Translocation

Question 1

What is the mass flow theory?

Answer 1

The currently accepted explanation of translocation

Question 2

What are sources?

Answer 2

Where the organic solutes are produced and are therefore at [high]
The source for sucrose is usually the mesophyll cell

Question 3

What are sinks?

Answer 3

Where organic solutes are used up and are therefore at low concentration, usually the meristems

Question 4

Describe the structure of the phloem

Answer 4

Each sieve tube element links to the next via a sieve plate which is perforated with pores, cytoplasmic connections in companion cells make sieve tubes alive
Little cytoplasm, no nucleus, no vacuole, few organelles, small mitochondria
Companion cells have a nucleus, mitochondria and other organelles

Question 5

Outline the mass flow theory

Answer 5

1- Sucrose pumped from the source by active transport, requires ATP and special carrier proteins
2- [Sucrose] increases, which lowers water potential
3- Water enters phloem by osmosis, increasing hydrostatic pressure at source
4- Active transport of sucrose from phloem to sink, where it’s used for respiration or converted to to starch
5- Low [sucrose], low water potential
6- Water leaves sieve tubes by osmosis and enters sink cells, decreasing hydrostatic pressure
7- Mass flow of sucrose down hydrostatic gradient

Question 6

What are 3 differences between the xylem and phloem?

Answer 6

Xylem vessels have no cross walls unlike sieve tubes that contain perforated cross walls.
Xylem cells are dead, Phloem cells are living.
Xylem transports water and mineral ions, Phloem transports organic solutes such as sucrose/ amino acids

Question 7

What are three supporting points for the mass flow theory?

Answer 7

• Hydrostatic pressure in phloem shown by the release of sap
• [sucrose] is higher in source than roots
• Companion cells have many mitochondria

Question 8

What are three points against the mass flow theory?

Answer 8

• The function of sieve plates is unclear as they would hinder the mass flow of sucrose, they may have a function in preventing bursting under pressure
• Not all solutes move at the same speed, which they should if moved by mass flow
• Sucrose is delivered at more or less the same rate to all sinks, rather than going faster with the lowest [sucrose] as the theory suggests