3.4.2 Mass transport in plants

• 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 cell walls with lignin → provides support / withstand tension / prevents water loss
  • ● Pits in side walls → allow lateral water movements
• Explain the cohesion-tension theory of water transport in the xylem

  1. Leaf:
  2. Water lost from leaf by transpiration - water evaporates from mesophyll cells into air spaces and water vapour diffuses through (open) stomata
  3. Reducing water potential of mesophyll cells
  4. So water drawn out of xylem down a water potential gradient
  5. Xylem:
  6. Creating tension (‘negative pressure’ or ‘pull’) in xylem
  7. Hydrogen bonds result in cohesion between water molecules (stick together) so water is pulled up as a continuous column
  8. Water also adheres (sticks to) to walls of xylem
  9. Root:
  10. Water enters roots 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 (eg. 1 minute)
  3. Calculate volume of water uptake in a given time:
  4. ● Use radius of capillary tube to calculate cross-sectional area of water (πr^2)
  5. ● Multiply this by distance moved by bubble
  6. 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 above, change one variable at a time (wind, humidity, light or temperature)
  • ○ Eg. set up a fan OR spray water in a plastic bag and wrap around the plant OR change distance of a light source OR change temperature of room
  • ● 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 different environmental variables affect transpiration rate - light intensity


  Increasing 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
• Suggest how different environmental variables affect transpiration rate - temperature

  Increasing temperature, increases rate of transpiration

  • Water molecules gain kinetic energy as temperature increases
  • So water evaporates faster
• Suggest how different environmental variables affect transpiration rate - wind intensity

  Increasing 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
• Suggest how different environmental variables affect transpiration rate - humidity

  Increasing 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 eg. sucrose in plants
• Suggest how phloem tissue is adapted for its function

  1. Sieve tube elements
  2. No nucleus / few organelles → maximise space for / easier flow of organic substances
  3. End walls between cells perforated (sieve plate)
  4. Companion cells
  5. 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 (where made, eg. leaves) to sink cells (where used / stored, eg. roots) 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 carbon-14
  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 eg. 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 below ring die as cannot get organic substances
• Suggest some points to consider when interpreting evidence from tracer & ringing experiments and evaluating evidence for / against the mass flow hypothesis

  • ● Is there evidence to suggest the phloem (as opposed to the xylem) is involved?
  • ● Is there evidence to suggest respiration / active transport is involved?
  • ● Is there evidence to show movement is from source to sink? What are these in the experiment?
  • ● Is there evidence to suggest movement is from high to low hydrostatic pressure?
  • ● Could movement be due to another factor eg. gravity?