It explains about solute transport

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?