3.4.2 mass transport in plants

Describe the function of the xylem

Transports water and mineral ions through the stem, up the plant to the leaves

How is the xylem tissue adapted for function

• cells joined with no end walls - water flows as a continuous column

• Cells contain no cytoplasm/ nucleus - easier 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 in the xylem (1. the 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

Explain the cohesion-tension theory in the xylem (2. Xylem)

• Water draw out of xylem creates tension in xylem

• Hydrogen bonds result in cohesion between water molecules (stick together), so water is pulled up as a continuous column

• Water also adheres to walls of xylem

Explain the cohesion-tension theory in the xylem (3. Root)

• causes water to enter roots by osmosis

How to set up a potometer

1. Cut a shoot underwater at a slant to prevent air from entering xylem

2. Assemble potometer with capillary tube submerged in a beaker of water

3. Insert shoot underwater

4. Ensure apparatus is water/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

How can a potometer be used to measure the rate of transpiration

Measuring water uptake

How can a potometer be used to measure the rate of transpiration (Step by step)

1. Record position of air bubble

2. Record distance moved in a certain amount of time

3. Calculate volume of water uptake (use radius of tube to calc pieR2, multiply by distance)

4. Calculate rate (divide by time taken)

Suggest limitations in using a potometer to measure rate of respiration

rate of water uptake may not be the same as rate of respiration.

• May be used for support/ turgidity.

• May be used in photosynthesis and produced during respiration

Rate of movement through shoot in potometer may not be the same as the rate through the shoot of the whole plant.

• Shoot in potometer has no roots and xylem is more narrow.

List 4 environmental factors that affect transpiration rate

• light intensity

• Temperature

• Wind intensity

• Humidity

How does the environmental variable (light intensity) affect transpiration rate

• increasing light intensity, increases rate of transpiration

• Stomata opens in light to let in CO2 for photosynthesis

• Allowing more water to evaporate faster

• Stomata close when its dark so there is a low transpiration rate

How does the environmental variable (temperature) affect transpiration rate

• increase in temperature, increases rate of transpiration

• Water molecules fain kinetic energy as temperature increases

• So water evaporates faster

How does the environmental variable (wind intensity) affect transpiration rate

• as wind intensity increases, it increases rate of respiration

• Wind blows away water molecules from around stomata

• Decreasing water potential of air around stomata

• Increasing water potential gradient so water evaporates faster

How does the environmental variable (humidity) affect transpiration rate

• increase in humidity, decreases rate of transpiration

• More water in air, so higher water potential

• Decreasing water potential gradient from leaf to air

• Water evaporates slower

What is the function of the phloem

transports organic substances (e.g sucrose in plants)

How is the phloem adapted for its function (Sieve tube elements)

• no nucleus/ few organelles - maximise space for easier flow of organic substances

• End walls between cells are perforated (sieve plate)

How is the phloem adapted for its function (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 to sink cells by mass flow

• Source cells = where made e.g leaves

• Sink cells = where used/ stored e.g roots

Explain the mass flow hypothesis for translocation

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) and 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

Explain mass flow in terms of hydrostatic pressure, water enter/ leave, low/ high water potential

• from high to low hydrostatic pressure

• At high hydrostatic pressure: water enters by osmosis, low water potential

• At low hydrostatic pressure: water leaves by osmosis, high water potential

Describe tracer experiments to investigate transport in plants

1. Lead supplied with a radioactive traced (e.g CO2 containing radioactive isotope)

2. Radioactive carbon incorporated into organic substances during photosynthesis

3. These move around the plant by translocation

4. Movement tracked using autoradiography or a Geiger counter

Describe ringing experiments to investigate transport in plants

1. Remove/ kill phloem by removing 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 the phloem)

4. Tissues below ring die as they cannot get organic substances