5. Membrane Transport

  1. primary active transport
    metabolic energy needed to drive the ion into the cell if moving against the concentration gradient, energy often comes from the hydrolysis of ATP

    • H+-ATPase - pump uses the energy of ATP to drive H+ ions out of the cytosol into the cell wall
    • creates a proton motive force across the plasma membrane
    • electrochemical energy stored in the proton gradient may be used to drive the uptake of another ion into the cell
  2. proton motive force
    • gradient of electrochemical potentials of hydrogen ions across the plasma membrane
    • represents stored free energy in the form of the proton gradient

    PMF = different in concentration of H+ ions across the membrane + difference in electrical potential across the membrane
  3. secondary active transport
    • the process by which electrochemical energy generated in primary active transport is used to drive the transport of the ion into the cell
    • occurs by symporters and antiporters
  4. symporter
    the transport of H+ and S are in the same direction
  5. antiporter
    the transport of H+ and S is in opposite directions
  6. symporter mechanism
    • hypothesis:
    • 1: H+ binds resulting in a conformational change that permits a molecule of the substate, S, to be bound
    • 2: substrate, S, binds causing a second conformational change that exposes the substrates to the inside of the cell
    • 3: the proton and molecule of S are released to the cell's interior, which restores the original conformation of the carrier and allows a new pumping cycle to begin
  7. anitporter transport
    • antiporters - proteins that couple the energetically downhill movement of protons to drive the active uphill (energetically) transport of a solute in the opposite direction
    • energy is derived from the proton gradient
    • as the proton moves back into the cell, its energy is released for the active transport of the solute which is moved out of the cell
  8. secondary transport is active
    • primary - ATP
    • secondary - proton motive force

    plants and fungi - sugars and amino acids are taken up by symport with H+
  9. plasma membrane transport
    • two electrogenic pumps:
    • H+- ATPase
    • Ca2+- ATPase

    direction of pumping is outward
  10. tonoplast transport
    • two types of electrogenic pumps that pump protons in the lumen of the vacuole:
    • V-ATPase - vacuolar hydrogen ion ATPase
    • H+-PPase - hydrogen ion pyrophosphatase

    also ABC transporters
  11. use of kinetic analysis to elucidate transport mechanisms
    • see Michaelis-Menten kinetics when look at kinetics of ion uptake
    • transporting protein is assumed to have an active site like an enzyme which binds to the ion transporting across the membrane
    • carrier protein exhibits specificity
    • vmax = number of molecules of specific carrier protein
    • km = affinity for protein or ion
    • lower km means higher affinity
  12. biphasic transport of sucrose
    rate of sucrose uptake shows regular saturable uptake increase as concentration rises from 0 to 10 mM, active carrier-mediated uptake

    concentrations about 10 nM, linear and non-saturable, inhibition of ATP synthesis blocks the saturable component, diffusion
  13. H+-ATPase generates electochemical energy
    • extrude protons from the cytosol - creates a membrane potential and pH gradient (PMF)
    • supply energy for secondary active transport
  14. possible mechanism of operation for the plasma membrane H+ -ATPase
    • 1. proteins binds H+ and is phosphorylated by ATP
    • 2. phosphorylation causes a conformational change releasing H+ to outside, opening the transport pathway to the outside and closing it to the cytoplasmic side
    • 3. protein is dephosphorylated and phosphate diffuses away - causes protein to return to original conformation
  15. Plasma membrane H+-ATPase regulation
    variety of isoforms - transport regulated in different ways for each tissue

    • activity determined by:
    • concentration of substrate (ATP)
    • pH
    • temperature
Card Set
5. Membrane Transport
plant physiology and biochemistry exam 5