Mass Transport in Plants

Describe the function of xylem tissue

Transports water (and mineral ions) through stem, up the plants to leaves of plants

Suggest how xylem tissue is adapted for its function

• cells joined with no end walls forming 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 → allows lateral water movement

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 concentration gradient

Xylem:

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

Roots:

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 a 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

   • Multiply this by distance moved by bubble

4. Calculate the 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 one the rate of transpiration

• carry out the above, change one variable at a time (wind, humidity, light or temperature)

  • e.g. 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 the rate of transpiration

• rate of water uptake might not be the same as the 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 (Temperature)

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 higher water potential

• Decreasing water potential gradient from leaf to air

• Water evaporates slower

Describe the function of phloem tissue

Transports organic substances e.g. sucrose in plants

Suggest how phloem tissue is adapted for its function:

1. Sieve tube elements

   • no nucleus / fewer organelles → maximise space for / easier flow of organic substances

   • End walls between 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 e.g. leaves) to sink cells (where used / stored, e.g. 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 e.g. 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 e.g. remove a ring of bark

2. Bulge forms on source side of ring

3. Fluid from bulge has higher concentration of sugars than below - shows sugar is transported into phloem

4. Tissues below ring die as cannot get organic substances

Suggest some points to consider when interpreting evidence from tracer 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? E.g. gravity?