Lesson 4

Question 1

What is translocation?

Answer 1

Bi-directional movement of soluble photosynthates

Question 2

What is difference between sink and source?

Answer 2

Source is where the photosynthates are produced and exported to other parts of plants
Sink is where photosynthates are received, stored or used for growth

Question 3

What are ringing experiments?

Answer 3

• Removal of bark from tree
• Bulge of accumulated sap grows as they can’t move past
• Shows that transport can be downwards and phloem

Question 4

What are aphid experiments?

Answer 4

• Whilst aphid is feeding, body can be cut off, leaving stylet penetrated in stem
• This way, pure phloem can be collected for analysis

Question 5

How can sink and source of plant be found?

Answer 5

• Plant can be supplied with radioactive carbon isotope
• Isotope is then used in photosynthesis to make sucrose
• Place photographic film under plant and sources of radioactivity can be seen as black
• Leaf will be fully black as it is source of sucrose with isotope so all isotopes will have passed through there and other areas with less amount of black are sinks
• Shows that translocation is bi-directional

Question 6

What is mass flow hypothesis?

Answer 6

Main theory to explain translocation: Sugars flow passively from areas of high concentration in the leaf (source) to areas of low concentration (such as a growing tissue) down a pressure gradient

Question 7

How does the mass flow hypothesis work?

Answer 7

• Sucrose from photosynthesis loaded by AT into sieve tubes using ATP
• Water enters sieve tubes along water potential grad. by osmosis
• Pressure in sieve tubes increase and sucrose moves down pressure grad. through phloem towards sinks
• Sucrose unloaded by AT into cells at sinks
• Water moves by osmosis out of phloem as sucrose is removed and pressure in phloem tissue becomes lower at the sink

Question 8

Why may mass flow not be the explanation for translocation?

Answer 8

• Observations show that sucrose transported at a rate of 25-100cm per hr but diffusion alone should be 0.2mm per day
• Sieve plates act as barrier for flow
• Sucrose and amino acids observed moving at different rates and directions
• So an active process may be involved

Question 9

What are some alternative theories to translocation?

Answer 9

• Streaming in cytoplasm of sieve tubes could be responsible for bi-directional movements
• Protein filaments have been observed passing through sieve pores, suggesting different solutes transported by different filaments