Question

Function of companion cells is

• A. providing energy to sieve elements for active transport
• B. providing water to phloem
• C. loading of sucrose into sieve elements by passive transport
• D. loading of sucrose into sieve elements.

Answer 1

Answer: (D)

Companion cells move sugar and amino acids into and out of the sieve elements. In 'source' tissue such as 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 seive tube. Loading sets up a water potential gradient that facilitates movement of sugar.