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Plants and Water (Movement of water (Osmosis (water molecules are always…

- - - - may require use of transport molecules and an input of energy
  - - - movement down the concentration gradient
  - - - movement down the concentration gradient
    - - they have free energy
      - water molecules can move through the selectively permeable membrane from both sides at the same time all the time
    - - proteins across the lipid bilayer
      - specific to water transport
        
        selectively permeable membrane
    - - water molecules will move in equal each way
        
        no net movement of water
      - when a solute is dissolved in water
        
        the solute attracts polar water molecules
        
        strengthens the hydrogen bonds and attraction between the remaining water molecules
        
        less able to move and diffuse
        
        water on one side of a selectively permeable membrane and sugar + water solution on the other
        
        net flow of water into the solution by osmosis
        
        down the concentration gradient
        
        'effective concentration'
  - - - i.e. water is pushed
      - increases the tendency of water molecules to move
      - free energy increases
        
        the effective concentration of the water is increased
        
        water and solute molecules move together in one direction
        
        down a pressure gradient
        
        mass flow
        
        e.g. when water moves through a pipe or tube
    - - i.e. suction
      - can pull water
        
        creates a pressure gradient
        
        e.g. a straw
  - - - surfaces which are good at attracting water will tend to stop the movement of water
        
        the presence of water attracting surfaces lowers the free energy
        
        decreases the effective concentration of water
    - - have a charged nature
        
        particularly attractive to water molecules
        
        can hold on to water
      - OM can attract water for the same reason
- - - - in solution
        
        including soil solutions and solutions in cells
        
        free energy is lowered
        
        Ψ is lower
  - - - movement from a higher Ψ to a lower Ψ
        
        down the Ψ gradient
  - - - in plant cells
        
        not soils or solutions
        
        pressure exerted by the cell wall on the living protoplast of the cell
        
        equal and opposite to the force of the protoplast pushing against the cell wall
        
        Flaccid cell in a wilting plant
        
        P = 0
        
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        in solution
        
        a pressure potential may be caused by physical forces
        
        suction of water up the plant
        
        due to transpiration
      - also called
        
        Turgor pressure
        
        Hydrostatic pressure
    - - pure water = 0
        
        as conc of solution increases
        
        π becomes more negative
        
        osmotic potential is always negative
        
        measure of how strongly a solution tends to pull
      - σ (sigma) = reflection coefficient
        
        multiply π by σ
        
        σ = 1
        
        perfect selectively permeable membrane
        
        a cell membrane is present
        
        π contributes to Ψ
        
        σ = 0
        
        no cell membrane present
        
        π does not contributes to Ψ
        
        π x 0 = 0
    - - measure of how tightly a soil/cell wall/seed holds water
        
        the tighter water is held, the more negative the matric potential
  - - - a cell full of water
      - a plant with sufficient water
      - helps the plant stay upright
    - - a cell with very little water
        
        protoplast has shrunk away from the cell wall
      - can be caused by drought conditions
      - plant wilts
    - - cell membrane is about to pull away from the cell wall as water moves out
    - - no matric potential and σ = 1
      - water potential is 0 in a turgid cell
        
        P and π are equal and opposite
        
        the amount of water moving in due to osmotic pressure is = and opposite to water moving out due to turgor pressure
        
        net movement of water in and out is 0
        
        if placed in a stronger external solution or a shortage of water
        
        water moves out of the cell
        
        solution inside the cell becomes more concentrated
        
        π in cell decreases
        
        Cell content does not push against cell wall as much
        
        P decreases
        
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- - - - oxygen out
      - carbon dioxide in
    - - produces a transpiration stream
        
        carries water + nutrients up the plant
      - the degree of stomatal opening controls the rate of transpiration
        
