3.4.2 - Mass transport in plants

Question 1

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

Answer 1

• Sucrose actively transported (into phloem)
• Lowering/reducing water potential
  / More negative water potential
• Water moves (into phloem) by osmosis (from xylem)

Question 2

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

Answer 2

• High (rate of) transpiration/evaporation
• Water lost through stomata
  /(High) tension in xylem
• (Causes) less water movement from xylem to phloem
  /Insufficient water potential in phloem to draw water from xylem

Question 3

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

Answer 3

• Water evaporates/is transpired (from leaves/ stalk/celery/plant)
• Water potential gradient/lower water potential creates tension/pulls up water
  / Osmosis creates tension/pulls up water
  ‘accept negative pressure for tension’
• Hydrogen bonds/cohesion/adhesion maintains column

Question 4

Describe the cohesion-tension theory of water transport in the xylem. (5)

Answer 4

• Water lost from leaf because of transpiration / evaporation of water (molecules) / diffusion from mesophyll / leaf cells
  /Transpiration / evaporation / diffusion of water (molecules) through stomata / from leaves
• Lowers water potential of mesophyll / leaf cells
• Water pulled up xylem (creating tension)
• Water molecules cohere / ‘stick’ together by hydrogen bonds
• (forming continuous) water column
• Adhesion of water (molecules) to walls of xylem

Question 5

Describe the mass flow hypothesis for the mechanism of translocation in plants. (4)

Answer 5

• In source / leaf sugars actively transported into phloem
• By companion cells
• Lowers water potential of sieve cell / tube and water enters by osmosis
• Increase in pressure causes mass movement (towards sink / root)
• Sugars used / converted in root for respiration for storage