- The source is where the sugar is produced, such as in the leaves.
- The sink is where the sugar is used, such as in the roots or flowers.
- The pressure flow hypothesis states that sugar is transported through the phloem, which is a specialised tissue in plants.
- Phloem consists of sieve elements and companion cells. The companion cells are responsible for loading the sugar into the sieve elements.
- The sugar is transported through the sieve elements as mass flow. This means that the sugar is dissolved in water and is transported by the movement of water.
The pressure flow hypothesis is supported by a number of observations:
- The sugar concentration in the phloem is higher than the sugar concentration in the xylem. This suggests that sugar is being transported from the leaves (source) to the roots (sink).
- The rate of phloem transport is proportional to the rate of photosynthesis. This suggests that the sugar produced by photosynthesis is being transported through the phloem.
- If the phloem is damaged, the transport of sugar is stopped. This suggests that the phloem is essential for the transport of sugar.
