Phloem unloading is the removal of sucrose and other assimilates from the sieve tubes at a sink, such as a growing root tip, a developing fruit or a storage organ.1. Sucrose leaves the sieve tube elements and moves into the surrounding sink cells. In many sinks this happens by diffusion through plasmodesmata, down a concentration gradient. In some sinks, such as developing seeds, sucrose is moved out by active transport.
2. In the sink cells, sucrose is used or converted into other substances. The enzyme invertase hydrolyses sucrose into glucose and fructose, which are used in respiration or for growth. In storage organs such as potato tubers, sugars are converted into starch.
3. Converting the sucrose keeps its concentration in the sink cells low. This maintains a concentration gradient from the sieve tube into the sink cells, so unloading can continue. Starch is also insoluble, so it does not affect the water potential of the storage cells.
As sucrose leaves the sieve tube, the water potential inside the sieve tube rises. Water therefore moves out of the sieve tube by osmosis, into the sink cells or the xylem. This lowers the hydrostatic pressure at the sink end of the phloem.The difference between the high hydrostatic pressure at the source, where sucrose is loaded, and the low pressure at the sink is what keeps the phloem sap moving.
2. In the sink cells, sucrose is used or converted into other substances. The enzyme invertase hydrolyses sucrose into glucose and fructose, which are used in respiration or for growth. In storage organs such as potato tubers, sugars are converted into starch.
3. Converting the sucrose keeps its concentration in the sink cells low. This maintains a concentration gradient from the sieve tube into the sink cells, so unloading can continue. Starch is also insoluble, so it does not affect the water potential of the storage cells.
As sucrose leaves the sieve tube, the water potential inside the sieve tube rises. Water therefore moves out of the sieve tube by osmosis, into the sink cells or the xylem. This lowers the hydrostatic pressure at the sink end of the phloem.The difference between the high hydrostatic pressure at the source, where sucrose is loaded, and the low pressure at the sink is what keeps the phloem sap moving.