Phloem is involved in the transport of organic food. According to the mass flow hypothesis, organic substances flow in solution form in sieve elements due to the development of an osmotically generated pressure gradient. As glucose is prepared at the source (by photosynthesis), it is converted to sucrose (a disaccharide). The sugar is then moved in the form of sucrose into the companion cells and then into the living phloem sieve tube cells by active transport. This process of loading at the source produces a hypertonic condition in the phloem.