Скачать презентацию Plant Physiology talk Six Solute transport Solute Скачать презентацию Plant Physiology talk Six Solute transport Solute

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Plant Physiology talk Six Solute transport Plant Physiology talk Six Solute transport

Solute transport • Plant cells separated from their environment by a thin plasma membrane 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 – 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 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

Membrane potential • Arise because charged solutes cross membranes at different rates • Create Membrane potential • Arise because charged solutes cross membranes at different rates • Create a driving force for ionic transport • Maintained by energydependent electrogenic pumps

Electrogenic pumps and membrane potential • Electrogenic pumps are ATPases (enzymes that split ATP) 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

Electrogenic pumps and membrane potential • To prove this • Add cyanide (CN) – 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 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 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 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 – 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 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 • Three types of membrane transporters enhance the movement of 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

Simple diffusion • Movement down the gradient in electrochemical potential • Movement between phospholipid 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 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 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.

Remember the aquaporin channel protein? • There is some diffusion of water directly across 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 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 • Binding to • Moderate speed Carriers – 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

Active transport • To carry out active transport: – The membrane transporter must couple 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 transport-Pumps • Movement against the electrochemical gradient • Unidirectional • Very slow Primary active transport-Pumps • 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 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 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

pump-mediated transport against the gradient (secondary active transport) • Two types: • (A) Symport: 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 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)

ABC transporters • Also known as the (ATP-binding cassette) superfamily. • ABC transporters all 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

ABC transporters • ABC transporters in Plant cells are specialized for pumping small compounds 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

Overview of Ion homeostasis in plant cells Overview of Ion homeostasis in plant cells

The Vacuole • Can be 80 – 90% of the plant cell • Contained 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.

The Vacuole In general, the functions of the vacuole include: • Isolating materials that 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 p. H • 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

Ion homeostasis in plant cells • Tonoplast antiporters move sugars, ions and contaminants to 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

Plasma membrane transporters Plasma membrane transporters

Ion transport in roots • As all plant cells are surrounded by a cell 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 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 transport in roots • Ion absorption in the root is more pronounced in 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.

Ion transport in roots • Once the Ion is in the symplast of the 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

Summary • The movement of molecules and Ions from one location to another is 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

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