Plant Energy Transport

Plants are classified as autotrophs because they manufacture their needed nutrients by photosynthesis, converting carbon dioxide and water to sugar fuels with the addition of energy from the Sun. In times of rapid photosynthesis, the main product is glucose, but it is usually converted to the larger sugar sucrose. These sugars that are synthesized in the leaves must be transported to other parts of the plant. Other structures in the plants such as roots and flowers require the energy but cannot manufacture it. Also, sugars may be stored in the roots and stem.

The sugar and other organic molecules are transported through the plant by means of a special layer of tissue called phloem. Phloem is composed of living cells that transport a water solution of sugars that we commonly call sap. This movement is modeled by the pressure-flow theory, a part of which is that the sugar-containing fluid is moved through sieve tubes by fluid pressure. By this means, nutrients can be moved from the photosynthetic site (the source) to the place where the sugar is being used (the sink) whether it is up or down the stem of the plant.

Pressure-Flow Theory for Nutrient Transfer

After sugars are produced in photosynthesis, these sugars must be transported to other parts of the plant for use in the plant's metabolism. Part of the pressure-flow theory is that the sucrose produced is moved by active transport into the companion cells of the phloem in leaf veins. This raises the concentration of sucrose molecules in the companion cells above that in the sieve tubes, so they can then move into the sieve tubes by diffusion. With the concentration of sucrose now greater in the sieve tubes than external to them, water molecules will move into the sieve tubes near those photosynthesis locations by osmosis. With a larger amount of water in the tube, its fluid pressure will be higher than at distant locations in the tube, and the pressure difference will cause flow in those directions.

At some distance from the photosynthetic source, there may be a region, say in a fruit, where sugar is needed. This theory suggests that the sucrose is transported into the fruit by active transport, raising the sugar concentration in the fruit relative to the sieve tube. In response to this concentration difference, water will follow the sugar into the fruit by osmosis.

The movement of the water from the sieve tube into the fruit lowers the fluid pressure at that location, continuing to produce the pressure gradient that leads to bulk flow of water to the fruit, carrying the dissolved sugar with it.

Phloem, the Medium for Plant Energy Transfer

In the stems of plants is a layer of living tissue called phloem that forms a medium for the movement of a sugar-rich fluid (sap) and which is therefore a key part of the energy transport within vascular plants. Part of the phloem is made up of sieve tubes which are constructed of specialized sieve tube cells with no nuclei. The sieve tubes permit the flow of sugar solution under the influence of a fluid pressure differential according to the pressure-flow theory. Phloem also contains companion cells for the sieve tubes which aid in the transport of sugars to the tubes.