Endocytosis

Endocytosis is the uptake of substances into a cell by enclosing them in membrane-bound vesicles formed from the plasma membrane. It includes several pathways distinguished by cargo and machinery.

Clathrin-mediated endocytosis: A selective uptake pathway in which clathrin-coated membrane pits form vesicles around receptor-bound cargo. It is the best-characterized route for concentrating specific cargo at the plasma membrane.
Plasma membrane: The selectively permeable lipid bilayer that surrounds a cell and separates its interior from the extracellular environment. Endocytic vesicles form by inward deformation of this cell boundary.
Low-density lipoprotein receptor: A cell-surface receptor that binds low-density lipoprotein particles and mediates their uptake. Its uptake pathway established how receptor-bound cholesterol enters cells.
Early endosome: A sorting compartment that receives newly internalized material and directs it toward recycling or degradation. Most endocytic cargo first reaches this compartment, where its next destination is selected.
Exocytosis: The release of substances from a cell when intracellular vesicles fuse with the plasma membrane. It moves vesicle contents outward, complementing endocytosis's inward transport.
Caveolae: Small, flask-shaped plasma-membrane invaginations enriched in cholesterol, sphingolipids, and caveolin proteins. These membrane domains can internalize selected cargo through a route distinct from clathrin-coated pits.
Lipid bilayer: A double layer of amphipathic lipids that forms the basic structure of biological membranes. Its flexibility and composition permit the membrane curvature needed to make vesicles.
Transferrin receptor: A cell-surface receptor that binds transferrin and internalizes iron through clathrin-mediated endocytosis. It illustrates repeated receptor recycling during nutrient acquisition.
Recycling endosome: A membrane compartment that returns selected internalized proteins and lipids to the cell surface. It restores receptors and membrane components after their internalization.
Pinocytosis: The uptake of extracellular fluid and dissolved substances in small vesicles. It is a fluid-focused form of endocytosis, contrasted with particle engulfment.

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