Module 3 - Mass transport in plants

Question 1

5 marks

Why is the diameter of a trunk smallest at midday

Answer 1

1. Midday= more sunlight
2. Stomata open in light= more water loss
3. Water evaporates more when warm (fall in WP- water is drawn out of xylem)
4. Hydrogen bonds between water molecules= cohesive= creates a column of water within xylem
5. Adhesion (water molecules stick to wall of xylem) = helps to pull the water column upwards
6. Xylem pulled inwards by tension/ faster flow of water

Question 2

3 marks

Explain the importance of the xylem being kept open as a continuous tube

Answer 2

1. There are no barriers to the movement of water
2. Due to cohesion between the H bonds
3. As water molecules evaporate, tension is created (cohesion-tension theory)

Question 3

2 marks

Explain one way sieve tube cells are adapted for mass transport

Answer 3

• Cytoplasm only at edges of the cell/ fewer organelles
• Easy flow of the sugars

Question 4

1 mark

Explain why values for the pressure in the xylem are negative

Answer 4

(Inside xylem) lower than atmospheric pressure / (water is under) tension;

Question 5

2 marks

One way in which seive cells are adapted for mass transport of sugars

Answer 5

1. Cytoplasm only at edge of the cell
2. Easy flow of the sugars

Question 6

2 marks

One way in which companion cells are adapted for mass transport of sugars

Answer 6

1. Lots of mitochondria
2. Release energy for active transport of sucrose into the phloem

Question 7

5 marks

Describe mass flow hypothesis

Answer 7

1. Sucrose actively transported into phloem by companion cells
2. This lower water potential of phloem
3. Water enters phloem via osmosis from xylem
4. Increase in water volume = increase in hydrostatic pressure= more liquid forced to sink cell
5. Sucrose actively transported to sink cell–> used in respiration or stored as insoluble starch

Question 8

5 marks

Describe the transport of carbohydrate in plants.

Answer 8

1. Sucrose actively transported into phloem cell
2. By companion cells
3. Lowers water potential in phloem and water enters from xylem by osmosis
4. Produces higher hydrostatic pressure
5. Mass flow to respiring cells
6. Unloaded/removed from phloem by active transport

Question 9

2 marks

Advantage of sunken stomata on xerophytic plants

Answer 9

• Reduces transpiration/ evaporation
• Fall in water potential gradient

Question 10

2 marks

Use knowledge of structure of leaf to explain why less water is loss through the upper surface of leaves than is lost through the lower surface

Answer 10

1. More stomata on the lower surface
2. Waxy cuticle on upper surface

Question 11

4 marks

Explain how xylem tissue is adapted for its function

Answer 11

1. Tubes with no end walls- continuous water columns
2. No cytoplasm / no organelles - allows easier water flow
3. Lignin- Withstand tension
4. Pits in walls- allow lateral movement / get round blocked vessels

Question 12

3 marks

Describe and explain three ways in which the leaves of xerophytic plants may be adapted to reduce water loss.

Answer 12

1. Thick, waxy cuticle waterproof/impermeable so reduces evaporation/transpiration.
2. Sunken stomata or hairy saturated layer of still air outside
3. Small leaves reduced SA for water loss
4. Reduced number of stomata- reduced SA for water loss
5. Leaves roll up in dry weather- less SA for water loss and stomata covered

Question 13

3 marks

Sorghum has few stomata per mm2 of leaf surface area. Explain how this is an adaptation to the conditions in which sorghum grows.

Answer 13

1. Grows in arid / dry conditions
2. Less surface area
3. Transpiration / water loss would be reduced

Question 14

2 marks

A student found the number of stomata per cm2 on the lower surface of a daffodil leaf. He removed a small, thin piece of lower epidermis and mounted it on a microscope slide. He examined the slide using an optical microscope. Explain why it was important that the piece of the epidermis that the student removed was thin.

Answer 14

1. Single layer of cells
2. So that light that can pass through

Question 15

2 marks

The stomata on the leaves of pine trees are found in pits below the leaf surface. Explain how this helps to reduce water loss.

Answer 15

1. Water vapour accumulates / increased humidity / reduced air movement (around stomata)
2. Water potential gradient reduced

Question 16

1 mark

A student investigated the rate of transpiration from a leafy shoot. She used a potometer to measure the rate of water uptake by the shoot. The student cut the shoot and put it into the potometer under water. Explain why.

Answer 16

Prevent air entering / continuous water column

Question 17

The student wanted to calculate the rate of water uptake by the shoot in cm3 per minute.
What measurements did she need to make?

Answer 17

1. Distance and time
2. Radius / diameter of capillary tube

Question 18

2 marks

The student measured the rate of water uptake three times.
(i) Suggest how the reservoir allows repeat measurements to be made
(ii) Suggest why she made repeat measurements.

Answer 18

1. Returns bubble to the start
2. Increases reliability / anomalous result can be identified

Question 19

2 marks

A potometer measures the rate of water uptake rather than the rate of transpiration. Give two reasons why the potometer does not truly measure the rate of transpiration.

