The pressure flow theory states that water with food molecules inside of it moves through the phloem of a plant through pressure. This theory is meant to explain how food moves around inside plants.
The pressure flow theory notes that pressure is created through water being pulled into the cells of the plant. The water cells are pulled into the plant when a diffusion gradient exists. This gradient creates a change in pressure, which forces osmosis.
While water is being drawn into the plant, all of the nutrients from photosynthesis also build inside of the phloem. As the two elements come together and continue to grow, turgor pressure builds, and the fluid carrying the nutrients from photosynthesis moves throughout the plant.
As the fluid moves throughout the phloem, different parts of the plant, such as the stem, removes sugars from it. Throughout the process of the sugars being removed from the water, the pressure inside the phloem decreases until the water is completely pure.
Once the pressure is low enough and the food has been removed, the water flows out of the plant through osmosis again or is sucked into other areas, such as the xylem, through a form of transpiration.