1. Plant Physiology talk Six Solute transport

2. Solute transport
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 bilayer and proteins.
The phospholipid sets up the bilayer structure
Phospholipids have hydrophilic heads and fatty acid tails.
The plasma membrane is fluid--that is proteins move in a fluid lipid background

5. Membrane potential
Arise because charged solutes cross membranes at different rates
Create a driving force for ionic transport
Maintained by energy-dependent electrogenic pumps

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

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
Three types of membrane transporters enhance the movement of solutes across plant cell membranes
Channels – passive transport
Carriers – passive 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.

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
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 borh 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. ABC transporters Also known as the (ATP-binding cassette) superfamily.
ABC transporters all have a similar structure, consisting of two ATP binding domains facing the cytosol and two transmembrane domains
Similar to the situation seen with ATP-driven ion pumps, the binding and hydrolysis of ATP by ABC transporters is thought to drive conformational changes that transport molecules across the membrane.
Kretzschmar et al (2011). Biochemical Society Essays Biochem. 50, 145–160

27. ABC transporters
ABC transporters in Plant cells are specialized for pumping small compounds out of cells.
In general, ABC transporters seem to be crucial for getting foreign substances (drugs and other toxins) out of cells, making them extremely important clinically
ABC transporters shuttle substrates as diverse as lipids, phytohormones, carboxylates, heavy metals, and chlorophyll catabolites
ABC transporters participate in a multitude of physiological processes that allow the plant to adapt to changing environments and cope with biotic and abiotic stresses.
Kretzschmar et al (2011). Biochemical Society Essays Biochem. 50, 145–160

28. Overview of Ion homeostasis in plant cells

29. The Vacuole Can be 80 – 90% of the plant cell
Contained within a vacuolar membrane (Tonoplast)
Contains:
Water, inorganic ions, organic acids, sugars, enzymes, and secondary metabolites.
Required for plant cell enlargement
The turgor pressure generated by vacuoles provides the structural rigidity needed to keep herbaceous plants upright.

30. The Vacuole
In general, the functions of the vacuole include:
Isolating materials that might be harmful or a threat to the cell
Containing waste products
Containing water in plant cells
Maintaining internal hydrostatic pressure or turgor within the cell
Maintaining an acidic internal pH
Containing small molecules
Exporting unwanted substances from the cell
Allows plants to support structures such as leaves and flowers due to the pressure of the central vacuole
In seeds, stored proteins needed for germination are kept in 'protein bodies', which are modified vacuole

31. 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

32. Plasma membrane transporters

33. Plasma membrane transporters

34. 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

35. 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

36. Ion transport in roots
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.

37. 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

38. Summary
The movement of molecules and Ions from one location to another is known as transport.
Plants exchange solutes and water with their environment and among tissues and organs
Both local and long distance transport are controlled by cellular membranes

39. Any Questions?