transport in phloem

what is an assimilate

These are molecules that are made as a result of photosynthesis eg sucrose

what is the transport of assimilates called

translocation

define what a source is

source are where the assimilates are produced where photosynthesis happens

define what a sink is

These are regions where assimilates are required

please provide examples of sinks and sources

sources= leaves and storage organs such as tubers

sinks= roots and shoots and storage organs

1. The glucose produced in the leaves is first converted to the disaccharide sucrose. This is because this is less likely to react with other molecules.

2. Sucrose is loaded into the phloem by an active process. To pump hydrogen ions out of the cytoplasm and into the spaces of the cell wall.

3. The hydrogen ions can now flow through a cotransporter protein down the concentration gradient back into the cell

4. This inward flow of hydrogen ions is coupled to an inward flow of sucrose into the companion cell

5. The companion cells have a large number of mitochondria which provides the ATP needed for the active transport of the hydrogen ions

6. folding on the cell membrane also increases the surface area for the protein involved

7. There is a high concentration of sucrose in the companion cells is high. The sucrose can now diffuse through the plasmodesmata from the companion cells into the sieve tube element cells

8. This lowers the water potential inside the sieve tube element causing water from the near by tissue eg xylem vessels to move into the phloem. causing the hydrostatic pressure to increase in the sieve tube element.

9. This increase in pressure causes the sap to move up or down

other method

. In the leaves sucrose is loaded into the phloem vessels by active transport by the companion cells. This decreases the water potential in the leaf phloem. 2. In the leaves ions are also actively transported out of the xylem vessels into leaf cells, which increases the water potential of the leaf xylem. 3. Water in the leaves therefore diffuses from xylem vessels to phloem vessels by osmosis, down its water potential gradient. 4. This osmosis increases the hydrostatic pressure in the phloem, so water and dissolved solutes are forced downwards to relieve the pressure. This is mass flow: the flow of water together with its dissolved solutes due to a force. 5. In the roots sucrose is unloaded from the phloem either by diffusion or by active transport into the cells of the root. 6. At the same time, ions are being pumped into the xylem from the soil by active transport, reducing the water potential in the xylem. 7. The xylem now has a lower water potential than the phloem, so in the roots water diffuses by osmosis from the phloem to the xylem. 8. Water and its dissolved ions are pulled up the xylem by tension from the leaves. This is also mass flow.