A process in which cell takes in materials from the outside by engulfing and fusing them with its plasma membrane.
Most of the extracellular materials that the cells absorb are large polar molecules (e.g. proteins and hormones) that cannot pass through the hydrophobic plasma membrane by simple diffusion.
There are two types of endocytosis, called phagocytosis, which literally means cell-eating, and pinocytosis, which literally means cell-drinking.
Word origin: endo: G., comb. form of éndon: within + cyte: G. kutos, hollow vessel + -osis: expressing state or condition.
Related forms: endocytotic (adjective), endocytic (adjective).
Related phrases: trans-endocytosis.
See also: plasma membrane.