Plant Physiology

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Plant Physiology

• Solute transport

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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

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Two perspectives formembrane 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

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Moving into cells and between compartments
requires membrane to be crossed

• Composed of a phospholipid (LipidPhosphate
  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.

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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
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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
cellsElectrogenic - H/K -ATPase animal gastric
mucosaElectroneutral
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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

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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

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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

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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

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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

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Electrochemical potential versuswater 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

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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

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Simple diffusion

• Movement down the gradient in electrochemical
  potential
• Movement between phospholipid bilayer components
• Bidirectional if gradient changes
• Slow process

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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

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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
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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

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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

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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

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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

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Primary active transport- Pumps

• Movement against the electrochemical gradient
• Unidirectional
• Very slow
• Significant interaction with solute
• Direct energy expenditure

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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

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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

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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)

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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)

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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

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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

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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.

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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