Endocytosis
Endocytosis is the process by which a cell engulfs extracellular material by invaginating its plasma membrane to form an intracellular vesicle, allowing large molecules, particles, or even other cells to be taken up without crossing the membrane directly. The three main types are phagocytosis (engulfment of solid particles), pinocytosis (engulfment of fluids and dissolved solutes), and receptor-mediated endocytosis (selective uptake via surface receptors such as clathrin-coated pits). Endocytosis requires energy (ATP) and plays critical roles in immune defense, nutrient uptake, and signal regulation.
Types of Endocytosis and Their Characteristics
| Type | Material Ingested | Vesicle Type | Energy Required | Cell Type / Example |
|---|---|---|---|---|
| Phagocytosis | Solid particles, bacteria, dead cells | Phagosome | Yes (ATP) | Macrophages, neutrophils |
| Pinocytosis | Extracellular fluid, small solutes | Macropinosome | Yes (ATP) | Most cell types |
| Receptor-mediated endocytosis | Specific ligands (LDL, hormones) | Clathrin-coated vesicle | Yes (ATP) | Liver cells, epithelial cells |
| Caveolae-mediated | Lipids, small molecules, pathogens | Caveolae vesicle | Yes (ATP) | Endothelial cells, adipocytes |
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Related Terms
Exocytosis
Exocytosis is the process by which a cell secretes or exports materials by fusing an intracellular vesicle with the plasma membrane, releasing its contents into the extracellular space. It is the reverse of endocytosis and requires ATP to drive vesicle trafficking along cytoskeletal tracks to the cell surface. Exocytosis is fundamental to neurotransmitter release at synapses, hormone secretion by endocrine cells, secretion of digestive enzymes by pancreatic acinar cells, and delivery of membrane proteins to the cell surface.
Lysosome
A lysosome is a membrane-bound organelle found primarily in animal cells that contains hydrolytic enzymes capable of breaking down all types of biological polymers, including proteins, nucleic acids, carbohydrates, and lipids. Lysosomes maintain an acidic internal pH of around 4.5–5.0, which is optimal for the activity of their digestive enzymes. They play a central role in autophagy, phagocytosis, and cellular homeostasis by recycling cellular components.
Active Transport
Active transport is the movement of molecules or ions across a cell membrane against their concentration gradient (from low to high concentration), requiring the expenditure of cellular energy in the form of ATP. Primary active transport uses ATP directly to power transport proteins called pumps, while secondary active transport uses the electrochemical gradient established by primary active transport to drive the movement of another solute. Active transport is essential for maintaining cellular ion balances, nutrient uptake, and nerve impulse transmission.
From Greek "endon" (within) and "kytos" (hollow vessel, cell) with the suffix "-osis" (process). The term was coined in the 1960s as electron microscopy enabled visualization of membrane vesicle formation.