Membrane Transport Lecture Notes

Overview

• Membrane transport is the movement of substances into and out of cells.
• The plasma membrane acts as a regulator, composed of phospholipids and proteins.
  • Phospholipids: polar, hydrophilic head and two non-polar, hydrophobic tails.
  • Forms a bilayer with heads facing outwards and tails facing inwards.
• Two fluids involved:
  • Cytosol: Inside the cell.
  • Interstitial fluid: Outside the cell.

Types of Membrane Transport

1. Passive Transport a type of membrane transport that does not require energy to move substances across cell membranes. Instead of using cellular energy, like active transport, passive transport relies on the second law of thermodynamics to drive the movement of substances across cell membranes.

2. Active Transport In cellular biology, active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient.

Passive Transport

• Does not require energy from the cell.
• Includes:
  • Simple Diffusion: Movement from high to low concentration without assistance.
    • Examples: Oxygen (O2) and Carbon Dioxide (CO2).
  • Facilitated Diffusion: Uses proteins to move charged or polar solutes.
    • Channel-Mediated: Uses ion channels (e.g., sodium ions).
      • Leak Channels: Continuously open.
      • Gated Channels: Open in response to stimuli.
    • Carrier-Mediated: Changes shape to move molecules like glucose.
  • Osmosis: Movement of water through a selectively permeable membrane.
    • Can occur through phospholipid bilayer or aquaporins.

Active Transport

• Requires cellular energy (ATP).
• Includes:
  • Primary Active Transport: Uses protein pumps to move ions against gradient.
    • Example: Sodium-potassium pump using ATP.
  • Secondary Active Transport: Uses energy from one molecule moving down its gradient to move another against its gradient.
    • Types:
      • Symport: Moves substances in the same direction.
      • Antiport: Moves substances in opposite directions.
  • Vesicular Transport: Uses vesicles for transporting large substances.
    • Types:
      • Exocytosis: Secretes materials out of the cell.
      • Endocytosis: Engulfs substances into the cell.
        • Phagocytosis: "Cell eating" of large particles.
        • Pinocytosis: "Cell drinking" of fluid droplets.
        • Receptor-Mediated Endocytosis: Specific molecule uptake via receptors.

Key Points

• Both endocytosis and exocytosis are energy-dependent processes.
• Membrane transport is crucial for maintaining cellular homeostasis.

This summary provides a comprehensive overview of the fundamental concepts of membrane transport mechanisms discussed in the lecture.