7. Transport in Plants

Question 1

7 differences between the vessels that transport phloem sap in plants and blood vessels in mammals

Answer 1

1. only sieve tubes (1 type) vs arteries, veins & capillaries (3 types)
2. sieve tubes are composed of cells vs blood vessels are composed of tissues
3. sieve tubes have sieve plates
4. sieve tubes have companion cells to fully function
5. veins have valves
6. not closed circulation for phloem vs closed circulation for blood vessels
7. unidirectional flow in blood vessels

Question 2

Describe the pathways by which water passes from the soil to the cells of the cortex. [4m]

Answer 2

• apoplastic pathway and symplastic pathway
• cell wall route; does not cross cell membranes
• entry into root hair cell by crossing partially permeable cell surface membrane
• then cytoplasmic route / within cytoplasm
• from cell to cell via plasmodesmata
• vacuolar route, crosses tonoplast

Question 3

Explain why transpiration occurs, even if it is a disadvantage to a plant. [3m]

Answer 3

• inevitable consequence of gas exchange
• occur even if stomata closed, through cuticle
• maintain transpiration pull to bring water for photosynthesis

Question 4

Explain, with reference to the structure of xylem vessel elements, why water does not take a symplastic pathway in the xylem to the leaves. [2m]

Answer 4

• xylem has no cytoplasm

- symplastic pathway through living cells

Question 5

Explain why the rate of movement of water in the xylem may slow down at night. [3m]

Answer 5

• stomata close
• only cuticular transpiration
• no photosynthesis
• rate of transpiration decreases
• evaporation from cell wall of spongy mesophyll cells decreases
• water potential gradient between root and leaf becomes less steep

Question 6

Describe the appearance of the phloem sieve tubes when viewed in longitudinal section. [2m]

Answer 6

• length greater than width
• sieve plates
• sieve pores
• peripheral cytoplasm
• thin wall
• no nucleus

Question 7

Transpiration

Answer 7

The loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by the diffusion of water vapour, down a water potential gradient, through the stomata

Question 8

3 structural differences between a guard cell and other lower epidermal cells

Answer 8

1. have chloroplasts
2. varying thickness of cell walls
3. no plasmodesmata

Question 9

Transpiration is a consequence of gas exchange in leaves.

Explain why the rate of transpiration is greater during the day than during the night. [3m]

Answer 9

• stomata only open in day and close at night
• opening of stomata to obtain enough CO2 for photosynthesis
• most water vapour lost via stomata
• only little water vapour lost via cuticle

Question 10

Suggest how the arrangement of cells in the leaf of a dicotyledonous plant contributes to the loss of water by transpiration. [3m]

Answer 10

• loosely packed spongy mesophyll cells, leading to presence of air spaces
• large SA for evaporation of water to air spaces
• cells arranged so that air spaces above stomata
• evaporation creates saturated air spaces
• thus creates water potential gradient between air in air spaces and air in external environment
• guard cells in epidermis form stomatal pore