1. Plant Physiology Solute transport

2. Plant cells separated from their environment by a thin plasma membrane (and the cell wall) Must facilitate and continuously regulate the inward and outward traffic of selected molecules and ions as the cell –Takes up nutrients –Exports wastes –Regulates turgor pressure –Send chemical signals to other cells

3. Two perspectives for membrane transport Cellular level –Contribution to cellular functions –Contribution to ion homeostasis (i.e., balance) Whole-plant level –Contribution to water relations –Contribution to mineral nutrition –Contribution to growth and development

4. Moving into cells and between compartments requires membrane to be crossed Composed of a phospholipid (Lipid+Phosphate group) bilayer and proteins. The phospholipid sets up the bilayer structure Phospholipids have hydrophilic heads and fatty acid tails. Such organization makes plasma membrane selectively permeable to ions and molecules.

5. Membrane potential Membrane potential is the difference in electrical potential between the interior and the exterior of a biological cell Arise because charged solutes cross membranes at different rates Create a driving force for ionic transport KCl Solution example K + and Cl - ions diffuse at different rates across the membranes Membranes are more permeable to K + than to Cl - Initially diffuse at different rates unless they achieve equilibrium Potential as a result of diffusion

6. Electrogenic pumps and membrane potential Electrogenic pumps are ATPases (enzymes that split ATP) ATPases use ATP energy to “pump” out protons (H+) to create charge gradients H+ gradients create a type of “battery” to power transport and maintain ion homeostasis Electroneutral? -Na + /K + -ATPase animal cells=Electrogenic - H + /K + -ATPase animal gastric mucosa=Electroneutral

7. Electrogenic pumps and membrane potential To prove this Add cyanide (CN) –Rapidly poisons mitochondria, so cells ATP is depleted –Membrane potential falls to levels seen with diffusion So membrane potential has too parts –Diffusion –Electrogenic ion transport Requires energy

8. Ion homeostasis within plant cells Plant cells segregate ions based upon: –Function or role –Potential toxicity This segregation creates a balance Creating and maintaining the balance may require energy

9. Ion homeostasis within plant cells Ion concentrations in cytosol and vacuole are controlled by passive (dashed) and active (solid) transport processes In most plant cells vacuole takes up 90% of the cell volume –Contains bulk of cells solutes Control of cytosol ion concs is important for the regulations of enzyme activity Cell wall is not a permeability barrier –It is NOT a factor in solute transport

10. Passive vs active transport Passive or active transport depends on the gradient in electrochemical potential The electrochemical potential has 2 parts –Concentration –Charge (Electrical) The two parts together dictate the electrochemical potential for a compartment of a cell

11. Passive v. active transport Passive transport –Movement down the electrochemical gradient –From a more positive electrochemical potential – to a more negative electrochemical potential Active transport –Movement against electrochemical gradient –From a more negative electrochemical potential – to a more positive electrochemical potential

12. Electrochemical potential versus water potential Just like water potential, solutes alone must follow the rules of the electrochemical potential and move passively If this is not what the cell or plant tissue needs, two components are required somewhere to counteract this natural tendency –Energy –Membrane transport proteins

13. Summary of membrane transport Facilitate the passage of ions and other polar molecules Arabidopsis thaliana contains 849 membrane proteins (4.8% of genome) Three types of membrane transporters enhance the movement of solutes across plant cell membranes –Channels – passive transport –Carriers – passive/active transport –Pumps- active transport

14. Simple diffusion Movement down the gradient in electrochemical potential Movement between phospholipid bilayer components Bidirectional if gradient changes Slow process

15. Channels Transmembrane proteins that work as selective pores –Transport through these passive The size of the pore determines its transport specifity Movement down the gradient in electrochemical potential Unidirectional Very fast transport Limited to ions and water

16. Channels Sometimes channel transport involves transient binding of the solute to the channel protein Channel proteins have structures called gates. –Open and close pore in response to signals Light Hormone binding Only potassium can diffuse either inward or outward –All others must be expelled by active transport. K + form the environment, opening of stomata Release of K + into xylem Closing of stomata

17. Remember the aquaporin channel protein? There is some diffusion of water directly across the bi- lipid membrane. Aquaporins: Integral membrane proteins that form water selective channels – allows water to diffuse faster –Facilitates water movement in plants Alters the rate of water flow across the plant cell membrane – NOT direction

18. Carriers Do not have pores that extend completely across membrane Substance being transported is initially bound to a specific site on the carrier protein –Carriers are specialized to carry a specific organic compound Binding of a molecule causes the carrier protein to change shape –This exposes the molecule to the solution on the other side of the membrane Transport complete after dissociation of molecule and carrier protein

19. Carriers Moderate speed –Slower than in a channel –100-1000 ions or molecules/second Binding to carrier protein is like enzyme binding site action Can be either active or passive Passive action is sometimes called facilitated diffusion Unidirectional

20. Active transport To carry out active transport: –The membrane transporter must couple the uphill transport of a molecule with an energy releasing event This is called Primary active transport –Energy source can be The electron transport chain of mitochondria The electron transport chain of chloroplasts Absorption of light by the membrane transporter Such membrane transporters are called PUMPS

21. Primary active transport- Pumps Movement against the electrochemical gradient Unidirectional Very slow Significant interaction with solute Direct energy expenditure

22. pump-mediated transport against the gradient (secondary active transport) Involves the coupling of the uphill transport of a molecule with the downhill transport of another (A) the initial conformation allows a proton from outside to bind to pump protein (B) Proton binding alters the shape of the protein to allow the molecule [S] to bind

23. pump-mediated transport against the gradient (secondary active transport) (C) The binding of the molecule [S] again alters the shape of the pump protein. This exposes the both binding sites, and the proton and molecule [S] to the inside of the cell (D) This release restores both pump proteins to their original conformation and the cycle begins again

24. pump-mediated transport against the gradient (secondary active transport) Two types: (A) Symport : –Both substances move in the same direction across membrane (B) Antiport: –Coupled transport in which the downhill movement of a proton drives the active (uphill) movement of a molecule –In both cases this is against the concentration gradient of the molecule (active)

25. pump-mediated transport against the gradient (secondary active transport) The proton gradient required for secondary active transport is provided by the activity of the electrogenic pumps Membrane potential contributes to secondary active transport Passive transport with respect to H+ (proton)

26. Ion homeostasis in plant cells Tonoplast antiporters move sugars, ions and contaminants to the cytoplasm from the vacuole Anion channels maintain charge balance between the cytoplasm and vacuole Ca channels work to control second messenger levels & cell signaling paths between vacuole and cytoplasm

27. Ion transport in roots As all plant cells are surrounded by a cell wall, Ions can be carried through the cell wall space with out entering an actual cell –The apoplast Just as the cell walls form a continuous space, so do the cytoplasms of neighboring cells –The symplast

28. Ion transport in roots All plant cells are connected by plasmodesmata. In tissues where large amounts of intercellular transport occurs neighboring cells have large numbers of these. –As in cells of the root tip Ion absorption in the root is more pronounced in the root hair zone than other parts of the root. An Ion can either enter the root apoplast or symplast but is finally forced into the symplast by the casparian strip.

29. Ion transport in roots Once the Ion is in the symplast of the root it must exit the symplast and enter the xylem –Called Xylem Loading. Ions are taken up into the root by an active transport process Ions are transported into the xylem by passive diffusion