5.4 Bulk Transport - Biology 2e | OpenStax

Key Concepts

• Understanding of bulk transport in cells, which includes moving large molecules and particles.
• Endocytosis and exocytosis are major processes.

Endocytosis

• Active transport process that moves particles into a cell via the plasma membrane.
• Common steps across types:
  • Plasma membrane invaginates to form a pocket around the target particle.
  • Pocket pinches off creating a vesicle containing the particle.

Types of Endocytosis

Phagocytosis ("cell eating")

• Cells engulf large particles or other cells.
• Example: Neutrophils engulfing microorganisms.
• Involves clathrin-coating, forming vesicles, and merging with lysosomes.

Pinocytosis ("cell drinking")

• Cells take in molecules and water from extracellular fluid.
• Results in smaller vesicles than phagocytosis.
• Potocytosis: A variation that involves caveolin instead of clathrin, forming smaller vesicles.

Receptor-mediated Endocytosis

• Uses receptor proteins in the plasma membrane.
• Specific substances bind to receptors, leading to vesicle formation.
• Inefficiencies can cause diseases (e.g., familial hypercholesterolemia due to defective LDL receptors).

Exocytosis

• Process of expelling material from the cell into extracellular fluid.
• Material is encased in a membrane, which fuses with the plasma membrane.
• Examples include secretion of extracellular matrix proteins and neurotransmitters.

Methods of Transport

• Diffusion: Passive, small-molecular weight material.
• Osmosis: Passive, water.
• Facilitated transport: Passive, sodium, potassium, calcium, glucose.
• Primary active transport: Active, sodium, potassium, calcium.
• Secondary active transport: Active, amino acids, lactose.
• Phagocytosis: Active, large macromolecules, whole cells.
• Pinocytosis/Potocytosis: Active, small molecules.
• Receptor-mediated endocytosis: Active, large quantities of macromolecules.

Important Figures

• Figure 5.21: Illustrates phagocytosis where the cell membrane surrounds and engulfs a particle.
• Figure 5.22: Details pinocytosis and the formation of smaller vesicles.
• Figure 5.23: Shows receptor-mediated endocytosis with clathrin-coated receptors.
• Figure 5.24: Example of exocytosis where vesicles fuse with the plasma membrane to release contents outside the cell.