Endocytosis

Endocytosis is a form of bulk transport in which a cell brings in large molecules, fluid or even whole cells. A patch of the plasma membrane folds inward around the material, forming a pocket that seals off as a membrane-bound vesicle inside the cytoplasm. Because it reshapes the membrane, it is an active process that uses the cell's energy.

There are three main types. In phagocytosis, or cell eating, a cell engulfs large particles such as bacteria; white blood cells like neutrophils and macrophages use it to destroy invaders, fusing the vesicle with a lysosome that digests its contents. In pinocytosis, or cell drinking, the cell gulps small droplets of extracellular fluid along with whatever is dissolved in them. In receptor-mediated endocytosis, receptors gathered in pits coated with the protein clathrin bind specific molecules, such as LDL cholesterol particles from the blood, so the cell can collect them selectively.

Faulty LDL receptors cause familial hypercholesterolemia, and some viruses and toxins enter cells by hijacking this route. The reverse process, in which vesicles fuse with the membrane to release material, is exocytosis.