Translocation of Phloem Sap It's summer, and this plant's leaves are making sugars that are being stockpiled in its taproot. A tissue called phloem transports the organic products of photosynthesis throughout the plant in a solution called phloem sap. This process is called translocation. In the summer, plants produce sugars in their leaves during photosynthesis. The sugar is then stored in the plant's taproot, which is the sugar sink. Sugar produced by the leaves is pumped into the sieve-tube members of the phloem by active transport. This raises the concentration of sugar, causing water to move into the phloem by osmosis. Water pressure at the sugar source increases, pushing water and sugar away from the sugar source and toward the sugar sink - a process called bulk flow. At the sugar sink, root cells remove sugar by active transport and convert the sugar into starch. Again water follows by osmosis. This lowers the water pressure at the sink. Thus, the phloem transports water and its sugar cargo from the sugar source (in the leaves), where there is a buildup of pressure, to the sugar sink (in the root), where the pressure is lower. Now the weather is changing. In autumn, the plant's leaves die and its metabolic processes slow down. The plant is dormant during the winter. In early spring, new leaves begin to grow. The growing leaves require much more sugar than they can produce through photosynthesis. Thus, the leaves are now the sugar sink. It is the taproot that is the sugar source. Its stored sugars are transported to the leaves where they are used for growth. In the spring, the direction is reversed, but the mechanism responsible for the translocation of phloem sap is the same. Sugar in the taproot is actively transported into the phloem. Water flows into the phloem by osmosis, increasing the water pressure. In the leaves, sugar is actively transported out of the phloem and water follows. Pressure flow drives water and sugar away from the sugar source and toward the sugar sink.