endocytoz

Endocytoz, also known as endocytosis, is a cellular process by which the plasma membrane envelops extracellular material, forming vesicles that ferry cargo into the cell. It is central to nutrition, receptor regulation, and innate immunity, and is powered by ATP and cytoskeletal remodeling, particularly actin. Endocytosis occurs through several pathways that share the basic mechanism of membrane invagination but differ in cargo size, selectivity, and molecular machinery.

Phagocytosis involves the intake of large particles like bacteria or dead cells, primarily by specialized cells such as macrophages and neutrophils, culminating in phagosome-lysosome fusion to degrade content. Pinocytosis is a nonspecific uptake of extracellular fluid and solutes; macropinocytosis, a form of pinocytosis, creates large vesicles via actin-driven membrane ruffles. Receptor-mediated endocytosis is highly selective; ligands bind to cell-surface receptors and are internalized via clathrin-coated pits, often with adaptors such as AP2 and vesicle scission mediated by dynamin. Caveolin- or raft-associated endocytosis uses caveolae, flask-shaped invaginations rich in cholesterol and caveolin.

After internalization, vesicles fuse with early endosomes, where cargo is sorted. Some receptors are recycled to the plasma membrane via recycling endosomes; others are directed to late endosomes and lysosomes for degradation or for trafficking to other organelles. The endocytic system intersects signaling: receptor abundance and sensitivity can be controlled by endocytosis, affecting pathways such as growth factor signaling and immune recognition.

Defects or alterations in endocytosis are associated with diseases including hypercholesterolemia, neurodegenerative disorders, and cancer, while many pathogens exploit endocytic routes to enter cells.