Biology 1020 lecture 9 Plasma Membrane Structure and Function

  1. Plasma membranes: the story so far..
    • Composition: Phospholipids, glycoproteins, glycolipids
    • Selective barrier
    • Contributions from endomembrane system
    • Connections to cytoskeleton
  2. Early model of plasma membrane structure: sandwich modelĀ  (Davson and Danielli 1930)
    (NOT TRUE)
    • Membranes were chemically analyzed
    • --> Proteins, lipids
    • --> Phospholipids
    • Lipid bilayer were sandwiched between proteins --> hydrophillic proteins could interact with aqueous extracellular space and cytoplasm
  3. Plasma membrane: Fluid mosaic model
    (Singer and Nicolson, 1972)
    • Fluid structure with a mosaic of proteins embedded in membrane
    • Hydrophobic R groups of amino acid in the fatty acid portion
    • Hydrophillic R groups associated with polar head groups and aqueous medium

    Evidence: Freeze fracture of membrane --> splits membrane in half the middle of the bilayer

    • Results: Membrane proteins in the two layers
    • - Inside of extracellular layer (little embedded proteins)
    • - Inside of cytoplasmic layer (lots of proteins)
  4. The fluid in the fluid mosaic model
    • Phospholipids move within the bilayer
    • Most of the lipids (107 times per second), and some proteins, drift laterally
    • Flip flops of lipids occurred seldomly (once per month)
  5. Membrane fluidity: Temperature
    • Temperature increases: Solid --> fluid
    • Temperature decreases: Fluid --> solid
  6. Membrane fluidity: Cholesterol
    Cholesterol stabilizes membrane fluidity

    • Warm temperatures
    • - Restraining movement
    • - Prevents increases in fluidity

    • Cool Temperature
    • - Maintain fluidity by preventing tight placement to form solid
  7. Membrane fluidity: fatty acid composition
    Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids
  8. Membranes must remain fluid to work properly. How do organisms cope with temperature changes?
    Homeoviscous adaptation
    • Change in the fatty acid composition to counter temperature effects.
    • - Drops in temperature --> fluidity decreases cell increases unsaturated fatty acids
    • - Rise in temperature --> fluidity increases cell increases saturated fatty acids
  9. To maximize unsaturated fatty acid content, when's the best time of the year to catch a marine fish?
  10. Membrane proteins (Glycoprotein): Location
    • Peripheral: Bound to the surface of the membrane
    • Integral (eg. integrins): penetrate the hydrophobic core (non-polar amino acids)

    Transmembrane proteins: Integral proteins that span the membrane
  11. Membrane proteins: Functions
    Intercellular joining: Long lasting (e.g. gap junctions and tight junctions)
  12. Membrane proteins: functions: attachment to cytoskeleton and ECM
    • Maintains cell shape.
    • to fibronectin on outside, and microfilament on the inside of the cell
  13. membrane proteins functions: cell-cell recognition
    • Recognition of surface molecules, typically glycoproteins (carbohydrates covalently bonded to proteins)
    • Differ among species, individuals, and even cell types in an individual. e.g. blood types, MHC antigens
  14. Cell-cell recognition
    • Major histocompatibility complex - MHC
    • - Cell surface proteins
    • - Recognized by immune system - T cells
    • - Self vs non-self
    • - Major antigens of Organ rejection
    • - Mate choice: Volatile body odours
  15. Membrane proteins functional types: Signaling
    • Shape of receptor fits chemical messenger
    • Message relayed inside cell (signal transduction)
  16. Membrane proteins functions: Enzymes
    • Catalyze reactions
    • Active sites exposed to accept substrates
  17. Membrane proteins functions: Transport
    • Move solutes, other molecules
    • Hydrophilic channels or shuttles (change shape)
    • Some use ATP
  18. Transport across the plasma membrane: Diffusion
    • Tendency for molecules to spread out evenly
    • Molecules move randomly
    • Populations of molecules move from high concentration to low concentration
    • Materials leave a cell if high concentration inside the cell, or materials enter a cell if the concentration is low in the cell.
  19. Active Transport
    • If a cell needs to accumlate molecules or expel to a higher concentration
    • => against the gradient
    • - Needs energy input
    • - Active transport
  20. Passive transport
    • With the gradient: high to low
    • Hydrocarbons (non polar) dissolve in lipid bilayer
    • Polar, large molecule don't cross easily --> can be assisted by proteins
  21. Passive transport across the plasma membrane
    Reaches a dynamic equilibrium as many molecules cross in one direction as in the other direction - no net difference

    Each solute diffuses down it's own concentration gradient
  22. Passive transport: Osmosis
    • Diffusion of water
    • From a region of lower solute concentraion ro a region of higher solute concentration
    • => total solute

    Because some water molecules are tightly clustered to solute, less water can cross membrane from th area of higher solute
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Biology 1020 lecture 9 Plasma Membrane Structure and Function
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