Cell Biology
Endocytosis and Exocytosis
Bulk transport into and out of cells — vesicle-mediated movement
Endocytosis: cell takes in material via vesicles formed from plasma membrane (endo = into). Exocytosis: cell releases material via vesicles fusing with membrane (exo = out). Both: bulk transport for large molecules or particles. Endocytosis types: phagocytosis (cells; "cell eating"), pinocytosis (fluids; "cell drinking"), receptor-mediated (specific molecules via receptor binding, e.g., LDL uptake). Exocytosis: secretion of hormones, neurotransmitters, digestive enzymes, antibodies. Energy required (ATP). Critical for: immunity, signaling, nutrient uptake, waste removal.
- EndocytosisMaterial in via vesicle from plasma membrane
- ExocytosisMaterial out via vesicle fusing with plasma membrane
- PhagocytosisEngulfing solid particles ("cell eating")
- PinocytosisEngulfing fluid droplets ("cell drinking")
- Receptor-mediatedSpecific molecules via receptor (e.g., LDL)
- EnergyATP required
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Why these matter
- Immune system. Phagocytosis defends against pathogens.
- Signaling. Receptor internalization regulates response.
- Nutrient uptake. Cholesterol, iron, etc.
- Neurotransmission. Vesicle release at synapses.
- Hormone secretion. Insulin, growth hormone, etc.
- Drug delivery. Targeting specific cells via receptors.
- Disease. Familial hypercholesterolemia (LDL receptor defect).
Common misconceptions
- Both endocytosis and exocytosis use same machinery. Different proteins.
- Vesicles random. Specific destinations marked by coat proteins.
- Phagocytosis only by immune cells. Many cell types do.
- Endocytosis = passive. Requires ATP.
- Endo = small, exo = large. Both can be size variable.
- Secretion is just exocytosis. Often regulated by complex signaling.
Frequently asked questions
How does endocytosis work?
Plasma membrane invaginates around material. Pinches off to form vesicle inside cell. Vesicle moves through cell, often fusing with lysosome for digestion. Energy required (ATP for membrane reorganization, vesicle transport). Common: receptor-mediated endocytosis. Ligand binds receptor → recruits clathrin (coat protein) → clathrin-coated pit → pinches off → uncoated vesicle.
What's phagocytosis?
"Cell eating." Engulfing large particles (>0.5 µm) — bacteria, dead cells, debris. Performed by specialized cells: neutrophils, macrophages, dendritic cells. Steps: cell extends pseudopodia around target; encloses in phagosome; fuses with lysosome → phagolysosome; degraded. Critical for immune defense and tissue cleanup.
What's receptor-mediated endocytosis?
Specific uptake using receptors. Ligand (e.g., LDL cholesterol) binds receptor on cell surface. Receptor-ligand complex clusters in clathrin-coated pit. Vesicle pinches off; clathrin removed; vesicle delivered to early endosome. Receptor often recycled to surface; cargo to lysosome (for digestion) or other destinations. Hundreds of types — for hormones, growth factors, iron, cholesterol, transferrin.
How does exocytosis work?
Vesicle (from Golgi or other source) carries cargo. Trafficked to plasma membrane via cytoskeleton. Vesicle fuses with membrane via SNARE proteins. Cargo released to extracellular space; vesicle membrane becomes plasma membrane. Constitutive: continuous secretion (e.g., extracellular matrix). Regulated: triggered by signal (e.g., neurotransmitter release, insulin secretion).
What's neurotransmitter release?
Specific exocytosis. Neuron action potential reaches synapse → Ca²⁺ enters → vesicles containing neurotransmitter fuse with membrane → release transmitter into synapse. Receptor on next neuron binds neurotransmitter. Speed: ~100 µs from action potential to release. Critical for: nervous system function. Dysregulation: many neurological disorders.
What's clathrin?
Coat protein for many vesicles. Triskelion shape (3-armed); polymerizes into hexagonal/pentagonal lattice. Coats budding pits → curves membrane → pinches off as clathrin-coated vesicle. After endocytosis: clathrin disassembles. Other coats: COPI, COPII (for ER-Golgi traffic). Coats define vesicle identity and destination.
How is bulk vs specific endocytosis different?
Pinocytosis: bulk fluid uptake; non-specific. Phagocytosis: large particles; relatively non-specific or via Fc/complement receptors (immune). Receptor-mediated: specific ligand binding; concentrative (concentrate cargo from low extracellular concentration). Different mechanisms, different cargoes, different cells use different types.