        if there is a good water supply
        
        partial or full closure of stomata will increase stomatal resistance
        
        reduce water movement out of the leaf
  - - - K⁺ mechanism
        
        1) K⁺ moves into stomatal guard cells
        
        moved by a proton pump
        
        Primary active transport
        
        H⁺₋ATPase
        
        2) an electrochemical gradient builds up
        
        allows passive diffusion of K⁺ into guard cells
        
        down a gradient of electrical charge from positive to negative
        
        through highly selective K⁺ channels in the plasmalemma
        
        secondary active transport
        
        3) Cl¯ enters the guard cells by co-transport with H⁺
        
        moves down a pH gradient
        
        Cl¯ balances the charge of K⁺
        
        4) increase concentration of K⁺ lowers the Ψ
        
        water moves into the guard cells by osmosis
    - - sucrose produced in photosynthesis in the guard cells lowers Ψ
        
        water moves into the guard cells by osmsosis
  - - - 2) K⁺ and Cl¯ move out of guard cells by diffusion down a concentration gradient
        
        decreases solute concentration and osmotic potential of guard cells
        
        more positive Ψ
        
        water moves out of guard cells by osmosis
  - - - low intercellular CO₂ conc promotes opening to take in more CO₂
      - high intercellular CO₂ conc promotes closing
      - sensed by guard cells
        
        mechanism is unknown
    - - 2 ways
        
        Photosynthetically Active Radiation (PAR)
        
        initiates photosynthesis
        
        lowers CO₂ levels
        important when light levels are high
        
        middle of the day
        
        very wide stomatal opening
        
        important for C4 plants
      - Blue light
        
        acts directly on guard cells
        
        independent of CO₂ conc
        
        blue light photosystem is sensitive but saturates at low light levels
        
        causes rapid opening at low light levels
        
        dawn
        
        shady conditions
        
        maximise light usage
        
        more important in C3 plants
    - - if water supply cannot keep up with transpiration, leaf cell turgour falls
        
        stomata close to conserve water
        
        overrides CO₂ conc and light level signals
    - - Abscisic acid
        
        water stress in roots causes higher levels of ABA in the stomatal guard cells
        
        ABA leads to a rapid inward movement of calcium ions, Ca+, into the guard cells
        
        followed by movement of K+ and Cl- ions out of the cell.
        
        increase the osmotic and water potential in the guard cell
        
        water moves out of the cell by osmosis
        
        reducing the turgidity of the guard cells and so closing the stomata
      - Non-drought conditions
        
        ABA moves up the plant in xylem sap at approximately pH 6.
        
        Leaf mesophyll cells have an approx pH of 7.4 ABA moves down a pH gradient into the more alkali leaf mesophyll cells and is held there or broken down.
        
        Little ABA gets to the stomatal guard cells so ABA levels low. Stomata stay open.
        
        pH in these conditions
        
        Nitrates are transported to the shoot in the xylem
        
        Nitrates, NO3+, decrease the pH in the xylem (nitric acid is HNO3).
        
        Nitrates are reduced by nitrate reductase, in the shoot, to form ammonium and be incorporated into amino acids
      - Drought conditions
        
        Xylem sap containing ABA increases in pH to around pH 7
        
        Leaf mesophyll cells have an approx pH of 7.4 Less difference in pH between the xylem sap and the leaf cells means that less ABA moves into the mesophyll cells and some stored ABA may be released.
        
        More ABA gets to the guard cells so levels of ABA increase. Increase in Ca2+ uptake leads to outward movement of K+, Cl- and water and stomata close.
        
        pH in these conditions
        
        Less nitrates transported to the shoot because dehydration of root cells reduces the activity of proton pumps involved in active transport of ions.
        
        Nitrate is reduced by nitrate reductase in the roots and assimilated into amino acids and organic acids.
        
