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 th...
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Vacuoles-digieduco

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Vacuoles (derived from a Latin word vacuus which means empty) are cavities present within the cytoplasm ; they occupy more than 90% of the volume of most mature plants. Each vacuole is filled with a liquid i.e. the cell sap and is surrounded by a membrane called ronoplast. The vacuoles appear colourless or pigmented in sections of living tissues, but in well-fixed material they look as clear areas bounded by the stained cytoplasm i.e. tonoplasm.

The composition of cell sap may vary in different cells and even in different vacuoles of the same cell. The main component of the cell sap is water-various substances either in true solution or in colloidal state are present in such water. Sugars, salts, proteins, organic acids and other soluble compounds, posphatides, tannins. flavonoid pigments, calcium oxalate etc. are present in plant vacuoles. Some Substances (e.g. tannins, protein bodies etc.) in the vacuole may occur in solid form and may even be crystalline. It is to be noted that, the materials present in the vacuoles are classified as ergastic-they are either by products of metabolism or they are reserve substances which may be utilised by the protoplast for vital functions. The vacuolar sap'is more or less viscous-this viscosity is probably due to colloidal nature of the sap which sometimes may appear in the form of true gels. There are two types of vacuoles with regard to pH-the relatively alkaline types of vacuoles stain reddish orange with neutral dye, while acid types stain bluish-magenta with the same dye. The concentration of the vacuolar sap varies, a substance may crystallize out if that accumulates beyond its saturation point.

The size and shape of vacuoles vary according to the stage of development and the metabolic state of the cell. Meristematic cells possess often numerous and small vacuoles. With growth and differentiation ofa cell the vacuoles enlarge and fuse forming one single vacuole which occupies the central part of the protoplast-as a result the protoplasmic materials are restricted to a peripheral position i.e. next to the cell wall.

There are various views regarding the origin of vacuoles : according to one vacuoles arise(1) from pre-existing vacuoles which multiply by fission, and after cell division each daughter cell receives a number of vacuoles ; (2) some workers think that vacuoles arise .. from Golgi vesicles ; (3) another view is that vacuoles originate by a de novo process, by attraction of water to a certain localised region in the cytoplasm and the formation of membrane around it ; (4) there is another hypothesis according to which vacuoles arise by dilation of endoplasmic reticulum, cisternae or vesicles derived from endoplasmic reticulum.

The vacuoles function in storage, in digestion and in regulation of the water and solute content of the cell (i.e. osmo-regulation). The vacuoles contain digestive enzymes that can break down cytoplasmic components and metabolites (Fahn, 1982), although some vacuoles may completely lack digestive enzymes. 

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