Biological Membranes: Transport I (Macrotransfer)

Biological Membranes: Transport I (Macrotransfer)

• Class: Foundation Year
• Course: Fundamentals of Human Biology
• Code: FUNBIO.9
• Lecturer: Dr Irene Oglesby
• Date: 16th October 2024

Learning Outcomes

• ALO1: Recall the difference between macrotransfer and microtransfer processes.
• ALO2: Differentiate between exocytosis and endocytosis.
• ALO3: Explain the mechanism of phagocytosis and pinocytosis.
• ALO4: Differentiate between receptor-mediated and fluid-phase pinocytosis.
• ALO5: Describe 4 stages in the secretory vesicle formation.
• ALO6: Differentiate between constitutive and regulated secretion.

Membrane Transport

• Macrotransfer: Transport of large molecules and larger items into and out of the cell.
  • Exocytosis: Movement out of the cell.
  • Endocytosis: Movement into the cell.
• Microtransfer: Passive and Active transportation.
  • Passive: Movement across the membrane without energy input.
  • Active: Movement across the membrane requiring energy input.

Exocytosis

• Process: Releasing large biomolecules through the plasma membrane.
• Tasks:
  • Increases the surface area of the plasma membrane.
  • Releases substances within vesicles into the exterior.
  • Incorporates vesicle membrane proteins into the plasma membrane.

Types of Exocytosis

• Constitutive: Non-Ca²⁺ dependent. Continuously occurs.
• Ca²⁺ dependent: Regulated exocytosis stimulated by signaling molecules.

Ca²⁺-mediated Exocytosis

• Requires presence of Calcium for vesicle release.
• Important Processes:
  • Neuronal communication - converting electrical to chemical signals (at the synapse)
  • Hormone release - e.g., insulin from beta cells.

Distinct Steps in Exocytosis

• 1. Vesicle Trafficking: Movement of vesicles from production site to release site.
• 2. Tethering: Formation of a bridging complex facilitated by proteins like Rab.
• 3. Docking: Attachment of vesicle membrane to target cell membrane.
• 4. Priming: Molecular rearrangements and ATP-dependent modifications.
• 5. Fusion: Phospholipid bilayers merge, driven by SNARE proteins, releasing large biomolecules.

Endocytosis

• Process: Cells absorb molecules like proteins from the exterior.
• Purpose: Uptakes polar & large molecules that can't pass through the hydrophobic plasma membrane.

Subcategories of Endocytosis

• Phagocytosis: Engulfing large particles (e.g., bacteria).
• Pinocytosis: Engulfing materials dissolved in the surrounding medium.

Pinocytosis

• Types:
  • Receptor-mediated: Molecules bind to receptors on the plasma membrane, then adsorbed.
  • Fluid-phase: No receptors are involved.

Phagocytosis

• Process:
  • Plasma membrane extends to engulf material.
  • Receptors can bind to antibodies on foreign bodies.
  • Formation of an intracellular vacuole(phagosome)
  • Fusion with lysosomes(formation of a secondary lysosome).
  • Digestion of engulfed material.

Macrophage Activity

• Macrophages can engulf 25% of their volume each hour via pinocytosis.
• This process removes 3% of their plasma membrane per minute.
• Exocytosis must balance endocytosis to maintain cell structure and function.

Reading Material

• Chapters 5 (Solomon 11th edition): Online resources for additional study on diffusion, sodium-potassium pump, membrane transport, and synaptic vesicle trafficking.