Carbohydrates produced by photosynthesis are translocated from source to sink through the phloem by a pressure flow mechanism.
learning outcomes
You should be able to:
Define and identify sources and sinks in plants as they relate to the products of photosynthesis.
Explain why energy and active transport are required to transport phloem sap and how the pressure flow model makes phloem sap flow possible.
Define and compare the processes of loading and unloading in phloem sap transport.
Explain the difference between a source and a sink.
A source is an organ that produces more carbohydrates that it requires. Sources can be photosynthetic (e.g., leaves) or storage organs with starch (e.g., seeds). A sink is an organ that does not produce enough carbohydrates for its own use and must import them from a source. Examples of sinks include roots and flowers.
How does loading of sucrose at the source result in bulk flow toward the sink?
Source cells load sucrose into sieve tubes of the phloem, decreasing their water potential. Water from adjacent xylem vessels enters the sieve tubes. This results in pressure inside the tubes, so the sap flows. At the sink, sucrose is removed, so the water potential inside the tubes increases. Water leaves to enter the xylem. The combination of these two events causes pressure flow.