Mass Transport 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
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
Xylem
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
Root
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:
Record position or air bubble
Record distance moved in a certain amount of time (eg. 1 minute
Calculate volume of water uptake in a given time:
Use radius of capillary tube to calculate cross-sectional area of water (pi r squared )
• Multiply this by distance moved by bubble
4. Calculate rate of water uptake - divide volume by time taken
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 CO, 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
The wind blows away water molecules from around the 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)
2. 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 trom 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. CO, 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
- 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
