Why Does Transport in Phloem require energy?

The sugar and other organic molecules are transported through the plant by means of a special layer of tissue called phloem. Phloem is composed of living cells that transport a water solution of sugars that we commonly call sap. This movement is modeled by thepressure-flow theory, a part of which is that the sugar-containing fluid is moved through sieve tubes by fluid pressure. By this means, nutrients can be moved from the photosynthetic site (the source) to the place where the sugar is being used (the sink) whether it is up or down the stem of the plant.