9.4- Translocation

Question 1

In what form are assimilates transported in plants?

Answer 1

• the leaves produce large amounts of glucose, which is needed by cells for respiration.
• the glucose is converted to sucrose for transport.
• once it reaches the cell it is converted back to glucose or starch for storage.

Question 2

What is translocation?

Answer 2

The process by which plants transport organic compounds in the phloem from sources to sinks (tissues that need them).

• active process that requires ATP.
• although glucose is made in photosynthesis, sucrose is the main assimilate transported.
• can be transported up and down.

Question 3

What are the main sources of assimilates in a plant?

Answer 3

• green leaves and stems.
• food stores in seeds when they germinate.
• storage organs like tap roots that are unloading their stores at the beginning of a growth period.

Question 4

What are the main sinks in plants?

Answer 4

• roots that are growing and/or actively absorbing mineral ions.
• actively dividing meristems.
• any parts that are laying down food stores such as developing seeds, fruits or storage organs.

Question 5

Why is sucrose the main assimilate transported in translocation?

Answer 5

It is not used in metabolism as readily as glucose, so it less likely to be used up in the transportation process.

Question 6

Explain the Phloem loading in translocation.

Answer 6

• sucrose travels down the apoplast route by diffusion down a concentration gradient.
• in companion cells= sucrose moved into cytoplasm in an active process.
• H+ ions actively pumped out of the companion cell using ATP and return to it down a conc gradient via co-transport protein.
• sucrose is co-transported which increases sucrose conc in companion cells and sieve tube elements.
  USE TEXTBOOK also learn unloading

Question 7

What is the evidence for translocation?

Answer 7

• advances in microscopy allow the adaptations of companion cells for active transport to be seen.
• in mitochondria of companion cells are poisoned, translocation stops.
• flow of sugars in phloem is alot faster than it would be with diffusion alone, suggesting an active process is driving the mass flow.
• aphids have shown there is a +ve pressure in phloem that forces the aap out through the stylet. Therefore pressure is lower closer to the sink and conc of sucrose higher near the source.