Transcript Solute transport - Lectures For UG-5

Plant Physiology
Solute transport
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
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
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.
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
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
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
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
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
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
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
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
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
Simple diffusion
• Movement down the gradient in
electrochemical potential
• Movement between phospholipid
bilayer components
• Bidirectional if gradient changes
• Slow process
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
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
K+ form the environment,
opening of stomata
• Only potassium can diffuse
either inward or outward
– All others must be expelled by
active transport.
Release of K+ into xylem
Closing of stomata
Remember the aquaporin
channel protein?
• There is some diffusion of
water directly across the bilipid 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
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
• Moderate speed
Carriers
– 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
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
Primary active transportPumps
• Movement against the
electrochemical gradient
• Unidirectional
• Very slow
• Significant interaction with
solute
• Direct energy expenditure
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
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
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)
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)
Overview of Ion homeostasis in
plant cells
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
Plasma membrane transporters
Plasma membrane transporters
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
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.
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