4. Ion Transport

  1. plant cell structure
    • enclosed by a porous cell wall
    • plasma membrane at inner boundary of the cell wall, encloses the cytoplasm
    • within the cytoplasm there is a large vacuole surrounded by tonoplast membrane
    • membrane is a phospholipid bilayer - ions and solutes are insoluble and less able to pass through
  2. plasma membrane
    • membranes are poorly permeable to ions because ions are relatively insoluble in lipids
    • Kj = cjmembrane/cjaqueous phase in equilibrium
    • Jj = Pj (cjo - cji). Pj is called the permeability coefficient
  3. membrane permeability
    the extent to which a membrane permits or restricts the movement of the substance

    • decreases as molecules increase in :
    • 1. size
    • 2. polarity

    oxygen and carbon dioxide are both non-polar molecules and pass through membranes rapidly, water is polar but passes through rapidly because it is small

    ions are charged molecules, sugars are large - would move slowly across membranes, except membrane transport proteins facilitate their movement
  4. passsive transport
    • the spontaneous downhill movement of molecules down a concentration gradient
    • can also be driven by the electric potential difference across the membrane
  5. electrochemical potential
    • 1. concentration difference
    • 2. electric potential difference across plasma membrane
  6. active transport
    • the ability to drive ions into the cell against the electrochemical gradient
    • must expend metabolic energy in the form of ATP
    • transport across relatively impermeable lipid membranes requires special transport proteins
  7. transport proteins
    • two main kinds:
    • carriers - can also carry out passive transport
    • channels

    pumps- really carriers that are involved in active transport
  8. carriers
    transmembrane proteins that can bind specific ions and other solutes and transport them across membranes as a result of a conformational change in the protein

    transport is slow because conformational change in the protein is required, much slower than channel

    can transport a much wider range of substrates than channels, can be either passive or active (pumps)
  9. pumps
    • membrane proteins that carry out primary active transport
    • most transport ions, those belonging to ATP-binding cassette family can carry large organic molecules

    • electrogenic - ion transport involving the net movement of charge across membrane, often hydrogen ion is principal ion
    • electroneutral - ion transport where there is no net movement of charge
  10. ABC transporters (ATP-Binding Cassette)
    • active transport proteins - considered to be pumps, energized by ATP
    • transport large organic molecules
    • act independently of any proton gradient
  11. channels
    transmembrane proteins that function as selective pores in the membrane

    • transport specificity depends on:
    • size of the pore
    • density of electrical charges on the surface of the interior lining

    • can involve transient binding of the solutes to the channel proteins
    • always passive
    • diffuse through rapidly
    • proteins have structures called gates that open and close in response to external signals - ex voltage changes
  12. aquaporins
    • channels that transport water
    • water-selective proteins
    • expression and activity appear to be regulated, possibly by protein phosphorylation, in response to water availability

    • role: the silencing of plant aquaporins has been linked to poor plant growth and even death of the plant
    • seems to serve to facilitate the transcellular symplastic pathway for water transport
    • aquaporins have gates that can stop the flow of water through the pore of the protein - can be for when the plant contains low amounts of water due to drought
    • gating carried out by interaction which causes a 3D change in the protein so that it blocks the pore and stops the flow of water through the pore

    • aquaporin gating:
    • dephosphorylation of certain serine residues - drought
    • protonation of specific histidine residues - flooding
Card Set
4. Ion Transport
plant physiology and biochemistry exam 5