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 2. 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: Leaf
- Water lost from leaf by transpiration - water evaporates from mesophyll cells into air spaces and water vapour diffuses through (open) stomata
- Reducing water potential of mesophyll cells
- So water drawn out of xylem down a water potential gradient.
Cohesion-Tension: Xylem (2)
- Creating tension (‘negative pressure’ or ‘pull’) in xylem
- Hydrogen bonds result in cohesion between water molecules (stick together) so water is pulled up as a continuous column
- Water also adheres (sticks to) to walls of xylem.
Cohesion-Tension Theory: Root (3)
- Water enters roots via osmosis.
Describe how to set up a potometer:
- Cut a shoot underwater at a slant →prevent air entering xylem
- Assemble potometer with capillary tube end submerged in a beaker of water
- Insert shoot underwater
- Ensure apparatus is airtight
- Dry leaves and allow time for shoot to acclimatise
- Shut tap to reservoir
- 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: Use radius of capillary tube to calculate cross-sectional area of water (πr2) 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:
● 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
Light Intensity:
Increases transpiration
1. Stomata open in light to let in CO2 for photosynthesis
2. Allowing more water to evaporate faster
3. Stomata close when it’s dark so there is a low transpiration rate
Temperature:
Increases rate of transpiration
1. Water molecules gain kinetic energy as temperature increases
2. So water evaporates faster
Wind Intensity:
Increases rate
of transpiration
1. Wind blows away water molecules from around stomata
2. Decreasing water potential of air around stomata
3. Increasing water potential gradient so water evaporates faster.
