2.3 transport across cell membrane

• fluid-mosaic model of membrane structure

  molecules free to move laterally in phospholipid bilayer

  many components - phospholipids, proteins, glycoproteins and glycolipids
• arrangement of components of a cell membrane

  phospholipids form a bilayer - fatty acid tails face inwards, phosphate heads face outwards

  proteins

  intrinsic / integral proteins in 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 tails
• 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
• role of cholesterol (sometimes present) in cell membranes

  restricts movement of other molecules making up membrane

  so decreases fluidity (and permeability) / increases rigidity
• 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 oxygen, steroid hormones

  move from an area of higher concentration to an area of lower concentration down a concentration gradient

  across phospholipid bilayer

  passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)
• 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 membrane occurs by osmosis

  water diffuses / moves

  from an area of high to low water potential / down a water potential gradient

  through a partially permeable membrane

  passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)
• water potential

  a measure of how likely water molecules are to move out of a solution pure (distilled) water has the maximum possible water potential (0 kPA)

  increasing solute concentration decreases water potential
• 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

  complementary substance binds to specific carrier protein

  ATP binds, hydrolysed into ADP + Pi, releasing energy

  carrier protein changes shape, releasing substance on side of higher concentration

  Pi released → protein returns to original shape
• describe how movement across membrane occurs by co-transport

  two different substances bind to and move simultaneously via

  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
• an example that illustrates co-transport

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

  establishing a concentration gradient of Na+ (higher in lumen than epithelial cell)

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

  via a co-transporter protein

  glucose moves down a concentration gradient into blood via facilitated diffusion
• movement of sodium

  the movement of sodium can be considered indirect / secondary active transport

  as it is reliant on a concentration gradient established by active transport
• describe different factors affect rate of movement across cell membranes

  increasing surface area of membrane increases rate of movement

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

  increasing concentration gradient increases rate of simple/facilitated diffusion and osmosis

  increasing concentration gradient increases rate of 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