A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Xylem, Phloem, and Transport in Plants — mark scheme explained
The short answer
In A-Level Biology, understanding the transport systems within plants is crucial for grasping how these organisms exchange substances with their environment. This section focuses on xylem and phloem, the primary tissues responsible for transporting water and organic substances, respectively.
The question
Explain how the cohesion-tension theory accounts for the upward movement of water in xylem. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Describe the main principles of the cohesion-tension theory.
- S2
The cohesion-tension theory states that water molecules stick together (cohesion) and to the walls of xylem vessels (adhesion). Transpiration from leaves creates a negative pressure or tension in the xylem, which pulls water up from the roots.
- S3
Step 2: Explain how these principles work together to move water upward.
- S4
The cohesive forces between water molecules maintain a continuous column of water within the xylem. The tension created by transpiration pulls this column upward, and adhesion helps prevent the column from breaking under tension.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Describe the main principles of the cohesion-tension theory.
- S2
The cohesion-tension theory states that water molecules stick together (cohesion) and to the walls of xylem vessels (adhesion). Transpiration from leaves creates a negative pressure or tension in the xylem, which pulls water up from the roots.
- S3
Step 2: Explain how these principles work together to move water upward.
- S4
The cohesive forces between water molecules maintain a continuous column of water within the xylem. The tension created by transpiration pulls this column upward, and adhesion helps prevent the column from breaking under tension.
Final answer: The cohesion-tension theory explains that water moves upward in xylem due to the cohesive forces between water molecules, the negative pressure (tension) created by transpiration, and the adhesive forces between water and xylem walls.
Common mistakes
- Confusing xylem with phloem functions. — Always remember that xylem transports water and minerals from roots to leaves, while phloem transports organic substances from sources to sinks.
- Misinterpreting the principles of the cohesion-tension theory. — Practice explaining each principle step-by-step. Cohesion keeps water molecules together, tension pulls water up due to transpiration, and adhesion helps maintain the column of water within xylem vessels.
- Failing to describe all steps of the mass flow hypothesis. — Always include all four steps: solute loading, water movement, solute unloading, and pressure flow. Practice describing each step clearly and concisely.
- Incorrectly interpreting experimental results from tracer experiments. — Practice analyzing data from tracer experiments and relate the movement of labeled substances to the steps of the mass flow hypothesis. Emphasize how the detection of tracers at sink tissues supports the theory.
- Misinterpreting the effects of ringing experiments. — Practice explaining that swelling indicates the accumulation of organic substances because they cannot move past the disrupted phloem. Emphasize that this provides evidence for the role of phloem in transporting organic substances.
- Failing to recognize correlations and causal relationships in plant transport. — Practice identifying and explaining the relationships between different variables. For example, explain how increased transpiration rate leads to greater tension in xylem, promoting water movement upward.
Where the marks go
- Full worked solution (all marking points)4 marks