Since the sieve tube element lacks organelles, the companion cell with its nucleus, mitochondria, ribosomes, enzymes etc., controls the movement of solutes and provides ATP for active transport in the sieve tube element. Strands of cytoplasm called plasmodesmata connect the sieve tube element and companion cell. Also, fibres provide support for the sieve tubes.
The final main element is phloem parenchyma, which is a packing tissue, and its main function is to fill the spaces between the other tissues. The cells are roughly spherical in shape, with flattened faces where they press against each other. If the cells are fully turgid and tightly packed as they normally are, parenchyma helps to maintain the shape and firmness of the plant.
The process called phloem translocation, involves the movement of organic substances around the plant. It requires energy to create a pressure difference and so is considered an active process.
Sucrose is loaded into the phloem at a source, usually a photosynthesising leaf. For this to occur, hydrogen ions are pumped out of the companion cell using ATP. This creates a high concentration of hydrogen ions outside the companion cell. Sucrose is moved into companion cells by active transport, against the concentration gradient.
However, the protein carrier involved in the loading, has two sites, one for sucrose and one for a hydrogen ion. When it is used to pump sucrose into the companion cell, hydrogen will move in the opposite direction, back down its concentration gradient. This is why a high concentration of ions is needed outside the cell.
The sucrose can then diffuse down the concentration gradient into the sieve tube element via the plasmodesmata that connects the companion cell with the sieve tube element. This lowers the water potential of the sieve element so water enters by osmosis.
At another point sucrose will be unloaded from the phloem into a sink (e.g. root). It is likely that the sucrose moves out by diffusion and is then converted into another substance to maintain a concentration gradient. Again, water will follow by osmosis.
This loading and unloading results in the mass flow of substances in the phloem. There is evidence to support this theory; the rate of flow in the phloem is about 10,000 times faster than it would be if it were due only to diffusion. The pH of the phloem sap is around 8 (it is alkaline due to loss of hydrogen ions). There is also an electrical potential difference across the cell surface (negative inside due presumably to the loss of positively charged ions).