Plasma Membrane & Fluid-Mosaic Model – Quick Review Notes

Page 1

  • Plasma (cell) membrane: physical barrier separating cell from environment; regulates material exchange, removes wastes.
  • Fluid-mosaic model: dynamic phospholipid\text{phospholipid} bilayer with proteins drifting laterally.
  • “Mosaic” = mix of phospholipids, cholesterol, proteins, carbohydrates.

Page 2

  • Schematic (Fig. 1) visualizes fluid-mosaic arrangement.

Page 3

  • Four principal membrane molecules: phospholipids, proteins, cholesterol, carbohydrates.
  • Bilayer = two opposing sheets of amphipathic phospholipids.
    • Hydrophilic phosphate heads contact intra- & extracellular fluids.
    • Hydrophobic fatty-acid tails face inward, exclude water.
  • Amphipathic nature drives self-assembly; analogy: sandwich (bread = heads, butter = tails).

Page 4

  • Phospholipid structure: glycerol + phosphate (head) + two fatty acids (tails: saturated &/or unsaturated).
  • Bilayer orientation: heads outward, tails inward → hydrophobic core barrier.

Page 5

  • Membrane proteins
    • Integral (mostly transmembrane): span or partially penetrate bilayer; may form channels.
    • Peripheral: loosely attached to membrane surface.
    • Functions: transport, enzymes, signal transduction, cell-cell recognition, intercellular joining, cytoskeleton/ECM attachment.
  • Carbohydrates (2–60 units) attach to proteins (glycoproteins) or lipids (glycolipids) forming glycocalyx → protection, cell recognition.
  • Cholesterol (animal cells): inserts between tails; moderates fluidity, prevents solidification, adds strength.

Page 6

  • Cholesterol also limits passage of some small molecules.
  • Plasma membrane functions
    1. Encloses cell; maintains integrity.
    2. Selective barrier (compartmentalization).
    3. Provides specialized functional properties.

Page 7

  • Selective (semi-) permeability
    • Hydrophobic core blocks many polar/large molecules.
    • Easily pass: nonpolar gases (O$2$, CO$2$), water (slowly), lipids.
    • Difficult: large polar molecules (glucose, amino acids) & ions (Na$^+$, K$^+$).
  • Transport proteins (e.g., channels) facilitate passage across bilayer.

Page 8

  • Channel proteins create hydrophilic tunnels for specific solutes (Fig. 3).
  • Transport categories
    • Passive: no cellular energy.
    • Active: requires energy (e.g., ATP).
  • Membrane selectivity vital for cell survival.

Page 9

  • Additional membrane-protein roles: advanced signaling via receptors & markers.
  • Receptors bind hormones/growth factors → trigger intracellular pathways.
  • Pathogens (e.g., HIV) exploit receptors to enter cells.