Translocation

Question 1

Define translocation

Answer 1

An active process in the which transports assimilates in the phloem from the source to the sink

Question 2

Give an example of a source in the plant

Answer 2

Leaves

Question 3

Give an example of a sink in the leaves

Answer 3

Roots
Merristem cells

Question 4

What is a source

Answer 4

Site where sucrose is loaded into the phloem

Question 5

What is a sink

Answer 5

The site where sucrose is unloaded from the phloem

Question 6

Describe the process of mass flow

Answer 6

1. Sucrose is actively loaded into the companion cell and diffuses into the sieve tube elements through the plasmodesmata by acitve transport
2. This reduces the water potential in the phloem and so water moves into the phloem from the xylem by osmosis
3. This increases the hydrostatic pressure at the top of the phloem creating a hydrostatic pressure gradient and so the water moves down the gradient, carrying the sucrose with it.
4. Now there is a concentration gradient of sucrose between the phloem and the companion cell so the sucrose diffuses into the companion cell through the plasmodesmata and is unloaded into the source
5. Now there is a high water potential in the phloem and the water moves back into the xylem by osmosis

Question 7

Describe the process of active loading

Answer 7

1. H⁺ ions are actively pumped from the companion cell to the leaf through a proton pump using active transport and ATP
2. Now there is a high concentration of H⁺ ions inside the leaf and so they need to move down the concentration gradient back into the companion cell
3. To do this, the H⁺ ions are cotransported, with sucrose molecules in the leaf, down the concentration gradient using a cotransporter protein back into the companion cell
4. The sucrose diffuses into the sieve tube elements via the plasmodesmata