Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 7.5 Bulk Transport Across the Plasma Membrane Occurs by Exocytosis and Endocytosis
Cells don’t just move small molecules. For bulk cargo—like proteins, polysaccharides, or even entire cells—they use specialized forms of vesicle transport.
This is where exocytosis and endocytosis take the stage.
1. Exocytosis: Pushing Things Out
- How it works:
- A vesicle containing cargo (e.g., proteins, hormones) buds off from the Golgi apparatus.
- It travels along the cytoskeleton toward the plasma membrane.
- The vesicle fuses with the membrane and releases its contents outside the cell.
- Examples:
- Insulin secretion from pancreatic cells
- Neurotransmitter release at nerve synapses
2. Endocytosis: Bringing Things In
- The cell membrane folds inward, forming a vesicle that captures external materials.
- Types of Endocytosis:
- Phagocytosis ("cellular eating"): Engulfs large particles like bacteria, forming a food vacuole that fuses with a lysosome.
- Pinocytosis ("cellular drinking"): Non-selectively engulfs extracellular fluid into small vesicles.
- Receptor-Mediated Endocytosis: Uses receptors to bind and internalize specific molecules (e.g., LDL cholesterol).
3. Recycling the Membrane
- Though exocytosis adds membrane and endocytosis removes it, the total surface area remains balanced.
- These processes also help in remodeling the membrane and controlling surface protein composition.
In a Nutshell
- Exocytosis exports large molecules via vesicle fusion with the plasma membrane.
- Endocytosis brings substances in by forming inward-folding vesicles.
- Three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated.
- Together, they handle bulk transport and help maintain membrane homeostasis.