mass transport in plants

    Cards (15)

    • Describe the function of xylem tissue
      Transports water (and mineral ions) through the stem, up the plant to leaves of plants
    • Suggest how xylem tissue is adapted for its function

      • cells joined with no end walls forming a long continuous tube - water flows as a continuous column
      • cells contain no cytoplasm / nucleus - easier water flow / no obstructions
      • thick cells wall with lignin - provides support / withstand tension / prevents water loss
      • pits in side walls - allows lateral water movements
    • Explain the cohesion-tension theory of water transport in the xylem
      1. Leaf
      • Water lost from leaf by transpiration - water evaporates from mesophyll cells into air spaces and water vapour diffuses through stomata
      • Reducing water potential of mesophyll cells
      • So water drawn out of xylem down a water potential gradient
      1. Xylem
      • Creating tension ('negative pressure' or pull) in xylem
      • Hydrogen bonds result in cohesion between water molecules so water is pulled up as a continuous column
      • Water also adheres to walls of xylem
      1. Root
      • Water enters root via osmosis
    • Describe how to set up a potometer
      1. Cut a shoot underwater at a slant - prevent air entering xylem
      2. Assemble potometer with capillary tube end submerged in a beaker of water
      3. Insert shoot underwater
      4. Ensure apparatus is watertight / airtight
      5. Dry leaves and allow time for shoot to acclimatise
      6. Shut tap to reservoir
      7. Form an air bubble - quickly remove end of capillary tube from water
    • Describe how a potometer can be used to measure the rate of transpiration
      Potometer estimates transpiration rate by measuring water uptake:
      1. record position of air bubble
      2. record distance moved in a certain amount of time
      3. calculate volume of water uptake in a given time:
      • use radius of capillary tube to calculate cross-sectional area of water
      • multiply this by distance moved by bubble
      1. calculate rate of water uptake - divide volume by time taken
    • Describe how a potometer can be used to investigate the effect of a named environmental variable on the rate of transpiration
      • carry out the practical, change one variable at a time (e.g. wind, humidity, light or temperature)
      • keep all other variables constant
    • Suggest limitations in using a potometer to measure rate of transpiration
      • rate of water uptake might not be same as rate of transpiration
      - water used for support / turgidity
      - water used in photosynthesis and produced during respiration
      • rate of movement through shoot in potometer may not be same as rate of movement through shoot of whole plant
      - shoot in potometer has no roots whereas a plant does
      - xylem cells very narrow
    • Suggest how light intensity and temperature affect transpiration rate
      Light intensity - increases rate of transpiration
      • stomata open in light to let in CO2 for photosynthesis
      • allowing more water to evaporate faster
      • stomata close when it's dark so there is a low transpiration rate
      Temperature - increases rate of transpiration
      • water molecules gain kinetic energy as temperature increases
      • so water evaporates faster
    • Suggest how different wind intensity and humidity affect transpiration rate
      Wind intensity - increases rate of transpiration
      • wind blows away water molecules from around stomata
      • decreasing water potential of air around stomata
      • increasing water potential gradient so water evaporates faster
      Humidity - decreases rate of transpiration
      • more water in air so it has a higher water potential
      • decreasing water potential gradient from leaf to air
      • water evaporates slower
    • Describe the function of phloem tissue
      transports organic substances e.g. sucrose in plants
    • Suggest how phloem tissue is adapted for its function

      1. sieve tube elements
      • no nucleus / few organelles - maximise space for / easier flow of organic substances
      • end walls between cells perforated (sieve plate)
      1. Companion cells
      • many mitochondria - high rate of respiration to make ATP for active transport of solutes
    • What is translocation?

      • movement of assimilates / solutes such as sucrose
      • from source cells to sink cells by mass flow
    • Explain the mass flow hypothesis for translocation in plants 

      1. At source, sucrose is actively transported into phloem sieve tubes / cells
      2. By companion cells
      3. This lowers water potential in sieve tubes so water enters from xylem by osmosis
      4. This increases hydrostatic pressure in sieve tubes (at source) / creates a hydrostatic pressure gradient
      5. So mass flow occurs - movement from source to sink
      6. At sink, sucrose is removed by active transport to be used by respiring cells or stored in storage organs
    • Describe the use of tracer experiments to investigate transport in plants

      1. Leaf supplied with a radioactive tracer eg. CO2 containing radioactive isotope 14C
      2. Radioactive carbon incorporated into organic substances during photosynthesis
      3. These move around plant by translocation
      4. Movement tracked using autoradiography or a Geiger counter
    • Describe the use of ringing experiments to investigate transport in plants

      1. remove / kill phloem e.g. remove a ring of bark
      2. Bulge forms on source side of ring
      3. Fluid from bulge has higher conc. of sugars than below - shows sugar is transported in phloem
      4. Tissues belong ring die as cannot get organic substances
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