Transport in Plants

Xylem
1. Transports water and mineral ions up a plant's stem to leaves
2. Root - xylem found in centre as a x
  1. Provides support for root
3. Stem - xylem found toward the outside
  1. Reduces bending
4. Leaf - xylem makes up network of veins
  1. Supports leaves
5. Adaptations
  1. Xylem vessels are long tube like structures
    1. Formed from vessel elements joined end to end
    2. No end walls
      1. Allows water to pass up through middle of cell easily
  2. Dead cells, containing no cytoplasm
  3. Woody substance lignin thickens cell walls
    1. Stops walls collapsing inwards
    2. Lignin in spiral patterns
      1. Flexibility
      2. Prevents stem from breaking
      3. Amount of lignin increases as cell gets older
      4. Supports xylem walls
  4. Small pits in walls where there is no lignin
    1. Allows water and mineral ions to move in/out of vessels
Phloem
1. Transports dissolved substances - sugars (like sucrose) up and down a plant
2. Root - phloem found in centre outside of xylem
  1. Supports root
3. Stem - phloem on outside
  1. Reduces bending
4. Leaf - phloem on inside making up network of veins
  1. Supports leaves
5. Adaptations
  1. Formed of cells arranged in tubes
  2. Purely transport tissue
    1. Contains phloem fibres, phloem parenchyma, sieve tube elements and companion cells
    2. Not used for support
  3. Sieve tube elements
    1. Living cells forming tube for transporting sugars through plant
      1. Joined end to end to form sieve tubes
    2. Sieve parts are end walls
      1. With holes to allow sugars to pass through them
      2. Sieve plates
    3. No nucleus, thin layer of cytoplasm and few organelles
      1. Sieve tube elements cannot survive on their own
    4. Cytoplasm of adjacent cells is connected through holes in sieve plates
  4. Companion cells
    1. Companion cell for every sieve tube element
      1. Helps it survive
      2. Carries out living functions for both itself and sieve tube elements
      3. Provides energy to actively transport sugars
Multicellular organisms
1. Small surface are to volume ratio
2. Uptake water, minerals and sugars
3. Get rid of waste substances
4. Diffusion would be too slow without a transport system
Water transport
1. Entering plant
  1. 1) Water enters through root hair cells
  2. 2) Passes through root cortex and endodermis
  3. 3) Reaches xylem
  4. Water moves down a water potential gradient
  5. Soil around roots has high water potential
  6. Leaves have lower water potential
    1. Water constantly evaporates from them
  7. Water potential difference creates a gradient
    1. Keeps water moving through plant from roots to leaves
2. Through roots
  1. Water travels by osmosis through roots via root cortex, into xylem by pathways
  2. Symplast pathway
    1. Goes through living parts (cytoplasm) of cells
    2. Cytoplasm of neighbouring cells connect through plasmodesmata
      1. Small channels in cell walls
  3. Apoplast pathway
    1. Goes through non-living parts of cells (cell walls)
      1. Walls absorbant = easy diffusion
    2. When reaching endodermis cells in root, path blocked by casparian strip
      1. Waxy, waterproof strip in cell walls
      2. Now takes symplast pathway
      3. After this water moves into xylem
    3. Useful as water has to go through cell membrane first
      1. Control if substances in water get through
3. Through leaves
  1. Water leaves xylem and moves into cells by apoplast pathway
  2. Water evaporates from cell walls into spaces between cells in leaf
  3. Stomata open = water moves out of leaf as water vapour into air
    1. Down water potential gradient
    2. Transpiration
4. Up plant
  1. Water movement from roots to leaves = transpiration stream
  2. Cohesion
    1. Water molecules stick together
    2. Causes column of water in xylem to move upwards as one
  3. Tension
    1. As water evaporates from leaves, this creates suction at top of xylem
    2. Pulls more water up into leaf
  4. Movement of water causes more water to enter stem through root cortex cells
  5. Adhesion
    1. Water molecules attracted to walls of xylem vessels
    2. Helps water rise up through vessels
  6. Air bubbles in xylem can block column of water from reaching cells
Transpiration
1. Evaporation of water from a plant's surface
  1. Happens as a result of gas exchange
2. Plant need to open stomata to let in CO2 for photosynthesis, to produce glucose
  1. Opening stomata lets water out
3. Higher concentration of water inside leaf than in surrounding air
  1. Water moves out down water potential gradient
4. Factors affecting transpiration rate
  1. Light
    1. More light = faster transpiration rate
    2. Stomata open wider the lighter it gets
      1. Allowing more water to evaporate
    3. Dark = stomata closed
      1. Little transpiration
  2. Temperature
    1. Higher temp = faster transpiration rate
    2. Warmer water molecules have more energy = evaporate from cells inside leaf faster
      1. Increases water potential gradient between inside and outside of leaf
        Water evaporates out of leaf faster
  3. Humidity
    1. Lower humidity = faster transpiration rate
