Translocation
Biology: IB Diploma Higher Level, topic 7.2.9
Translocation
Translocation is the process where sugars produced in photosynthesis are transported from the leaves to other parts of a plant. The steps involved are:
1) Active transport of sucrose
- Sucrose is produced during photosynthesis in the leaves.
- The leaves are called the source.
- Sucrose is actively transported into the companion cells in the phloem.
- Energy is provided for active transport from mitochondria in the companion cells.
- This process involves H+ ions being actively transported out of companion cells.
- The H+ ions then diffuse back into the companion cells with sucrose. This is known as co-transport.
2) Diffusion of sucrose
- The loading of sucrose into the phloem causes the concentration of sucrose to increase.
- Sucrose diffuses from the companion cells into the sieve tube elements.
3) Osmosis
- The increase in the concentration of sucrose in the phloem causes water potential in the phloem to decrease.
- There is a water potential gradient between the outside of the phloem and the inside.
- Water diffuses into the phloem by osmosis.
4) Unloading of sucrose
- At the bottom of the phloem, sucrose concentration is low because it is being used up in the cells.
- The cells at the bottom of the phloem are called the sink.
- Sucrose diffuses out of the phloem and into the sink cells.
- This lowers the water potential of the sink cells.
5) Osmosis
- Water diffuses down its water potential gradient out of the phloem by osmosis.
- The diffusion of water into the phloem at the source and out of the phloem at the sink creates a hydrostatic pressure gradient.
- The hydrostatic pressure gradient allows mass transport of sucrose from source to sink.
- This is called mass flow.
Test yourself on Translocation
Which of the following statements about translocation are true?
During translocation, what type of cell is sucrose co-transported into?
Steps Involved in Translocation