Sieve tube members contain 2-3 times more sucrose than mesophyll cells. At the sink end, the phloem unloads sucrose. These occur spontaneously down a chemical gradient. At equilibrium there is no net movement. Movement of a substance involves binding to a protein, a conformational change in the protein, and then dissociation of the substance on the other side of the membrane.
Active transport involves movement of substances against a chemical gradient.
The movement of water through plants is often explained by the TACT hypothesis. This effect can explain most of the movement of water in plants. This process is called bulk flow.