The process by which animal cells internalize particulate material (such as cellular debris and microorganisms), macromolecules (such as proteins and complex sugars), and low-molecular-weight molecules (such as vitamins and simple sugars). Cells engage in at least three different types of endocytosis: phagocytosis where cells engulf particulate material, receptor-mediated-endocytosis of macromolecules, and potocytosis of small molecules.
Some of the essential nutrients that a cell needs are scarce in the environment. The cells overcome this problem by expressing high-affinity receptors, or binding sites, on the membrane surface. Each type of receptor is specific for either macromolecules or molecules. These endocytic receptors are capable of concentrating their ligand at the cell surface before carrying it into the cell, thus increasing the efficiency of uptake.
In all three endocytic pathways the internalization step begins with the invagination of plasma membrane and the conversion of this membrane into a closed vesicle called an endosome. Each of the pathways has its own set of molecules that control internalization. These molecules assemble at the cell surface and physically deform the membrane into the shape of a vesicle. The vesicle, the endosome, then detaches and migrates to other locations within the cell. The same cell-surface assemblage of molecules also attracts endocytic receptors that are moving around on the cell surface, causing them to cluster over the site of internalization. Receptor clustering, which is essential for efficient uptake, is sometimes stimulated by ligand binding.
Endosomes that are generated by the phagocytic and receptor-mediated endocytic pathways often fuse with lysosomes that contain many different hydrolytic enzymes. Small molecules, by contrast, do not need further processing, so during potocytosis they are delivered directly to the cytoplasm. See Cell membranes, Lysosome
Phagocytosis is a receptor-mediated process where the receptors function as adhesive elements that bond the plasma membrane to the particle. The adhesive interaction of the phagocytic receptors with the membrane stimulates invagination. A critical molecule in this activity is actin, the same protein that provides power for muscle contraction. Surface membranes contain actin-binding proteins that link the phagocytic receptor to the actin cytoskeleton of the cell. Thus, when a particle binds to its endocytic receptor, a signal cascade is initiated that stimulates the recruitment of actin filaments to the site of phagocytosis. See Cytoskeleton, Phagocytosis, Signal transduction
The clathrin-coated pit is a segment of cell membrane that is specialized for receptor-mediated endocytosis. Each pit can be recognized by the presence of a polygonal lattice on the cytoplasmic surface of the membrane. This lattice shapes the plasma membrane into a coated vesicle that immediately uncoats and fuses with endosomes. The endosome functions as a switching area that directs membrane and content molecules to specific locations within the cell.
Potocytosis uses membrane proteins that are anchored by lipid rather than protein as endocytic receptors. The lipid anchor causes the attached proteins to migrate in the plane of the membrane and cluster in a membrane specialization called a caveola. Clustering ensures that any ligand bound to these receptors will be concentrated in this location. When caveolae close, they create a tiny compartment of uniform size that is sealed off from the extracellular space. When the ligand dissociates from its receptor, it reaches such a high concentration that it naturally flows through water-filled membrane channels into the cell.
The closed caveolar compartment appears to be a unique space for the cell. It is transient, does not merge with other organelles, and can selectively concentrate extracellular molecules or ions and deliver them to the cytoplasm. In addition to importing molecules, cells can also use this space to store and process incoming or outgoing messengers that affect cell behavior. See Cell (biology), Cell permeability