A level Biology 7.2 Transport mechanisms - Exam style question

Question

The diagram shows some structures used for transport in the phloem.

Which statement is correct?

A. Cell Y is the sink cell. B. Cell Z can be above or below cell Y in the plant. C. Cell Z must be in a photosynthetic tissue. D. Cell Y allows glucose to move to the sieve tube via the companion cell.

▶️ Answer/Explanation

Solution

Ans: B

In phloem transport, the source (cell Y) and sink (cell Z) can be in any relative position – roots, leaves, or stems. Transport can be upward or downward depending on plant needs. A is wrong (Y is source), C is wrong (sinks aren’t always photosynthetic), and D is wrong (sucrose, not glucose, is transported).

Question

Which conditions are needed to allow the mass flow of sucrose in phloem sieve tubes?

▶️ Answer/Explanation

Solution

Ans: B

The mass flow of sucrose in phloem sieve tubes occurs due to the pressure flow hypothesis, which requires:

Active loading of sucrose into sieve tubes at the source (e.g., leaves), creating a high solute concentration.
Water uptake by osmosis, increasing hydrostatic pressure.
Unloading of sucrose at the sink (e.g., roots or fruits), reducing pressure and maintaining flow.

The image likely depicts these conditions, with option B correctly representing the necessary steps for mass flow.

Question

Which description of adhesion and cohesion is correct?

A. Adhesion refers to the force between the water molecules due to hydrogen bonding. Cohesion refers to the force between water molecules and the xylem vessel walls. B. Adhesion refers to the reduced friction between hydrophobic lignin walls and the water molecules. Cohesion refers to the force between water molecules and the xylem vessel walls. C. Adhesion refers to the reduced friction between hydrophobic lignin walls and the water molecules. Cohesion refers to the force between water molecules due to hydrogen bonding. D. Adhesion refers to the force between the water molecules and the xylem vessel walls. Cohesion refers to the force between water molecules due to hydrogen bonding.

▶️ Answer/Explanation

Solution

Ans: D

Adhesion is the attraction between water molecules and the xylem vessel walls (due to polar interactions), while cohesion is the attraction between water molecules themselves (due to hydrogen bonding). Option D is the only choice that accurately describes both terms, making it correct.

Question

Carrier proteins in the cell surface membranes of companion cells are involved in the transfer of assimilates to phloem sieve tubes. The diagram represents the use of two types of carrier protein in this process.

What are the substances labelled X and Y?

▶️ Answer/Explanation

Solution

Ans: A

In the phloem loading process, assimilates (mainly sucrose) are actively transported into sieve tubes from companion cells. The mechanism involves:

Substance X (H⁺ ions): Proton pumps in the companion cell membrane actively transport H⁺ out, creating a proton gradient.
Substance Y (sucrose): Sucrose is co-transported back into the companion cell along with H⁺ ions via a sucrose-H⁺ symporter.

Thus, the correct identification is X = H⁺ ions and Y = sucrose, corresponding to option A.

A level Biology 7.2 Transport mechanisms – Exam style question – Paper 1

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