Phloem cells carries sugars to all parts of the plant body.
In vascular plants, phloem is the living tissue that carries organic nutrients, particularly sucrose, a sugar, to all parts of the plant where needed. The phloem is mainly concerned with the transport of glucose and starch made during photosynthesis.
Active transport is the passage of a substance (salt or ion) from its lower to higher concentration (opposite to what happens in diffusion), using energy from the cell, through a living cell membrane. Active transport is in a direction opposite to that of diffusion. This process requires enzymes and a 'proton–pump' embedded in the plasma membrane. The pumps are energized by ATP molecules–a cellular energy storage molecule. This is possible due to phloem cells.
Leaves make food by the process of photosynthesis. The food made by leaves is in the form of simple sugar (glucose). Other types of substances are called plant hormones are released from the tips of roots and shoots. Now every part of the plant needs food and other substances. So these substances are to be transported to all the parts of the plants like roots, stems, growing regions etc. The movement of food from leaves to other parts of the plant is called translocation.
Phloem loading and unloading of sucrose
In plants, phloem translocates the food and other substances. Phloem consists of sieve tubes and companion cells. Sieve tubes are living cells which contain cytoplasm but do not have nucleus. So its function is supported by companion cell. Each sieve tube has a companion cell next to it. Food is prepared in the Mesophyll cells of a leaf. Food is translocated in the form of sucrose. The movement of water and dissolved minerals in xylem is always upward (from soil to leaves). The movement of food can be upward as well as downward depending upon the needs of the plants.
Plant cells use energy stored in the proton gradient and membrane potential to drive the transport of many different solutes. For example, the membrane potential generated by proton pumps contributes to the uptake of K+ by root cells. In the mechanism called cotransport, a transport protein couples the downhill passage of one solute (H+) to the uphill passage of another (NO3−). The "coattail" effect of cotransport is also responsible for the uptake of the sugar sucrose by plant cells. A membrane protein cotransports sucrose with the H+ that is moving down its gradient through the protein.