3.4.2 Mass transport in plants

Question 1

What is the cohesion-tension theory of water transport in the xylem:

Answer 1

How water moves up the xylem against gravity via the transpiration stream

• Water evaporates from the leaves via the (open) stomata due to transpiration
• Reducing water potential in the cell and increasing water potential gradient
• Water drawn out of xylem, creating tension
• Cohesive forces between water molecules pull water up as a column by hydrogen bonds
• Water lost enters the roots via osmosis
• Water is moving up, against gravity
• Water is also adhesive so sticks to the walls of xylem

Question 2

What are the adaptations of the xylem:

Answer 2

• Elongated cells arranged end to end to form a continuous column
• Hollow due to lignification so no cytoplasm/nucleus to slow water flow
• End walls break down for flow
• Thick cell walls with lignin
• Rigid so less likely to collapse under low pressure
• Waterproof preventing water loss
• Pits allow lateral water movements
• Narrow lumen increases height water can rise due to cohesion tension/ capillary action

Question 3

What is translocation:

Answer 3

Movement of solutes/ assimilates from source to sink/ one place to another
E.g. sugars made from photosynthesis in the leaves are transported to the site of
respiration

Question 4

What happens at the source?

Answer 4

• High concentration of solute
• Active transport loads solutes from companion cells to sieve tubes of the phloem
• Lowering the water potential inside the sieve tubes
• Water enters sieve tubes by osmosis from xylem and companion cells
• Increasing pressure inside sieve tubes at the source end

Question 5

What happens at the sink:

Answer 5

• Low concentration of solute
• Solutes removed to be used up e.g. enzymes hydrolyse
• Increasing the water potential inside the sieve tubes
• Water leaves tubes via osmosis
• Lowering pressure inside sieve tubes

Question 6

What is the mass flow hypothesis:

Answer 6

High hydrostatic pressure at the source, low hydrostatic pressure at the sink therefore maintaining a hydrostatic pressure gradient
- Solutes move down
- Sugars used in respiration/convert into starch for storage

Question 7

What are the adaptations of the phloem:

Answer 7

• Sieve tube elements have no nucleus and few organelles
• Companion cell for each sieve tube element to carry out the living functions for the sieve cells
  i.e. ATP for active transport of solutes

Question 8

How do you use tracers as evidence for for and against the mass flow hypothesis?

Answer 8

• Supply plant with radioactive tracer such as 14C in CO2 to a photosynthesising leaf by pumping the radioactive CO2 into a container surrounding the leaf
• 14C is incorporated into the organic substances produced by the leaf e.g. sugars via photosynthesis
• Organic substances undergo translocation
• Autoradiography – plant killed and placed in a photographic film, film turns black where the radioactive substance is present
• Identifies where radioactive substance has moved to and thus where the organic substances have moved to via translocation from source to sink
• Can show this over time by taking autoradiographs at different times

Question 9

How do you use potometer to investigate the effect of a named environmental variable on the rate of transpiration?

Answer 9

Potometer estimates the transpiration rate by measuring water uptake

• Assemble potometer with capillary tube end submerged in a beaker of water
• Insert shoot underwater (prevent air entering xylem)
• Ensure apparatus is watertight and airtight
• Dry leaves and allow time for the shoot to acclimatise
• Shut off tap to reservoir
• Remove the end of the capillary tube from the water beaker until one air bubble has formed, then put the tube back into the water
• Record the position of the air bubble
• Use a stopwatch to record time e.g. one minute
• Record distance moved per unit time
• Rate of air movement = estimate of transpiration rate
• Change one variable at a time and keep all other variables constant (wind, humidity, light and temperature)

Question 10

How does light affect transpiration rate:

Answer 10

The higher the light intensity, the faster the transpiration rate (positive correlation)
- Because 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

Question 11

How does temperature affect transpiration rate?

Answer 11

• The higher the temperature, the faster the transpiration rate (positive correlation)
• Water molecules gain kinetic energy as temperature increases
• Move faster
• Water evaporates faster

Question 12

How does humidity affect transpiration rate?

Answer 12

• The lower the humidity, the faster the transpiration rate (negative correlation)
• Because as humidity increases, more water is in the air so it has a higher water potential
• Decreasing the water potential gradient from leaf to air
• Water evaporates slower

Question 12

How does humidity affect transpiration rate?

Answer 12

• The lower the humidity, the faster the transpiration rate (negative correlation)
• Because as humidity increases, more water is in the air so it has a higher water potential
• Decreasing the water potential gradient from leaf to air
• Water evaporates slower