GCEBiologyTransport Systems2024

Which of the following correctly describes the role of companion cells in phloem transport?

AThey provide structural support to the sieve tube elements
BThey supply energy and metabolic support to the adjacent sieve tube elementsCORRECT
CThey actively transport water into the phloem from the xylem
DThey produce the callose that lines the sieve plates
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Why the answer is B, and why the others tempt you.
Companion cells are metabolically very active and possess numerous mitochondria; they supply ATP and other metabolic products to the enucleate sieve tube elements, which cannot sustain their own metabolism. This symplastic relationship is essential for the active loading and unloading of sugars into the phloem. Structural support in phloem is provided by phloem fibres, not companion cells.
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