2.3 Transport across cell membranes

• Describe the fluid-mosaic model of membrane structure


  The basic structure of all cell membranes (cell-surface membranes & membranes around eukaryotic organelles) is the same ● 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 tails face inwards, phosphate heads face outwards

  ● Proteins

  ○ Intrinsic / integral proteins span bilayer eg. channel and carrier proteins

  ○ Extrinsic / peripheral proteins on surface of membrane

  ● Glycolipids (lipids with polysaccharide 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

  
• 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 to water
• Explain the role of cholesterol (sometimes present) 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 act as receptors / antigens → involved in cell signalling / recognition
• Describe how movement across membranes occurs by simple diffusion

  ● Lipid-soluble (non-polar) or very small substances eg. O2, steroid hormones

  ● Move from an area of higher conc. to an area of lower conc. down a conc. gradient

  ● Across phospholipid bilayer

  ● Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances
• Explain the limitations imposed by the nature of the phospholipid bilayer

  ● Restricts movement of water soluble (polar) & larger substances eg. 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

  ● Through specific channel / carrier proteins

  ● Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)
• Explain the role of carrier and channel proteins in facilitated diffusion

  ● Shape / charge of protein determines which substances 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 substance 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 high to low water potential (ψ) / down a water potential gradient

  ● Through a partially permeable membrane

  ● Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)
• What does Water potential measure

  Water potential is a measure of how likely water molecules are to move out of a solution Pure (distilled) water has the maximum possible ψ (0 kPA), increasing solute concentration decreases ψ
• Describe how movement across membranes occurs by active transport

  ● Substances move from 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 substance 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

  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 its concentration gradient is often coupled with the movement of another down its concentration gradient
• Describe co-transport - absorption of sodium ions and glucose (or amino acids) by cells lining the mammalian ileum


  ● Na+ actively transported from epithelial cells to blood (by Na+/K+ pump)

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

  ● Na+ enters epithelial cell down its concentration gradient with glucose against its concentration gradient - by a co-transporter protein

  ● Glucose moves down a conc. gradient into blood via facilitated diffusion

  
• What does co-transport show

  The movement of sodium can be considered indirect / secondary active transport, as it is reliant on a concentration gradient established by active transport
• Rate of movement across cell membrane affected

  ● Increasing SA of membrane increases rate of movement

  ● Increasing number of channel / carrier proteins increases rate of facilitated diffusion / active transport

  ● Increasing conc. gradient increases rate of simple / facilitated diffusion and osmosis

  ● Increasing conc. gradient increases rate facilitated diffusion

  ○ Until number of channel / carrier proteins becomes a limiting factor as all in use / saturated ● Increasing water potential gradient increases rate of osmosis
• Explain the adaptations of some specialised cells in relation to the rate of transport across their internal and external membranes

  ● Membrane folded eg. microvilli in ileum → increase in surface area

  ● More protein channels / carriers → for facilitated diffusion (or active transport - carrier proteins only)

  ● Large number of mitochondria → make more ATP by aerobic respiration for active transport