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.
Comparison of Constitutive and Regulated Exocytosis
| Feature | Constitutive Exocytosis | Regulated Exocytosis |
|---|---|---|
| Trigger | None (continuous) | Signal (Ca²⁺ influx, hormones) |
| Timing | Constant, ongoing | On-demand, rapid |
| Cargo | Membrane proteins, extracellular matrix | Neurotransmitters, hormones, enzymes |
| SNARE proteins | Used for fusion | Used for fusion (more tightly regulated) |
| Example | Fibroblast collagen secretion | Neurotransmitter release at synapse |
| Cell types | All cell types | Neurons, endocrine cells, mast cells |
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Related Terms
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.
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 "exo" (outside) and "kytos" (hollow vessel, cell) with the suffix "-osis" (process). The term was introduced alongside "endocytosis" in the 1960s to describe the complementary outward membrane fusion process.