has 8Å maximum width. Again cellulose molecules are regularly arranged in bundles-each such bundle forms an elementary fibril. Each bundle of elementary fib ..read more

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

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 ..read more