VESICULAR TRANSPORT

Vesicular transport mechanisms are involved in the transport of macromolecules such as large protein molecules which can neither pass through the membrane by diffusion nor by active transport mechanisms. The vesicular transport mechanisms include endocytosis, exocytosis, and transcytosis. Endocytosis is the process in which the substance is transported into the cell by infolding of the cell membrane around the substance and internalising it. It is further categorized into three types: Pinocytosis, Phagocytosis, and Receptor-mediated endocytosis.  Exocytosis is reverse of endocytosis, i.e., by this process the substances are expelled from the cell without passing through the cell membrane. In this process, the substances which are to be extruded are collected in the form of granules or vesicles which move towards the cell membrane. Their membrane then fuses to the cell membrane. The area of fusion breaks down releasing the contents to the exterior and leaving the cell membrane intact. Release of hormones and enzymes by secretory cells of the body occurs by exocytosis. The process of exocytosis requires Ca2+ and energy along with docking proteins. Vesicular transport within the cell is called transcytosis or cytopempsis. It is quite similar to exocytosis and endocytosis. Three basic steps involved in this process are: vesicle formation, vesicle transportation and docking in the cell.