3.4.2 - Translocation exam pack

Question 1

Describe how high pressure is produced in the leaves

Answer 1

MP1 - sucrose is actively transported from mesophyll cells to sieve tube element, via companion cells. This reduces water potential in sieve tube element

MP2 - so, by the process of osmosis, water moves from xylem into sieve tube element (phloem) down water potential gradient.

MP3 - this causes high hydrostatic pressure in phloem because volume of water in it has increased.

Question 2

During an experiment for translocation , the scientist ensured that the rate of photosynthesis is constant. Why is this important

Answer 2

To sure that the sucrose produced by all the plants remains constant

If the concentration of sucrose increases, rate of translocation will also increase.

Question 3

Describe the mass flow hypothesis for the mechanism of translocation in plants.

Answer 3

1. Translocation is a passive process
2. Sucrose is actively transported into the phloems sieve tube
3. This lowers the water potential in the sieve tube.
4. So water moves into the phloem from the xylem by osmosis down water potential gradient
5. This causes high hydrostatic pressure in the phloem
6. This causes sucrose to be transported from source to sink down hydrostatic pressure gradient and it is a bulk movement
7. Sucrose is used for either respiration or stored as starch