Biology Chapter: Endocytosis and Exocytosis

Bulk Transport: Endocytosis and Exocytosis

• Vesicular/Bulk Transport: Large molecules and particles are moved across the plasma membrane via exocytosis and endocytosis.

Endocytosis: Entering the Cell

• Mechanism: Endocytosis is a process where the cell takes in large particles and macromolecules. A section of the plasma membrane folds inward, forming a vesicle that encloses the substance. Protein coats sometimes form on the inner surface of the membrane, helping select the material for transport.

• Types of Endocytosis:

  • Phagocytosis ("Cell Eating"): Large particles are engulfed by the cell membrane, forming a phagosome. The phagosome fuses with a lysosome to digest the contents. This can involve pseudopods.
  • Pinocytosis ("Cell Drinking"): Small droplets of extracellular fluid are taken into the cell in a tiny vesicle, nonspecifically. No receptors are involved in this process.
  • Receptor-Mediated Endocytosis: Specific molecules bind to receptors on the plasma membrane. This leads to the formation of a protein-coated vesicle to capture and concentrate the substances. Receptors are recycled to the plasma membrane.

Exocytosis: Leaving the Cell

• Mechanism: Exocytosis is an active process for transporting molecules outward from the cell to the extracellular space.
• Steps (Figure 2.5):
  • A membrane-bound vesicle containing the molecule to be secreted moves towards the plasma membrane.
  • At the membrane, vesicle-associated proteins (v-SNAREs) bind to plasma membrane proteins (t-SNAREs).
  • The vesicle and plasma membrane fuse, creating a pore.
  • The contents of the vesicle are released to the exterior of the cell.

Clinical Application: Hypercholesterolemia

• Familial Hypercholesterolemia: A genetic disorder where cells lack LDL receptors. This leads to cholesterol accumulation in the blood, which can cause atherosclerosis (hardening of the arteries) and an increased risk of stroke and heart disease.