MT 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

  Leaf

  1 Water lost from leaf by transpiration- water evaporates from mesophyll cells into air spaces + water vapour diffuses through stomata

  2 Reducing water potential of mesophyll cells

  3 So water drawn out of xylem down a water potential gradient

  xylem

  4 Creating tension in xylem

  5 Hydrogen bonds result in cohesion between water molecules so water is pulled up as a continuous column

  6 Water also adheres to walls of xylem

  Root

  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

  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
• Suggest how different environmental variables affect transpiration rate Temp

  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

  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

  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

  ● 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

  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 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

  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?