Transport of Sugars

Translocation is the movement of carbohydrates through phloem from a source, such as leaves, to a sink, a site of carbohydrate utilization. Translocation is described by the pressureflow hypothesis, as follows:

1. Sugars enter sieve-tube members. Soluble carbohydrates, such as fructose and sucrose, move from a site of production, such as the palisade mesophyll, to sieve-tube members by active transport. This develops a concentration of solutes (dissolved substances, sugars in this case) in the sieve-tube members at the source that is higher than that at the sink (a root, for example).

2. Water enters sieve-tube members. As a result of the movement of solutes into the sievetube members, the concentration of water inside the cell becomes less than in the area outside the cell. As a result, water diffuses into these cells, moving down the water concentration gradient.

3. Pressure in sieve-tube members at the source moves water and sugars to sieve-tube members at the sink through sieve tubes. When water enters the sieve-tube members in the leaves (or other source), pressure builds up because the rigid cell wall does not expand. As a result, water and sugars move by bulk flow through sieve tubes (through sieve plates between sieve-tube members).

4. Pressure is reduced in sieve-tube members at the sink as sugars are removed for utilization by nearby cells. As water and sugars move by bulk flow from source to sink, pressure begins to build at the sink. However, a sink is an area where carbohydrates are being utilized. Thus, sugars are removed from the sieve-tube members (by active transport), which increases the concentration of water within the sieve-tube members. Water then diffuses out of the cell (moving down the water concentration gradient), relieving the pressure.

There is a physiologically important result when sugars are stored as starches. Starch is essentially insoluble in water. Thus, any cell can act as a sink if it removes soluble sugars from its cytoplasm by converting them to starch. Doing so would have the same effect as breaking down the sugars for energy. Similarly, any cell can act as a source if it breaks down starch into soluble glucose molecules. For example, when photosynthesis activity is low (during nights or cold winters), roots in plants can act as a sugar source when stored starches are broken down to sugars.