1.11 Endocytosis and Exocytosis

These are processes of active, bulk transport used to move large molecules, bacteria, and waste materials across the cell membrane that cannot pass through the lipid bilayer via diffusion.

Both processes require energy (ATP) and involve the formation and fusion of vesicles with the cell membrane. For a deeper understanding of how the membrane structure facilitates this, see Cell Wall and Plasma Membrane→ and Membrane Transport Mechanism→.

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Endocytosis

Endocytosis is the general term for the process of bringing substances into the cell. It involves the cell membrane folding inward (invaginating) to surround the target material and budding off to form an intracellular vesicle.

There are two main subtypes of endocytosis.

1. Phagocytosis ("Cell Eating")

• Definition: The ingestion of large, solid particles like other cells or bacteria.
• Process:
  1. Extensions of the cytoplasm called pseudopodia ("false feet") surround the target particle.
  2. The pseudopodia fuse, enclosing the particle within a large vesicle called a phagosome or food vacuole.
• Examples:
  • Amoeba uses phagocytosis for feeding.
  • In higher animals, immune cells like white blood cells (phagocytes) use it as a defensive mechanism to destroy pathogens and antigens.

2. Pinocytosis ("Cell Drinking")

• Definition: The ingestion of extracellular fluid and its dissolved solutes, such as ions, enzymes, and hormones.
• Process: The cell membrane invaginates (folds inward) to trap a droplet of fluid, then buds off to form a small vesicle called a pinosome.

Receptor-Mediated Endocytosis

A specific type of pinocytosis.

• Process: Macromolecules bind to specific receptors on the cell surface. These receptors cluster in regions called coated pits (often coated with the protein clathrin). The pit then invaginates and forms a coated vesicle, bringing the specific target molecule (ligand) into the cell.
• Example: Cholesterol absorption by cells.

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Exocytosis

Exocytosis is the process of moving materials from inside the cell to the outside. It is a form of active transport.

• Process:
  1. Substances are packaged into vesicles within the cell (often by the Golgi apparatus).
  2. The vesicle is transported to the cell membrane.
  3. The vesicle's phospholipid bilayer fuses with the cell's plasma membrane.
  4. The contents of the vesicle are released into the extracellular fluid.
• Examples:
  • Release of neurotransmitters at a synapse between nerve cells (see Cell Signaling→).
  • Secretion of hormones or enzymes from glandular cells.
  • Release of waste products.

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