        Nitrogen is transported in the xylem in forms other than nitrates (amino acids and organic acids) so the pH is increased from the ‘normal’.
- - - - plant cells are full of water
        
        support and growth
        
        chemical reactions take place in solution
    - - the rest evaporates from leaves
        
        Transpiration
        
        leaves can loose their weight in water every hour
        
        the evaporation of water is also used to cool the leaves
      - The movement of water up the plant is largely responsible for the movement of nutrients up the plant
  - - - Hydrogen sulfide H2S
      - Ammonia NH3
    - - so they stick together due to attraction between negative and positive ends
        
        hydrogen bonding
        
        this raises the melting and boiling point
        
        cohesion
      - water molecules are attracted to other polar molecules
        
        will wet solid surfaces such as cell walls
        
        adhesion
      - capillary movement up a thin tube
        
        adhesion to the wall material
        
        cohesion to other water molecules
      - makes water a good solvent for ions and polar organic molecules
- - - - Some plants can dry out to a crisp and recover a few hours after being wetted
      - no special mechanism to prevent water loss or increase uptake
        
        rely on specialised biochemistry
        
        enzymes
        
        cell membranes
        
        photosynthetic pigments
        
        exactly how this is done is unknown.
    - - 10% water content and can survive at this low water content for a long period
        
        Drying results in an increase in ABA levels
        
        brings about changes essential for reversible desiccation
        
        example in maize ABA leads to the activation of a gene for a specific dehydration protein; maize mutants lacking this protein have seeds which do not dry out but germinate on the cob
  - - - produce very long tap roots
      - Mycorrhizae
    - - Some plants can increase the osmotic pressure in their cells
        
        accumulation of inorganic ions in cell vacuoles or of metabollically inactive organic solutes such as proline or betaine in the cytoplasm
        
        reduce the water potential in their cells
    - - Xerophytes, plants adapted to living in arid conditions, may save water by
        
        closing stomata rapidly and completely before cell damage occurs
        
        having fewer stomata
        
        possession of thick and highly waterproof cuticle to reduce water loss
        
        reduce transpiration
        
        small, thick leaves
        
        release of volatile oils to reduce leaf temperature
        
        sunken stomata or rolled leaves to reduce the water potential difference between the inside and the outside the leaf by holding a layer of humid air just outside the stomata
- - - - ΔΨ = difference between 2 point in the soil
        
        ΔΨ = ΔP + σ Δπ + Δm
        
        no pressure or osmotic potential in the soil so ΔΨ=Δm
        
        matric potential is controlled by the solid-water interfaces of soil particles
        
        drier soil has a greater affinity for water
        
        more negative m
        
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        field capacity
        
        fully wetted soil after draining
        
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        plant takes up water from the soil
        
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        Clay particles
        
        charged
        
        small
        
        develop a greater matric potential than sandy soil
        
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    - - 2 possible routes
        
        symplast (simple)
        
        from cell interior to cell interior through the cytosplasm
        
        water moves from soil to root cell to root cell down a Ψ gradient
        
        water is moved into the xylem
        
        apoplast (around)
        
        along cell walls
        
        living cell walls are made up of a loose network of cellulose fibrils embedded in a jelly like matrix
        
        similar to soils
        
        high proportion of solid to liquid
        
        no selectively permeable membranes
        
        numerous irregular minute spaces
        
        ΔΨ=Δm
      - 3) Water movement into the xylem
        
        In all but the very tips of roots, the central stele of the root will be surrounded by a mostly water proof endodermis
        
        water must pass through the endodermis to reach the xylem
        
        from root hair
        
        to the cortex
        
        through the endodermis
        
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        apoplast route
        
        blocked by a layer of waterproof suberin
        
        water must cross a selectively permeable membrane into the symplast route
        
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        4) Movement along xylem vessels
        
        xylem cells
        
        ΔΨ = ΔP
        
        low ration of solid to liquid
        
        Δm is minor
        
        dead cells
        
        σ = 0
        
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        water movement is dependent on pressure in the xylem and cannot just be a result of root pressure
        
        Cohesion-tension theory
        
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        Cavitation / Embolism
        
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        5) movement from leaf to air
        
        water from leaf cells evaporate into air spaces in the leaf
        
        high humidity
        
        can then move out of the leaf by diffusion down a Ψ gradient
        
        intercellular spaces in the leaf
        
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        Atmosphere
        
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        6) resistance to water movement + water flow
        
        rate of water movement through a plant is limited by the capacity of the routes through the plant
        
        resistance to water movement from soil increased by
        
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        resistance in cell membranes
        
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        resistance of water movement out of the leaves
        
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