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Phloem Transport in Plants
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 (such as roots)
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 of
nutrients, storage areas such as the roots and stems can 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 Model
Sugar from a source is actively transported into phloem. After it enters the phloem, water
follows by passive osmosis. The buildup of water within the phloem creates the positive
pressure that pushes the phloem sap away from the source. Sugar is removed from the phloem at
the sink by active transport and water follows by passive osmosis and returns to the xylem. The
removal of the sugar and water at the sink produces a lower osmotic pressure at the sink than at
the source ensuring that phloem sap will continue to run from source to sink.