The process of translocation of sugars from the source to the sink is known as a pressure flow hypothesis. Glucose is synthesized by photosynthesis and it is changed into sucrose. Sucrose moves into the companion cells and then into the sieve tube cells through the active transport process. The increase of sugar content in the source region makes the phloem to become hypertonic. The water moves from the adjacent xylem into the phloem through the process of osmosis. Once the osmotic pressure is increasing in the source region, the phloem sap moves to the region of lower osmotic pressure. The osmotic pressure of the sink has to be lower than that of the source.
The phloem sap moves out of the phloem into the cells that make use of the sugars by the active transport process. The sugars at the sink region are converted into energy, cellulose and starch. When the sugars are moved out from the source, the osmotic pressure reduces at that region and hence the water moves out of the phloem.
To conclude, the sugars move from the phloem after they are loaded into the sieve tubes by the active transport. The phloem is loaded to create the water potential gradient that can facilitate the movement of sap.
The phloem comprises of long sieve tubes that have sieve plates at the end of each cell. The sieve plates possess small holes. The cytoplasmic strands traverse through the sieve plate holes and form continuity in those filaments. The pressure flow starts when the hydrostatic pressure in the sieve tubes increases and the movement of sap occurs in the phloem. At the sink region, the sugars in the phloem are actively transported out of it, which get transformed into complex carbohydrates. The solute removal creates a higher water potential in the phloem, while the water moves out of the phloem into the nearest xylem vessels.
An experiment called ‘girdling’ helps in identifying the plant tissues that are involved in the transport of food materials. The bark that exists till the phloem layer towards inside, in the trunk region is removed. This stops the food movement downwards. After some time, it can be observed that the part of the stem that is above the removed region will be swollen. This simple experiment illustrates that the phloem is responsible for the transport of food and the food transport occurs in one direction itself, down towards the roots. This experiment can be easily performed by everyone.