Many substances are transported across the cell membrane by endocytosis and exocytosis.
It involves intake of materials in the form of carrier vesicles formed by invagination of small regions of plasma membrane.
It is of three types : (i) phagocytosis (ii) pinocytosis and (iii) receptor-mediated endocytosis.
(i) Phagocytosis : It is the process by which extracellular substances (bacteria, dead tissue, foreign particles etc.) are engulfed by the cells. The substance makes contact with the cell membrane, which then invaginates. The endocytic vesicle pinches off from the cell membrane and fuses with another intracellular vesicle e.g., lysosome, from which the ingested substance is released into ICF.
(ii) Pinocytosis : Fluid materials having high molecular weight such as proteins, amino acids, fats, insulin, lipoproteins, etc., in the form of globules of fluid enter the cytoplasm by invagination of plasma membrane. Pinocytosis is initiated when the plasma membrane is stimulated by certain chemical substances. A liquid droplet at the point of contact forms infolding of cell membrane in sac like structure into which this fluid food is drawn in. Globules of fluid materials are called pinosomes which are pinched off from the plasma membrane inside the cytoplasm in the form of small pinocytic vesicles.
(iii) Receptor-mediated endocytosis : The material to be transported first binds to a receptor, and then the receptor-substance complex is ingested by endocytosis. For example, transport of iron and cholesterol into the cell.
Substances secreted by the cell are trapped within vesicles or granules which fuse with the cell membrane and release their contents to the extracellular fluid. This leaves the contents of the vesicles or granules outside the cell. Hormones, digestive enzymes and synaptic transmitters are examples of substances transported out of the cell by this process.