Transport across membranes

• Describe how movement across membranes occurs by simple diffusion


  -lipid-soluble or very small substances (steroid hormones)

  -move from one area of higher concentration to area of lower concentration, down con gradient

  -across phospholipid bilayer

  -passive so doesn’t require energy from atp/respiration (only kinetic energy of substance)
• limitations of phospholipid bilayer


  -restrict movement of water soluble and large substances

  -due to hydrophobic fatty acid tails in interior of bilayer
• describe movement across membranes by facilitated diffusion


  water-soluble/polar/charged (slightly larger) substances (glucose/amino acids)

  -move down conc grad

  -through specific channel/carrier proteins

  -passive so doesn’t require energy from ATP
• how do carrier proteins facilitate diffusion of flighty larger substances


  -complementary substances attaches to binding site

  -protein change shape to transport substance
• how do channel protein facilitate diffusion


  -hydrophilic pore filled with water

  -may be gated so can open/close
• describe how movement across membranes occurs by osmosis


  -water diffuses

  -from high to low water potential/down water potential grad

  -thrpugh partially permeable membrane

  -passive so doesn’t require energy from ATP
• what is water potential


  -measure of how likely water molecules are to move OUT of a solution

  -pure distilled water has maximum possible water potential

  -increasing solute conc decreases water potential
• Describe how movement across membranes occurs by active transport


  -substances move from low conc to higher conc

  -against conc grad

  -requiring hydrolysis of ATP and specific carrier proteins
• describe role of carrier proteins


  -complementary substance binds to specific carrier protein

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

  -pi released so protein return to original shape.
• importance of hydrolysis of atp in active transport


  -atp binds and hydrolysed into atp + pi

  -carrier protein changes shape and release substance on side of higher conc
• how does movement across membranes occurs by co-transport


  -2 diff substances bind to and move simultaneously via co transport protein (type of carrier)

  -movemement of one substance AGAINST conc grad is often coupled with movement of another DOWN conc grad
• explain adaptations of specialised cells in relation to rate of transport across internal/external membranes


  -cell membrane folded (increase SA)

  -more protein channels (for facilitated diffusion)

  -large number mitochondria (more atp for respiration for active transport)
• what increases rate of movement across membranes

  -increasing SA of membrane

  -increasing number of channel or carrier protein

  -increasing conc grad of simple diffusion

  -Increasing conc grad of facilitated diffusion (until number of channel/carrier proteins become limiting factor/saturated)

  -increasing water potential grad