Translocation is the movement of organic substances (assimilates), such as sucrose and amino acids, through the phloem. It is an active process that requires energy.Assimilates move from a source to a sink.1. A source is where assimilates are made or released, for example photosynthesising leaves, or a storage organ such as a potato tuber when it breaks down its stored starch in spring.
2. A sink is where assimilates are used or stored, for example growing root and shoot tips, developing fruits and seeds, and storage organs while they are being filled.The same organ can be a source at one time and a sink at another, so phloem can carry assimilates up or down the plant.
Sugar is transported as sucrose rather than glucose. Sucrose is soluble, so it can be carried in solution, but it is less reactive than glucose because it is a non-reducing sugar. It is therefore not used up by metabolism on the way.Evidence that assimilates move in the phloem1. If a plant is given carbon dioxide containing radioactive carbon-14, radioactive sugars soon appear in the phloem. Their position is shown by placing the plant on X-ray film, a technique called autoradiography.
2. If a ring of bark (containing the phloem) is removed, the stem swells above the ring as sugars build up there.
3. Aphids feeding on phloem sap show that the sap contains sucrose.
2. A sink is where assimilates are used or stored, for example growing root and shoot tips, developing fruits and seeds, and storage organs while they are being filled.The same organ can be a source at one time and a sink at another, so phloem can carry assimilates up or down the plant.
Sugar is transported as sucrose rather than glucose. Sucrose is soluble, so it can be carried in solution, but it is less reactive than glucose because it is a non-reducing sugar. It is therefore not used up by metabolism on the way.Evidence that assimilates move in the phloem1. If a plant is given carbon dioxide containing radioactive carbon-14, radioactive sugars soon appear in the phloem. Their position is shown by placing the plant on X-ray film, a technique called autoradiography.2. If a ring of bark (containing the phloem) is removed, the stem swells above the ring as sugars build up there.
3. Aphids feeding on phloem sap show that the sap contains sucrose.