Biology 9700 · AS & A Level · Structure of transport tissues
Structure of transport tissues — practice question
Sugars move through phloem sieve tubes from a source, for example a mature leaf, to a young leaf that functions as a sink. That young leaf also requires water and dissolved mineral ions, and these are delivered to it in xylem vessels.
(a)[3]
As the young leaf matures, the amount of sugar absorbed by the leaf falls to zero, whereas its water requirement rises. Suggest and explain why the amount of sugar absorbed by the developing leaf falls to zero over time, but its water requirement rises.
(b)[4]
The features shown in Table 2.1 occur in one or more of the three cell types: companion cell, phloem sieve tube element, and xylem vessel element. Complete Table 2.1 by putting a tick (✓) if the feature is present and a cross (✗) if it is absent.
Worked solution & mark scheme
This 7-mark question has a full step-by-step worked solution and mark scheme. One marking point: “The leaf switches from sink to source; sugars are exported.” …