Mass transport in plants

Describe the function of the Xylem

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 and nucleus → easier water flow / no obstructions

• Thick cell walls with lignin → provides support / withstand tension / prevents water loss

• Pits in the sides of the wall → allows lateral movements (sideways movement)

Explain the Cohesion-tension theory of water transport in the xylem

1. Water lost from leaf by transpiration - water evapourates from mesophyll cells into air spaces and water vapour diffuses through (open) stomata

2. Reducing water potential of Mesophyll Cells

3. So water draw out of xylem down a water potential gradient

4. Creating tension (‘negative pressure’ or ‘pull’) in xylem

5. Hydrogen bonds results in cohesion between water molecular (stick together) so water is pulled up as a continuous column

6. Water also adheres (sticks to) to walls of xylem

7. 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 (e.g. 1 min)

3. Calculate volume of water uptake in a given time:

   • use radius of capillary tube to calculate cross-sectional area of water (pi r²)

   • Multiply this by distance moved by bubble

4. 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

• Change 1 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 light intensity affects the rate of transpiration