translocation and mass flow hypothesis

Question 1

describe the function of phloem tissue.

Answer 1

transports solutes (mainly sugars) around the plant (movement in both directions)

Question 2

suggest how phloem tissue is adapted for its function.

Answer 2

sieve tube elements form a tube structure through the plant - a sieve plate is between them allowing for movement from one to the next.
no nucleus/few organelles to maximise space for flow of substances (easier flow)
companion cells are found alongside sieve elements and carry out living functions for the cells and help with loading and unloading.

Question 3

what is translocation?

Answer 3

movement of solutes from source cells to sink cells by mass flow.

Question 4

explain the mass flow hypothesis for translocation in plants.

Answer 4

• at source cell, companion cells actively transport sucrose into phloem sieve tubes.
• this lowers the water potential in phloem and causes water to enter from xylem into phloem via osmosis.
• pressure increases at source end of phloem.
• sucrose moves by facilitated diffusion from sieve tube element, through companion cell to sink cell.
• this increases water potential at sink end, water moves from high to low water potential (phloem to xylem) which reduces pressure at sink end.
• at sink sucrose is removed via active transport to be used in respiration.

Question 5

evidence for and against mass flow.

Answer 5

• is the phloem (as opposed to xylem) involved?
• is respiration/active transport involved?
• is there movement from source to sink?
• could movement be due to another factor?
• is movement from high to low pressure?