CBSE Class 10 Answered

Translocation through the phloem is dependent on metabolic activity of the phloem cells. The water containing food molecules flows under pressure through the phloem. The pressure is created by the difference in water concentration of the solution in the phloem and the relatively pure water in the nearby xylem ducts. At their source i.e. the leaves, sugars are pumped by active transport into the companion cells and sieve elements of the phloem. As sugars (and other products of photosynthesis) accumulate in the phloem, water enters by osmosis. Turgor pressure builds up in the sieve elements. As the fluid is pushed down (and up) the phloem, sugars are removed by the cortex cells of both stem and root (the sinks) and consumed or converted into starch. Starch is insoluble and exerts no osmotic effect. Therefore, the osmotic pressure of the contents of the phloem decreases. Finally, relatively pure water is left in the phloem, and this leaves by osmosis and/or is drawn back into nearby xylem vessels by the suction of transpiration-pull. Thus it is the pressure gradient between source (leaves) and sink (shoot and roots) that drives the contents of the phloem up and down through the sieve elements.