Provide energy to sieve elements for active transport
Provide water to phloem
Load sucrose into sieve elements by passive transport
Load sucrose into sieve elements by active transport
Solution
Companion cells move sugar and amino acids into and out of the sieve elements. In "source" tissue such as the leaf, companion cells use transmembrane proteins to take up sugar and amino acids by active transport. Movement of sugars in the phloem begins at the source, where sugars are loaded (actively transported) into a sieve tube. Loading sets up a water potential gradient that facilitates the movement of sugar.