Explain how sucrose is transported from the phloem to the sieve tube element. (5)

Protons are actively transported using ATP out of the companion cell into the cell wall, creating an electrochemical gradient. These protons then re-enter the cell by facilitated diffusion through a co-transport protein in the cell membrane, which transports a molecule of sucrose with every proton transported. Sucrose enters the companion cell against the concentration gradient, increasing the concentration of sucrose in the cell. Sucrose diffuses down a concentration gradient from the companion cell into the sieve tube element through plasmodesmata.