Transport Across Cell Membranes

• Describe the fluid-mosaic model of membrane structure

  molecules free to move laterally in the phospholipid bilayer (fluid), many components including phospholipids/proteins./glycoproteins and glycolipids (mosaic)
• Describe the arrangement of the components of a cell membrane

  phospholipids form a bilayer, intrinsic proteins span the bilayer (e.g channel and carrier proteins), extrinsic proteins on the surface of the membrane, glycolipids are found on the exterior surface, glycoproteins are found on the exterior, cholesterol 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 to interior, hydrophilic phosphate heads point towards water
• Explain the role of cholesterol (sometimes present) in cell membranes

  restricts the movement of other molecules making up membrane, decreases fluidity / increases rigidity
• Suggest how cell membranes are adapted for other functions

  fluid --> membrane can bend for vesicle formation/phagocytosis

  glycoproteins and glycolipids act as receptors/antigens --> involved in cell signalling and recognition
• Describe how movement across membranes occurs by simple diffusion

  lipid-soluble (non-polar) or very small substances (e.g O2 or steroid hormones) move from an area of higher concentration to an area of lower concentration down a gradient. Passive - doesn't require energy from ATP
• Explain the limitations imposed by the nature of the phospholipid bilayer

  restricts movement of water soluble (polar) and larger substances due to hydrophobic fatty acid tails in interior of bilayer
• Describe how movement across membranes occurs by facilitated diffusion

  water-soluble/larger substances move down a concentration gradient through specific channel/carrier proteins. passive - doesn't require energy from ATP (only kinetic energy of substances)
• Explain the role of carrier proteins in facilitated diffusion


  Shape/charge of a protein determines which substances move

  carrier proteins facilitate diffusion of larger substances --> complementary substance attaches to binding site, protein changes shape to transport substance

  
• Explain the role of channel proteins in facilitated diffusion

  Channel proteins facilitate the diffusion of water-soluble substances, hydrophilic pore filled with water. maybe gated - can open and close

  
• Describe how movement across membranes occurs by osmosis

  Water diffuses from an area of high to low water potential (ψ) down a water potential gradient through a partially permeable membrane
• What is water potential?

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

  substances move from area of lower to higher concentration/ against a concentration gradient, requiring the 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 to 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 (carrier protein)

  Movement of one substance against its concentration gradient is often coupled with the movement of another down its concentration gradient
• Describe an example that illustrates co-transport

  Absorption of sodium ions and glucose by cells lining the mammalian ileum

  1. Na+ actively transported from epithelial cells to blood by Na+/K+ pump - establishing a conc. gradient of Na+ higher in the lumen than in the epithelial cell
  2. Na+ enters epithelial cell down the conc. grad. with glucose against its concentration gradient via a co-transporter protein
  3. Glucose moves down a conc. gradient into blood via facilitated diffusion

  
• Describe how surface area affects the rate of movement across cell membranes

  Increasing surface area of membrane increases rate of movement
• Describe how the number of channel or carrier proteins affects the rate of movement across cell membranes

  increasing the number of channel/carrier proteins increases the rate of facilitated diffusion/active transport
• Describe how differences in gradients of concentration affects the rate of movement across cell membranes

  Increasing concentration gradient increases rate of simple diffusion and osmosis

  Increases rate of facilitated diffusion until the number of channel/carrier proteins becomes a limiting factor as all in use
• Describe how differences in water potential gradient affect the rate of movement across cell membranes

  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 (e.g microvilli in ileum) - increase in surface area

  More protein channels/carriers - for facilitated diffusion

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