Answer 19

1. Water used for support / turgidity
2. Water used in photosynthesis
3. Water produced in respiration
4. Apparatus not sealed / ‘leaks

Question 20

4 marks

Describe the mass flow hypothesis for the mechanism of translocation in plants

Answer 20

1. In source sucrose actively transported into phloem
2. By companion cells
3. Lowers water potential of sieve cell and water enters by osmosis
4. Increase in pressure causes mass movement towards sink / root
5. Sugars used / converted in root for respiration for storage

Question 21

3 marks

Describe the relationship between humidity and transpiration rate

Answer 21

1. increased humidity leads to decreased transpiration
2. high humidity means more water in the air, increased water potential
3. reduced diffusion gradient / water potential gradient
4. less evaporation

Question 22

2 marks

Explain how each of the following is related to the function of xylem tissue.
(i) Xylem tissue contains hollow tubes.
(ii) Lignin is present in xylem cell walls.

Answer 22

i) Continuous column, easy water flow
ii) Maintains column of water/ provides stregnth

Question 23

6 marks

Describe the cohesion-tension theory of water transport in the xylem

Answer 23

1. Water lost from leaf because of transpiration / evaporation of water molecules + through stomata from leaves
2. Lowers water potential of mesophyll / leaf cells
3. Water pulled up xylem creating tension
4. Hydrogen bonds between water molecules= cohesive
5. forming continuous column of water
6. Adhesion of water molecules to walls of xylem- (water molecules stick to wall of xylem) = pulls the water column upwards

Question 24

1 mark

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.

Answer 24

Plant has roots
OR
xylem cells very narrow

Question 25

# 2 marks Describe 2 differences between the structure of the xylem and the phloem

Answer 25

1. Xylem has pits, phloem does not 2. Xylem has lignin, phloem does not 3. Phloem contain sieve plates, xylem does not 4. Phloem contains cytoplasm, xylem does not

Question 26

# 1 mark Name the bonds that hold the molecules of water together

Answer 26

Hydrogen

Question 27

# 1 mark Give the term that describes water molcules being attracted to other water molecules

Answer 27

Cohesion

Question 28

# 3 marks Explain the mechanism used by water to enter the root hair cells if theres a great conc of mineral ions in the root cells than in the soil

Answer 28

1. More mineral ions means lower water potential in root cells than soil 2. Water moves from soil to root down a water potential gradient 3. By osmosis

Question 29

# 1 mark Unit to compare distribution of stomata on leaves

Answer 29

Stomata per mm^2

Question 30

# 2 marks Other than the distribution of stomata, suggest and explain two xerophytic features the leaves of this plant might have.

Answer 30

1. Hairs so ‘trap’ water vapour and water potential gradient decreased 2. Stomata in pits/grooves so ‘trap’ water vapour and water potential gradient decreased 3. Thick waxy cuticle layer so increases diffusion distance 4. Waxy layer/cuticle so reduces evaporation/transpiration 5. Rolled/folded/curled leaves so ‘trap’ water vapour and water potential gradient decreased 6. Spines/needles so reduces surface area to volume ratio

Question 31

# 2 marks Give two reasons why it was important that the student counted the number of stomata in several parts of each piece of leaf tissue.

Answer 31

1. Distribution may not be uniform so it is a representative sample 2. To obtain a reliable mean

Question 32

# Purpose: to prove that mass flow happens. How are tracers used to monitor Mass Flow? | To track the movement of sucrose in a plant through the phloem

Answer 32

Radioactive label C14. Incorporated into sucrose. Movement can be traced using autoradiography xrays

Question 33

# To prove that phloem (not xylem) carries sugars like sucrose in a plant. How are Ringing experiments used to monitor Mass Flow?

Answer 33

Bark is removed [phloem removed]. Swelling occurs as sucrose has nowhere to flow. * Sugars move through the phloem, not the xylem. * This supports the mass flow theory: sugars move from source (leaves) to sink (roots).

Question 34

# 3 marks Explain how sucrose-transport proteins in leaf cells enable the production of a pressure gradient in the phloem.

Answer 34

1. Sucrose actively transported into phloem 2. Causes decrease in water potential inside phloem 3. Water enters by osmosis into phloem from xylem

Question 35

Explain how a dicotyledonous plant is adapted to gas exchange

Answer 35

Large SA: air spaces in spongy mesophyll, Short DP: through stomata to mesophyll cells

Question 36

# 4 marks Why does rate of transpiration increase during midday

Answer 36

1. Rate of transpiration increases due to increased temperature and light intensity 2. Increased temp increases kinetic energy causing more water loss, increases water potential gradient 3. Increased light intensity causes stomata to open, allowing gas exchange 4. Stomata close during night/ dark

Question 37

# 3 marks Explain why water enters root hair cells from the soil.

Answer 37

1. The soil has more water than the root cells. 1. The root cells have more minerals (+ less water), which means a lower water potential. 1. Water moves by osmosis, from high water potential (soil) to low water potential (root cells).

Question 38

# 5 marks How are radioactive tracers used to investigate mass flow in the phloem?

Answer 38

1. Radioactive carbon-14 (¹⁴C) is supplied to the plant as CO₂, which is used in photosynthesis. 1. The ¹⁴C becomes part of sucrose, which is transported in the phloem. 1. Autoradiography is used: plant is killed, frozen, and placed on X-ray film. 1. Dark areas on the film show where the radioactive sucrose has travelled. 1. Confirms movement of sugars and supports the mass flow hypothesis