Overview of Membranes and Transport Processes

Jan 12, 2025

B2.1 Membranes and Transport Lecture Notes

Plasma Membrane Overview

  • Terminology: Plasma membrane = cell membrane.
  • Structure: Composed of two layers of phospholipids (lipid bilayer).
  • Function: Controls what enters and leaves the cell; semi-permeable/selectively permeable.
  • Permeability Factors: Based on size and charge of molecules.
    • Large and charged/polar molecules are less permeable.
    • Small, nonpolar molecules like oxygen pass easily.

Molecular Movement

  • Molecular Motion: Molecules are always in motion, leading to collisions and spreading out.
  • Diffusion: Passive movement from high to low concentration, no energy required.
    • Occurs with or without a membrane.
    • Continues until equilibrium is reached.

Proteins in Membrane

  • Types:

    • Peripheral Proteins: Attached to the surface, not spanning the whole membrane.
    • Integral Proteins: Span across the membrane (transmembrane), involved in transport.
  • Functions (Mnemonic: JET RAT):

    • Join cells together.
    • Enzymatic activity.
    • Transport: channel proteins and protein pumps.
    • Recognition or attachment.
    • Transduction for signaling.

Specific Transport Proteins

  • Channel Proteins: Facilitate passive transport without energy.

    • Specific to molecules (e.g., glucose channels).
    • Allow polar molecules and ions to pass.
  • Aquaporins: Specialized channels for water (osmosis).

    • Facilitate faster water transport.
    • Cells regulate their permeability to water via aquaporins.

Types of Transport

  • Osmosis: Net movement of water across a membrane.

    • Passive, no energy required.
    • Water moves toward high solute concentration.
  • Facilitated Diffusion: Passive transport via channel proteins.

    • Movement from high to low concentration.
    • Does not require energy, but uses specific channels.
  • Active Transport: Movement from low to high concentration.

    • Requires energy (ATP).
    • Uses protein pumps, specific for different molecules.

Selective Permeability

  • Membrane Control: Selective permeability through production of specific channel proteins or pumps.
    • Example: Manufacture of glucose channels if needed.

Membrane Recognition & Structure

  • Glycoproteins: Proteins with carbohydrate chains for cell recognition and joining.
  • Glycolipids: Lipids with carbohydrate chains, specific to eukaryotic cells.

Fluid Mosaic Model

  • Description: Current model of the cell membrane.
    • Fluid: Components can move.
    • Mosaic: Composed of various parts.
  • Key Points:
    • Phospholipid bilayer and protein components.
    • Understand both functional and structural aspects.
    • Practice drawing the model.