The Phospholipid Bilayer

  • Cell membranes are asymmetrical bcuz the two sides of a cell membrane face different environments and carry out different functions
    • The polar phosphate regions are oriented towards the aqueous external or internal environments
    • The nonpolar hydrocarbon fatty acid regions face each other inside the membrane
  • Proteins may be loosely attached to inner/outer membrane or extend into the membrane
    • Some are transmembrane
  • Phospholipids (and some proteins) are amphipathic
    • Have polar (hydrophilic) and nonpolar (hydrophobic) regions

Fluid Mosaic Model

  • Membrane is made up of of amphipathic proteins embedded in fluid bilayer of phospholipids”
  • Membrane is fluid bcuz of weak hydrophobic interactions in interior of membrane that allow proteins to move vertically
  • Proteins and lipids move in membrane which stays fluid until temp decreases
    • Fluidity of membrane depends on # of saturated and unsaturated fatty acids
      • Fluidity affects permeability and protein transport
    • Unsaturated hydrocarbons of some phospholipids help keep membranes fluid at lower temp.
      • Double bonds stop carbons and fatty acids from packing together
      • Cholesterol helps keep membrane fluid at low temp and rigid at high temp
        • Lipids get excited (more energy) at high temp → increased fluidity & permeability
      • Functional & structural: lipids provide structure but proteins determine function


  • The cell membrane is amphipathic with a hydrophobic interior, so only nonpolar molecules can pass through → selective permeability

What Can Pass Through?

  • Ability of molecule to pass through depends on size, charge, and SP of membrane
  • Small, uncharged, nonpolar molecules can freely pass among the membrane bcuz repel water like hydrophobic inside layer
    • Do not need channel or carrier proteins
    • N2, O2, CO2, H2
  • Large, polar molecules and all ions (hydrophilic) are impermeable and can only move across the membrane through EMBEDDED CHANNEL AND TRANSPORT PROTEINS
    • Ex: ions, monosaccharides (glucose), amino acids
    • Some uncharged, polar molecules (H2O) can pass thru the membrane in small amounts

Plasma Membrane Proteins

  • Channel proteins: have hydrophilic channels like a tunnel for certain molecules
  • Carrier proteins: bind to molecules (ex: glucose) and change shape to shuttle them across,
  • Transport Proteins: use energy (ATP) to transport materials across membrane against gradient
  • Recognition proteins: give each cell unique identification → distinction between self/foreign cells & normal/infected cells
    • Ex: glycoproteins
  • Receptor proteins: binding sites for hormones or other trigger molecules
  • Enzymes:
  • Anchor proteins: attach cells to other cells or provide anchors for internal filaments

Types of Transport Proteins                                                                                                                              

  • Integral proteins: amphipathic, penetrate hydrophobic interior, some transmembrane
  • Transmembrane proteins: all across membrane
  • Peripheral protein: not in lipid bilayer: bound to membrane surface → hydrophilic

Cell Walls (found in plants)

  • Cell walls provide a semipermeable barrier that regulates the movement of molecules into the cell
    • Cell walls stop plants, fungi, some protists, and prokaryotes from bursting in hypotonic solution