5.4 Bulk Transport - Biology 2e | OpenStax

Learning Objectives

• Describe endocytosis, including phagocytosis, pinocytosis, and receptor-mediated endocytosis.
• Understand the process of exocytosis.

Bulk Transport Overview

• Cells need to transport large molecules and particles in addition to small ions and small molecules through membranes.
• Bulk transport requires energy since large particles cannot pass through the membrane even with energy.

Endocytosis

• Endocytosis: Active transport into the cell by enclosing materials in vesicles formed from the plasma membrane.

Types of Endocytosis

1. Phagocytosis (cell eating)

   • Involves the engulfing of large particles or whole cells.
   • Example: Neutrophils in human body engulf microorganisms.
   • Process involves clathrin-coated vesicles merging with lysosomes.

2. Pinocytosis (cell drinking)

   • Involves ingestion of extracellular fluid and molecules.
   • Smaller vesicles are formed compared to phagocytosis.
   • Does not need to merge with lysosomes.

3. Potocytosis

   • Variation of pinocytosis using caveolin instead of clathrin.
   • Involves transcytosis, where substances are transported through the cell.

4. Receptor-mediated Endocytosis

   • Involves receptors on the plasma membrane specific to certain substances.
   • Example: Removal of LDL (bad cholesterol) from blood.
   • Can be hijacked by viruses and toxins to enter cells.

Exocytosis

• Exocytosis: The process of expelling materials from the cell into the extracellular fluid.
• Involves vesicles fusing with the plasma membrane to release contents outside the cell.
• Examples: Secretion of extracellular matrix proteins, neurotransmitter release.

Methods of Transport and Energy Requirements

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