Unit 3- MT in Plants =)

Question 1

Explain why, in summer, the diameter of a branch is smaller at noon than at midnight. (4)

Answer 1

1. ↑ temp at noon –> ↑ transpiration rate
2. ↑ evaporation of water from surface of mesophyll cells
3. ↓ Ψ in cells
4. water moves from xylem –> cells in leaves
5. creating tension–> pulls water up xylem
6. pulls xylem vessels in –> ↓ diameer of trunk
7. x photosynthesis at night- stomata may close for periods –> ↓ transporation + ↓ tension

Question 2

The mass flow hypothesis is used to explain the movement of substances through phloem.
Use your understanding of the mass flow hypothesis to explain how pressure is generated inside this phloem tube. (3)

Answer 2

• sucrose a.t. into phloem
• ↓ Ψ
• water moves from xylem into phloem by osmosis

Question 3

Phloem pressure is reduced during the hottest part of the day.
Use your understanding of transpiration and mass flow to explain why. (3)

Answer 3

• ↑ rate of transpiration
• water lost through stomata
• ↓ water moves from xylem to phloem (∵ insufficient Ψ in phloem to draw water from xylem)

Question 4

A different student used coloured water to investigate the movement of water in leaf stalks of celery.
Explain why coloured water moved up the stalk. (3)

Answer 4

• water evaporates from leaves
• ↓ Ψ creates tension
• H bonds maintain column

Question 5

The student measured the distance the coloured water had travelled in eight celery stalks.
Suggest the most appropriate measure for this set of results and why. (1)

Answer 5

Median
- outliers present
- small sample size (only 8)

Question 6

Ulva lactuca is an alga that lives on rocks on the seashore. It is regularly covered by seawater.
Unlike plants, Ulva lactuca does not have xylem tissue.
Suggest how Ulva lactuca is able to survive without xylem tissue. (1)

Answer 6

• short diffusion pathway
• / surface permeable to water

Question 7

The rate of water movement through a shoot in a potometer may not be the same as the rate of water movement through the shoot of a whole plant.
Suggest one reason why. (1)

Answer 7

• x roots (∴ x resistance)
• xylem cells v. narrow
• xylem may be damaged when cut

Question 8

Describe and explain the relationship between humidity and transpiration rate. (3)

Answer 8

• ↑ humidity ↓ transpiration
• ↑ water in the air (–> ↑ Ψ)
• ↓ Ψ gradient
• ↓ evaporation

Question 9

A stem was cut from a transpiring plant. The cut end of the stem was put into a solution of picric acid, which kills plant cells. The transpiration stream continued. Suggest an explanation for this observation. (2)

Answer 9

• transpiration in xylem- dead cells
• process is passive

Question 10

Explain why the values for the pressure in the xylem are negative. (1)

Answer 10

lower than atmospheric pressure

Question 11

How would the total area of the stomatal openings differ if the investigation was repeated on a dull day? (1)

Answer 11

lower

Question 12

Explain the increase in the flow rate between 1000 and 1400 hours. (2)

Answer 12

• ↑ temp ↑ rate of transporation
• ↑ ke of water molecules //
• ↑ light ↑ rate
• ↑ stomata open –> ↑ flow rate ∵ cohesion between water molecules

Question 13

The diameter of the trunk decreased during the same period, reaching its minimum when the flow rate was highest.
Use your knowledge of the cohesion-tension theory to suggest an explanation for this decrease. (2)

Answer 13

• adhesion of H2O molecules to xylem wall
• creates tension as water pulled up stem
• pulls xylem in

Question 14

Use your knowledge of gas exchange in leaves to explain why plants grown in soil with very little water grow only slowly. (2)

Answer 14

• stomata close
• ↓ CO2 absorbed for ↓ photos