    2. Water potential gradient increases between leaf and air if air surrounding plant is dry
      1. Lower in air, higher in leaf
      2. Moves down water potential gradient
    3. Increase in water potential gradient = increases transpiration
  4. Wind
    1. Increase in wind = faster transpiration rate
    2. Air movement blows away water molecules from around stomata
      1. Water potential in air is now more lower than leaf
    3. Increases water potential gradient
      1. Increases rate of transpiration
5. Potometers
  1. Estimates transpiration rates
  2. Measures water uptake by a plant
  3. Use to estimate how different factors affect transpiration rates
  4. Process
    1. 1) Cut shoot underwater at a slant
      1. Prevents air entering xylem which can block water column
      2. Increases surface area for water uptake
    2. 2) Assemble potometer in water and insert shoot underwater
    3. 3) Remove potometer from water, but keeping end of capillary tube in beaker of water
    4. 4) Check potometer is watertight and airtight
    5. 5) Dry leaves
      1. Make sure water potential around leaves isn't higher than it should be
    6. 6) Allow time for shoot to acclimatise, then shut tap
    7. 7) Remove end of capillary tube from beaker until one air bubble has formed
      1. Then put end of tube back into water
    8. 8) Record starting position of air bubble
    9. 9) Start stopwatch and record distance the bubble moves per unit of time
    10. Keep conditions constant throughout experiment
      1. Factors etc
6. Xerophytic plants
  1. Examples
    1. Cacti
    2. Pine trees
    3. Prickly pears
  2. Stomata are sunk in pits
    1. Trapping water vapour
      1. Reduce transpiration by lowering water potential gradient
  3. Hair layer on epidermis
    1. Traps moist air round stomata
      1. Reduces water potential gradient between leaf and air
  4. Curled leaves
    1. Traps moist air
    2. Lowers exposed surface area for losing water
    3. Protects stomata from wind
  5. Less stomata
    1. Fewer places for water vapour to diffuse out of leaf
  6. Thick waxy layer on epidermis
    1. Layer is waterproof
      1. Water cannot pass through to evaporate
  7. Spines for leaves
    1. Reduces surface area for water loss
Translocation
1. Movement of assimilates in plant
  1. Sugars
2. Requires energy
3. Happens in phloem
4. Moves assimilates from source to sink
  1. Source = where assimilates are produced
    1. High concentration
  2. Sink = where assimilates are used up
    1. Low concentration
  3. Enzymes maintain concentration gradient from source-sink
    1. Changing dissolves substances at sink into something else/by breaking them down
      1. = lower concentration at sink than source
5. Examples
  1. Sucrose
    1. Source = leaves
    2. Sink = food storage organs and meristmems in roots/stems/leaves
  2. Potatoes
    1. Sucrose converted to starch in sink
      1. Lower concentration of sucrose at sink than in phloem
6. Mass flow
  1. Hypothesis
    1. 1) Source
      1. Active transport loads assimilates into sieve tubes of phloem
      2. Lowering water potential inside sieve tubes
        Water enters tubes by osmosis
        Creates high pressure inside sieve tubes at source end of phloem
    2. 2) Sink
      1. Assimilates removed from phloem to be used up
        Increases water potential inside sieve tubes
        Lowers pressure inside sieve tubes
    3. 3) Flow
      1. Result of pressure gradient from source end to sink end
      2. Gradient pushes assimilates along sieve tubes to where they are needed
  2. Evidence
    1. Support
      1. Removing ring of bark from woody stem = bulge forms above ring
        Includes phloem but not xylem
        Fluid in bulge has higher concentration of sugars than below the ring
        Sugars cannot move past the area where bark has been removed
        Evidence downward flow of sugars
      2. Investigating pressures in phloem using aphids
        Aphids pierce phloem, bodies removed, mouthparts left behind allowing sap to flow out
        Sap flows out quicker nearer leaves than further down stem
        Evidence for pressure gradient
      3. Metabolic inhibitors in phloem = translocation stops
        Inhibitors stop ATP production
        Evidence active transport is involved
    2. Against
      1. Sugar travels to many different sinks
        Not just to one with highest water potential
        As model suggests
      2. Sieve plates would create barrier to mass flow
        A lot of pressure would be needed for assimilates to get through at reasonable rate
    3. Experiment
      1. Showing mass flow hypothesis
      2. Two containers A and B
        Lined with selectively permeable membrane like cells
      3. Top tube connecting A and B represents phloem
      4. Bottom tube connecting A and B represents xylem
      5. A = source end
        Contains concentrated sugar solution
      6. B = sink end
        Contains weak sugar solution
      7. Water enters A by osmosis
        Increases pressure = sugar solution flows along top tube
      8. Pressure increases in B
        Forcing water out and back through bottom tube

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	Created by Gemma Bradford over 11 years ago