Question

How the product of photosynthesis is translocated?

Answer 1

The rate of translocation in angiosperms (flowering plants) is approximately 1 meter per hour. In conifers it is generally much slower, but even so this is far too fast to be accounted for by diffusion. Instead, the sap flows, like a river of dilute syrup water. What is the force that drives the flow of material in the phloem? It is pressure, generated in the sieve elements and companion cells in source tissues. In leaves, sugar is synthesized in mesophyll cells (the middle layer of the leaf), and is then actively pumped into the phloem, using metabolic energy. By using energy, the sugar is not only transferred to the phloem but is also concentrated. When a solute such as sugar is concentrated inside cells, water enters the cells by osmosis . Since the plant cells have a rigid cell wall, this influx of water creates a great deal of internal pressure, over ten times the pressure in an automobile tire. The pressure causes sap to move out through the pores of the sieve element, down the tube.

At the other end of the transport stream, in the sinks, sugar is constantly leaving the phloem and being used by surrounding cells. Some is consumed as an energy source, some is stored as sugar or starch, and some is used to make new cells if the sink tissue is growing. Since sugar leaves the phloem in the sink, water exits too (again by osmosis) and the pressure goes down. Therefore, there is a difference in pressure between source and sink phloem. This causes the solution to flow, just as water flows along a pressure gradient in a garden hose. This process is known as the pressure-flow mechanism.