Photosynthesis occurs primarily in the leaf (see Figure 10.1). The carbohydrate products of photosynthesis (mainly sucrose) diffuse to the nearest small vein (composed of xylem and phloem), where they are actively transported into sieve tube elements of the phloem. The movement of carbohydrates and other solutes through the phloem is called translocation.
745
focus your learning
Phloem sap is transported, or translocated, from sources to sinks in plants.
Translocation in plants is explained by the pressure flow model.
Two steps in translocation, loading and unloading, involve active transport, which requires energy.
The products of photosynthesis are called photosynthates, and the content of the phloem is called the phloem sap (see Table 34.1). Translocation of phloem sap throughout the plant is by bulk flow, from sources to sinks:
A source is an organ (such as a mature leaf or a storage root) that produces, by photosynthesis or by digestion of stored reserves, more sugars than it requires.
A sink is an organ (such as a root, flower, developing fruit, or immature leaf) that consumes sugars for its own growth and storage needs of the plant.
Sources and sinks can change roles. For example, the storage roots of cassava (see Chapter 33) are sinks when they accumulate carbohydrates but are sources when their stored carbohydrates are mobilized to nourish other organs when the plant grows.