Transport across cell membranes

• Describe the fluid-mosaic model of membrane structure:

  • Molecules free to move laterally in phospholipid bilayer
  • Many components - phospholipids, proteins, glycoproteins and glycolipids
• Describe the arrangement of the components of a cell membrane:

  • Phospholipids form a bilayer - fatty acid tail faces inwards, phosphate heads face outwards
  • Proteins:

  -Intrinsic/integral proteins span bilayer e.g. channel and carrier proteins

  -Extrinsic/peripheral proteins on surface of membrane

  • Glycolipids (lipids with polysaccharides chains attached) found on exterior surface
  • Glycoproteins (proteins with polysaccharide chains attached) found on exterior surface
  • Cholesterol (sometimes present) bonds to phospholipid hydrophobic fatty acid tails
• Explain the arrangement of phospholipids in a cell membrane:

  • Bilayer, with water present on either side
  • Hydrophobic fatty acid tails repelled from water so point away from water/to interior
  • Hydrophilic phosphate heads attracted to water so point towards water
• Explain the role of cholesterol in cell membranes:

  • Restricts movement of other molecules making up membrane
  • So decreases fluidity (and permeability)/increases rigidity
• Suggest how cell membranes are adapted for other functions:

  • Phospholipid bilayer is fluid - membrane can bend for vesicle formation/phagocytosis
  • Glycoproteins/glycolipids are receptors/antigens - involved in cell signalling/recognition
• Describe how movement across membranes occurs by simple diffusion:

  • Lipid-soluble (non-polar) or very small substances e.g. O2
  • Move from an area of higher conc. to an area of lower conc. down a concentration gradient
  • Across phospholipid bilayer
  • Passive - doesn't require energy from ATP/respiration
• Explain the limitations imposed by the nature of the phospholipid bilayer:

  • Restricts movement of water soluble (polar) and larger substances e.g. Na+/glucose
  • Due to hydrophobic fatty acid tails in interior of bilayer
• Describe how movement across membranes occurs by facilitated diffusion:

  • Water-soluble (polar)/slightly larger substances
  • Move down a concentration gradient (higher to lower)
  • Through specific channel/carrier proteins
  • Passive doesn't require energy from ATP/respiration
• Explain the role of carrier and channel proteins in facilitated diffusion:

  • Shape/charge of protein determines which substances can move
  • Channel proteins facilitate diffusion of water soluble substances

  -Hydrophilic pore filled with water

  -May be gated - can open/close

  • Carrier proteins facilitate diffusion of (slightly larger) substances

  -Complementary substances attaches to binding site

  -Protein changes shape to transport substance
• Describe how movement across membranes occurs by osmosis:

  • Water diffuses/moves
  • From an area of higher to lower water potential/down a water potential gradient
  • Through a partially permeable membrane
  • Passive - doesn't require energy from ATP/respiration
• Describe how movement across membranes occurs by active transport:

  • Substances move from an area of lower to higher concentration/against a concentration gradient
  • Requiring hydrolysis of ATP and specific carrier proteins
• Describe the role of carrier proteins and the importance of the hydrolysis of ATP in active transport:

  1. Complementary substances binds to specific carrier protein
  2. ATP binds, hydrolysed into ADP + Pi, releasing energy
  3. Carrier protein changes shape, releasing substance on side of higher concentration
  4. Pi released = protein returns to original shape
• Describe how movement across membranes occurs by co-transport:

  • Two different substances bind to and move simultaneously via a co-transporter protein (type of carrier protein)
  • Movement of one substance against concentration gradient is often couple with the movement of another down its concentration gradient
• Describe an example that illustrates co-transport:

  1)- Na+ actively transported from epithelial cells to blood

  - Establishing a conc. gradient of Na+ (higher in lumen than epithelial cell)

  2)- Na+ enters epithelial cell down its concentration gradient with glucose against its concentration gradient

  - Via a co-transporter protein

  3)- Glucose moves down a conc. gradient into blood via facilitated diffusion
• Explain the adaptations of some specialised cells in relation to the rate of transport across their internal and external membranes:

  • Membrane folded e.g. microvilli in ileum = increases surface area
  • More protein channels/carriers = for facilitated diffusion (or active transport)
  • Large number of mitochondria = make more ATP by aerobic respiration for active transport