movement of sucrose

• By which mechanism does sucrose move along the phloem?

  Mass transport.
• Describe mass transport.

  A solution of sucrose, amino acids, and other assimilates flows along the tube. This solution is called sap, it can flow either up or down the plant as needed. The flow is caused by a difference in hydrostatic pressure between the 2 ends of the tube causing a pressure gradient. Water enters the tube at the source, increasing the pressure, and it leaves the tube at the sink which reduces the pressure.
• Why, in the early spring, can the roots be described as a source?

  The roots is where energy is stored as starch, this can be converted into sucrose and moved into other parts of the plant in order to enable growth in the spring.
• Where can the source of the plant be described in the late spring, summer and early autumn?

  The lead. Sugars are made during photosynthesis, these are converted into sucrose and loaded into the phloem sieve tubes. The sucrose is transported to other areas of the plant, that may be growing, like the meristems, or areas such as the roots for storage
• A sink is anywhere that removes sucrose from the phloem sieve tubes. Tre sucrose could be used for respiration and growth in a meristem or it could be converted to starch for storage in a root. Where sucrose is being used in the cells, it can be diffused out of the sieve tube through the plasmodesmata or active transport. The removal of sucrose from the sap makes the water potential higher, so water moves out of the sieve tube element into the surrounding cells- reducing the hydrostatic pressure in the sink.
• water entering the sieve tube at the source increases the hydrostatic pressure. Water leaving the sieve tube at the sink reduces the hydrostatic pressure. therefore a pressure gradient is set up along the tube and the sap flows across the gradient. this could be in either direction, depending on where sucrose is being produced and is needed. it is even possible that sap could be flowing in opposite directions in different tubes at the same time.