Endocytosis is the process by which cells internalize substances from their external environment. It is how cells get the nutrients they need to grow and develop. Substances internalized by endocytosis include fluids, electrolytes, proteins, and other macromolecules. Endocytosis is also one of the means by which white blood cells of the immune system capture and destroy potential pathogens including bacteria and protists. The process of endocytosis can be summarized in three basic steps.
The Basic Steps of Endocytosis The plasma membrane folds inward (invaginates) forming a cavity that fills with extracellular fluid, dissolved molecules, food particles, foreign matter, pathogens, or other substances. The plasma membrane folds back on itself until the ends of the in-folded membrane meet. This traps the fluid inside the vesicle. In some cells, long channels also form extending from the membrane deep into the cytoplasm. The vesicle is pinched off from the membrane as the ends of the in-folded membrane fuse together. The internalized vesicle is then processed by the cell. There are three primary types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis is also called "cell eating" and involves the intake of solid material or food particles. Pinocytosis, also called "cell drinking", involves the intake of molecules dissolved in fluid. Receptor-mediated endocytosis involves the intake of molecules based upon their interaction with receptors on a cell's surface.
