5.4 Bulk transport: endocytosis and exocytosis

Cells use endocytosis and exocytosis to move large molecules and particles across the plasma membrane. Unlike channels and carriers that handle small molecules and ions, bulk transport uses vesicles to shuttle cargo that's too big for those mechanisms. These processes are also central to cell communication, since they're how cells secrete signaling molecules and take in information from their environment.

Bulk Transport Mechanisms

Endocytosis and exocytosis in transport

Endocytosis brings material into the cell. The plasma membrane folds inward around extracellular material, pinches off, and forms an intracellular vesicle containing the ingested cargo. That cargo can include nutrients, signaling molecules, or even pathogens.

Exocytosis works in the opposite direction. Intracellular vesicles travel to the plasma membrane, fuse with it, and release their contents outside the cell. This is how cells secrete hormones, neurotransmitters, and extracellular matrix components.

Both processes require energy (they're active transport) and both reshape the plasma membrane itself. Endocytosis removes membrane surface area, while exocytosis adds it back. This balance helps regulate cell size and membrane composition.

Types of endocytosis

Phagocytosis ("cell eating") handles large particles, typically bigger than 0.5 μm. Think bacteria, dead cells, or cellular debris. The cell extends actin-driven membrane protrusions (pseudopods) that wrap around the particle and engulf it, forming a phagosome. This is mostly performed by immune cells like macrophages and neutrophils.

Pinocytosis ("cell drinking") takes in small amounts of extracellular fluid along with whatever solutes are dissolved in it. The vesicles formed are small (under 0.2 μm) and pinch off from the membrane. Pinocytosis is further divided into macropinocytosis (larger, ruffled membrane folds) and micropinocytosis (smaller, more routine uptake). Most cell types perform pinocytosis continuously.

Receptor-mediated endocytosis is the most selective form. Here's how it works:

1. Specific ligands (like LDL cholesterol particles) bind to receptors on the cell surface.
2. Ligand-receptor complexes migrate and cluster in regions called coated pits, which are lined on their cytoplasmic side with clathrin protein arranged in a polyhedral lattice.
3. The coated pit invaginates and pinches off to form a clathrin-coated vesicle.
4. Once inside the cell, the clathrin coat is removed, and the vesicle delivers its contents for processing.

This mechanism allows cells to concentrate and internalize specific molecules very efficiently, even when those molecules are at low concentrations outside the cell.

Process and importance of exocytosis

Exocytosis follows a defined sequence:

1. The Golgi apparatus packages molecules into secretory vesicles.
2. Vesicles are transported along the cytoskeleton toward the plasma membrane.
3. SNARE proteins mediate the actual fusion. v-SNAREs on the vesicle membrane bind to t-SNAREs on the target (plasma) membrane, pulling the two membranes together until they merge.
4. The vesicle contents are released into the extracellular space.

Exocytosis serves several critical functions:

• Secretion of signaling molecules: Hormones (insulin, growth hormone), neurotransmitters (acetylcholine, dopamine), and digestive enzymes (pepsin, trypsin) are all released this way.
• Building the extracellular matrix: Components like collagen and proteoglycans are secreted via exocytosis to maintain tissue structure.
• Membrane maintenance: Newly synthesized membrane proteins and lipids are delivered to the plasma membrane through vesicle fusion, allowing the cell to renew and modify its surface.

Examples of endo-exocytosis transport

• Substances transported via endocytosis:
  • Nutrients (proteins, lipids, carbohydrates)
  • Signaling molecules (hormones, growth factors, cytokines)
  • Pathogens (viruses; bacteria via phagocytosis)
  • Plasma membrane components (receptors and lipids during membrane recycling)
• Substances transported via exocytosis:
  • Hormones (insulin, growth hormone)
  • Neurotransmitters (acetylcholine, dopamine, serotonin)
  • Digestive enzymes (pepsin, trypsin, lipase)
  • Extracellular matrix components (collagen, fibronectin, proteoglycans)

A useful way to keep these straight: endocytosis generally brings in what the cell needs or what the immune system needs to destroy, while exocytosis sends out what other cells need to receive.