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Transcript
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