Mass transport in plants

Describe the function of the Xylem

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 and nucleus → easier water flow / no obstructions

• Thick cell walls with lignin → provides support / withstand tension / prevents water loss

• Pits in the sides of the wall → allows lateral movements (sideways movement)

Explain the Cohesion-tension theory of water transport in the xylem

1. Water lost from leaf by transpiration - water evapourates from mesophyll cells into air spaces and water vapour diffuses through (open) stomata

2. Lowers water potential of Mesophyll Cells

3. So water draw out of xylem down a water potential gradient

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

5. Hydrogen bonds results in cohesion between water molecular (stick together) so water is pulled up as a continuous column

6. Water also adheres (sticks to) to walls of xylem

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 (e.g. 1 min)

3. Calculate volume of water uptake in a given time:

   • use radius of capillary tube to calculate cross-sectional area of water (pi r²)

   • Multiply this by distance moved by bubble

4. Rate of water uptake = volume / time taken

Describe how a potometer can be used to investigate the effect of a named environmental variable on the rate of transpiration

• Change 1 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 Light Intensity affects the rate of transpiration

As light intensity increases, rate of transpiration increases,

• Stomata open in light to let in CO₂ for photosynthesis

• Allowing more water to evapourate faster

• Stomata close when it’s dark so there’s a low transpiration rate

Suggest how Temperature affects the rate of transpiration

As temp increases, rate of transpiration increases,

• Water molecules gain kinetic energy as temperature increases

• So water evapourates faster

Suggest how Wind intensity affects the rate of transpiration

As wind intensity increases, rate of transpiration increases,

• Wind blows away water molecules from around stomata

• Decreasing water potential of air around stomata

• Increasing water potential gradient so water evapourates faster

Suggest how Humidity affects the rate of transpiration

As humidity increases, rate of transpiration decreases,

• 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 e.g. 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 of 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

Transfer of Sucrose into Sieve Elements from Photosynthesising Tissue

• Sucrose is made from photosynthesis in chloroplast-containing cells

• Sucrose diffuses from photosynthesising cells into companion cells through facilitated diffusion - down a conc. gradient

• H⁺ are actively transported from companion cells into the cell wall spaces using ATP

• H⁺ diffuse down their conc. gradient into sieve tube elements through carrier proteins

• Sucrose molecules are co-transported with H⁺ into sieve tube elements via co-transport proteins

Mass Flow of Sucrose Through Sieve Tube Elements

• Sucrose is actively transported into sieve tubes from photosynthesising cells - this lowers the water potential

• Water moves from xylem into sieve tubes by osmosis - this increases hydrostatic pressure in the sieve tubes at the source

• Sucrose is removed from sieve tubes by respiring cells (used in respiration or stored as starch)

• These sink cells have low sucrose levels, so sucrose is actively transported into them - this lowers the water potential

• Water moves from sieve tubes into sink cells by osmosis - this lowers the hydrostatic pressure at the sink

• A pressure gradient is created (high at source, low at sink)

• Sucrose solution flows from source to sink by mass flow down this hydrostatic pressure gradient

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 - sugar is transported in phloem

4. Tissues below ring die as can’t get organic substances