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Cell Membranes and Transport Notes

Cell Membranes and Transport

Plasma Membrane Structure

  • Common to all cells; forms outer limit.
  • Partially permeable barrier controlling entry/exit.
  • 7-10nm thick; visible in detail with electron microscope.
  • Phospholipid bilayer with varying protein content.
  • Eukaryotic cells: membranes surround organelles (single or double - envelope).
  • Internal membranes (RER, SER, Golgi) also partially permeable.

Components of Plasma Membrane

Phospholipid Bilayer

  • Basis of the membrane.
  • Heads: Polar, hydrophilic (phosphate and glycerol).
  • Tails: Non-polar, hydrophobic (fatty acids).
  • Barrier to water-soluble substances.
  • Fluidity affected by:
    • Saturated/unsaturated fatty acids: More unsaturated = more fluid.
    • Temperature: Increase = more fluid, Decrease = less fluid.

Cholesterol

  • Stabilizes bilayer; scattered within membrane.
  • Regulates fluidity and mechanical stability.

Protein Molecules

  • Extrinsic (Peripheral): On surface or partly embedded.
    • Functions: structural support, cell receptors, recognition sites.
  • Intrinsic (Integral): Span the bilayer.
    • Functions: carriers, hydrophilic channels, enzymes.

Protein Functions

  • Transport proteins: Hydrophilic channels for ions/polar molecules.
  • Glycoproteins: Carbohydrate chains attached; stabilize membrane, aid adhesion.
  • Receptor molecules: Bind substances (e.g., hormones).
  • Antigens: Initiate immune response; cell-cell recognition.
  • Enzymes: Catalyze reactions within the membrane.

Glycolipids

  • Phospholipids with carbohydrate chains attached; cell-cell interactions, receptors.
  • Do not act as antigens.

Fluid Mosaic Model

  • Proposed by Singer and Nicolson (1972).
  • Fluid: Phospholipids and proteins in constant motion.
  • Mosaic: Scattered proteins embedded in bilayer.

Evidence for Fluid Mosaic Model

  • Freeze-fracture electron microscopy: Trans-membrane proteins.
  • Antibody tagging: Proteins move freely.

Membrane Permeability

  • Lipid soluble substances: Permeable (small, non-polar).
  • Water soluble substances: Not freely permeable; pass through transport proteins (polar).
  • Large molecules: Cannot pass through.
  • Selectively permeable.

Factors Affecting Permeability

  • Temperature: Higher temp = more fluid = faster diffusion.
  • Organic solvents: Dissolve phospholipids, destroying membrane.

Transport Across Plasma Membrane

Diffusion, Facilitated Diffusion, Osmosis, Active Transport, Bulk Transport (Endocytosis & Exocytosis).

Diffusion

  • Passive process (no ATP required).
  • Movement down a concentration gradient.
  • Affected by:
    • Concentration gradient: Greater gradient = faster diffusion.
    • Surface area: Greater area = faster diffusion. $Rate of diffusion = {\Surface area \times Difference in concentration \over Diffusion distance}$$
    • Diffusion path: Shorter distance = faster diffusion.
    • Molecule size: Smaller = faster diffusion.
    • Temperature: Higher = faster diffusion.
    • Membrane nature: Pores, composition.

Facilitated Diffusion

  • Passive, no ATP required.
  • For large/charged molecules via transport proteins.
    • Channel proteins: Pores lined with polar groups; may be gated.
    • Carrier proteins: Change shape to transport specific molecules.

Co-transport

  • Type of facilitated diffusion.
  • Molecules/ions brought into cells together via co-transport carrier proteins.

Osmosis

  • Diffusion of water across a partially permeable membrane.
  • Water moves from high to low water potential ($\psi$).
  • Aquaporins: Special protein channels for water.
  • Solute potential ($\psi_s$): Always negative; decreases with more solutes.
  • Pressure potential ($\psi_p$): Always positive; increases with pressure.
  • $\psi = \psis + \psip$

Tonicity

  • Hypertonic: Higher solute concentration outside cell --> water moves out (crenation/plasmolysis).
  • Hypotonic: Lower solute concentration outside cell --> water moves in (haemolysis/turgid).
  • Isotonic: Equal solute concentration --> no net movement.

Osmosis in Animal/Plant Cells

  • Animal cells lack cell walls so are affected dramtically by osmosis. If too much water enters the cell, it bursts (haemolysis). In hypertonic solutions they shrivel (crenation). Plant cells, by contrast, have cell walls that prevent bursting. However, in hypertonic solutions they undergo plasmolysis because they lose water.

Incipient Plasmolysis

  • Cell membrane loses contact with cell wall; pressure potential = 0.

Active Transport

  • Requires ATP and carrier proteins (protein pumps).
  • Moves molecules against concentration gradient.
  • Mechanism: Molecule binds, ATP phosphorylates carrier, protein changes shape, molecule released.
  • ATPase enzyme involved.
  • Uniport, symport, antiport systems.
  • Affected by: number of carrier proteins, factors affecting aerobic respiration.
  • Cyanide inhibits cytochrome oxidase, preventing ATP synthesis.

Bulk Transport

  • Endocytosis: Uptake of material via vesicle formation.
    • Phagocytosis: Uptake of solids (