Sugar Transport
Sugars, which are formed by the plant during photosynthesis, are an
essential component of plant nutrition. Like water, sugar (usually in the form
of sucrose, though glucose is the original photosynthetic product) is carried
throughout the parts of the plant by the vascular system. Phloem, the
vascular tissue responsible for transporting organic nutrients around the
plant body, carries dissolved sugars from the leaves (their site of production)
or storage sites to other parts of the plant that require nutrients. Within the
phloem, sugars travel from areas of high osmotic concentration and high
water pressure, called sources, to regions of low osmotic concentration and
low water pressure, called sinks. (Osmotic concentration refers the
concentration of solutes, or sugars in this case; where the concentration of
solutes is highest, so is the osmotic concentration).
Sources
The nutrient-rich regions that supply sugars for the rest of the plant are
called the sources. Sources include the leaves, where sugar is generated
through photosynthesis. When they are high in supplies, the nutrient storage
areas, such as the roots and stems, can also function as sources. In the
sources, sugar is moved into the phloem by active transport, in which the
movement of substances across cell membranes requires energy expenditure on the
part of the cell.
Sinks
Sinks are areas in need of nutrients, such as growing tissues. When they are
low in supply, storage areas such as the roots and stems cane function as sinks.
The contents of the phloem tubes flow from the sources to these sinks, where the
sugar molecules are taken out of the phloem by active transport.
Pressure Flow
The mechanism by which sugars are transported through the phloem, from sources
to sinks, is called pressure flow. At the sources (usually the leaves),
sugar molecules are moved into the sieve elements (phloem cells) through
active transport. Water follows the sugar molecules into the sieve elements
through osmosis (since water passively diffuses into regions of higher solute
concentration). This water creates turgor pressure in the sieve elements,
which forces the sugars and fluids down the phloem tubes toward the sinks. At
the sinks, the sugars are actively removed from the phloem and water follows
osmotically, so that conditions of high water potential and low turgor pressure
are created, driving the pressure flow process.
