Endocytosis
It is the bulk uptake of material through the cell membrane by formation of a vacuole or vesicle is known as endocytosis. A part of the cell membrane invaginates and then detaches itself from the surface, forming an isolated vesicle within the cell. This process is known as phagocytosis. If the contents are fluid, the vacuole formed during phagocytosis is known as phagosome and the one formed in pinocytosis is called a pinosome. When the particles are taken into cells by ciliary movement, the process is spoken of as phagotrophy and the vacuole framed is named the food vacuole.
a) Phagocytosis: Phagocytosis is the ingestion of solid particles into a phagosome. It is an important aspect of digestion in protozoans and in all invertebrates in which intracellular digestion occurs. However, in most animals, phagocytosis is important chiefly as a defence mechanism against invading organisms. Phagocytes possess enzymes protienases and lipases, which enable them to digest proteins and lipids.
Phagocytosis occurs in amoeboid cells wherever found, and the cells concerned are called phagocytes. Phagocytes may be fixed or free. Fixed amoeboid cells occur in special reticuloendothelial epithelia in vertebrates. These epithelia are found lining the channels or cavities in the liver, spleen, lymphatic tissue and bone marrow. The fixed phagocytes remove bacteria, cellular debris, and other foreign material from the passing fluid.
The freely moving amoeboid cells in blood or perivisceral fluid are called leucocytes or white blood cells. Leucocytes in mammalian blood are of five main kinds, all of which at some stage, are phagocytic. Most prominent in the blood in this respect are the neutrophils and monocytes.
Many lysosomes fuse with the phagosome, discharging their hydrolytic enzymes into it.
b) Pinocytosis: Pinocytosis is the formation of small channels in the cell membrane followed by the separation of small vacuoles, or pinosomes, in the cytoplasm. Pinocytosis occurs in amoeba and amoeboid cells, such as leucocytes, brush border cells of the kidney, epithelial cells of the intestine, reticuloendothelial cells, Kufler cells of the liver, cells lining the walls of the capillaries, macrophages, fibroblasts and some other cells.
c) Phagotrophy: The intake of food by ciliates like paramecium is spoken of as phagotrophy. It refers to uptake of particulate or liquid matter into an organism possessing a cytosome resulting in the formation of a food vacuole.
Endocytosis may be considered as a special case of active transport because it requires the expenditure of energy by the cell and enables material to enter the cell even though they are present in the extracellular fluid in a lesser concentration than that within the cell.