Plants use sunlight to produce sugar molecules in their leaves. Some of this sugar is needed by cells in the roots. Would you expect plants to rely on diffusion to transport the sugar molecules from leaves to roots, and why? A. Yes, because diffusion will happen naturally, so the plant does not have to spend energy on it.

B. Yes, because all materials in living tissues move by diffusion.

C. No, because it would take too long for the sugar molecules to diffuse that far.

D. No, because diffusion only operates over small distances, not over long distances.

C. No, because it would take too long for the sugar molecules to diffuse that far.

Explanation:

Plants are very large organisms and have small surface area to volume ratios, therefore diffusion alone would not be sufficient to transport substances throughout the whole plant, to places where they are required. Therefore, plants have special structures like the phloem and xylem which transport substances within the plant.

The phloem is responsible for transporting food produced in the plant leaves to other parts of the plant such as the root where they are needed.  Phloem is composed of cells that transport the sugars manufactured in the leaves as a sugar solution called sap. The sucrose produced in the leaves is transported by active transport into the companion cells of the phloem in leaf veins and then into the sieve tubes by diffusion. Movement of water molecules by osmosis into the sieve tubes creates a fluid pressure difference which will cause flow in the direction of pressure difference such as the roots where they are needed.