mass transport in plants Flashcards
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
Leaf
1. Water lost from leaf by transpiration - water evaporates from mesophyll cells into air spaces and water vapour diffuses through (open) stomata
2. Reducing water potential of mesophyll cells
3. So water drawn out of xylem down a water potential gradient
Xylem
4. Creating tension (‘negative pressure’ or ‘pull’) in xylem
5. Hydrogen bonds result in cohesion between water molecules (stick
together) so water is pulled up as a continuous column
6. Water also adheres (sticks to) to walls of xylem
Root
7. 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 watertight / 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
Suggest how different environmental variables 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
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 eg. sucrose in plants
Suggest how phloem tissue is adapted for its function
- Sieve tube elements
● No nucleus / few organelles → maximise space for / easier flow of organic substances
● End walls between cells perforated (sieve plate) - 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 (where made, eg. leaves) to sink cells (where used / stored, eg. roots) by mass flow
Explain the mass flow hypothesis for translocation in plants
- At source, sucrose is actively transported into phloem sieve tubes / cells
- By companion cells
- This lowers water potential in sieve tubes so water enters (from xylem) by osmosis
- This increases hydrostatic pressure in sieve tubes (at source) / creates a hydrostatic pressure gradient
- So mass flow occurs - movement from source to sink
- 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
- Leaf supplied with a radioactive tracer eg. CO2 containing radioactive isotope 14C
- Radioactive carbon incorporated into organic substances during photosynthesis
- These move around plant by translocation
- Movement tracked using autoradiography or a Geiger counter
Describe the use of ringing experiments to investigate transport in plants
- Remove / kill phloem eg. remove a ring of bark
- Bulge forms on source side of ring
- Fluid from bulge has higher conc. of sugars than below - shows sugar is transported in phloem
- 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?
