Endocytosis refers to a biological process whereby cells absorb material outside their cell membranes. Endocytosis is the opposite of exocytosis, and always involves the formation of a vesicle from part of the cell membrane. Endocytosis can be of three forms:

  1. Pinocytosis (literally, cell drinking) is the invagination of the cell membrane to form a pocket filled with extracellular fluid. The pocket then pinches off to form a vesicle, and the vesicle ruptures to release its contents into the cytosol.
  2. Receptor-mediated endocytosis is similar to pinocytosis, except it is prompted by the binding of a large extracellular molecule - such as a protein - to a receptor on the cell membrane. The invagination that results engulfs the protein, which is then released into the cytosol.
  3. Phagocytosis is the process by which cells ingest large objects, such as prey cells or large chunks of dead organic matter. These are sealed off into large vacuoles. Lysosomes then merge with the vacuole, turning it into a digestive chamber. The products of the digestion are then released into the cytosol. Macrophages are cells of the immune system that specialize in the destruction of bacteria by phagocytosis.