A-Level · Biology · AQA · Mark scheme decoded

AQA A-Level Biology: Xylem, Phloem, and Transport in Plants — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Biology specificationlast verified 3 July 2026

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.]

4 marks

Mark scheme, decoded

How the examiner actually awards the marks on this topic.

Gradora's own decode of the marking approach — not the exam board's published mark scheme.

How marks are awarded

For descriptive questions, marks are typically awarded for clear and concise explanations that demonstrate a deep understanding of the topic. For evaluative questions, marks are given for identifying both strengths and limitations of theories and providing evidence to support your assessment. For interpretive questions, marks are awarded for accurately analyzing data and drawing appropriate conclusions.

What the command words demand

Explain: Provide a detailed description of the concept, including relevant principles and relationships.
Describe: Give a clear account of the process or mechanism without necessarily providing an explanation.
Evaluate: Assess the strengths and limitations of a theory or hypothesis based on evidence.
Interpret: Analyze data or experimental results to draw conclusions about plant transport.

Model answer

A full-mark response to the question above, worked through step by step.

Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, provide detailed answers, and check your work.

Describe the main principles of the cohesion-tension theory.2 marks
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.
Explain how these principles work together to move water upward.2 marks
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.

Work through every step correctly and you earn all 4 marks.

Another worked example

Describe the steps involved in the mass flow hypothesis of phloem transport.

6 marks

Explain solute loading.2 marks
Solute loading involves actively transporting sugars and other organic substances into sieve tube elements at source tissues, such as leaves during photosynthesis.
Describe water movement.2 marks
The high concentration of solutes in the sieve tubes creates a water potential gradient, causing water to move into the phloem from surrounding cells. This increases the pressure within the sieve tubes.
Explain solute unloading.1 mark
At sink tissues, such as roots or developing fruits, organic substances are unloaded from the sieve tubes. This reduces the solute concentration and causes water to move out of the phloem, reducing the pressure.
Describe pressure flow.1 mark
The difference in pressure between source and sink tissues drives the movement of sap through the phloem, ensuring efficient transport of organic substances throughout the plant.

Final answer: The mass flow hypothesis involves solute loading at source tissues, water movement into the phloem due to a concentration gradient, solute unloading at sink tissues, and pressure flow driven by the difference in pressure between source and sink tissues.

Work through every step correctly and you earn all 6 marks.

Common mistakes

Confusing xylem with phloem functions.
Why it happens: Students may mix up the roles of xylem and phloem, leading to incorrect answers in questions about plant transport.
Fix: 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.
Why it happens: Students may not fully understand how cohesion, tension, and adhesion work together to move water through xylem.
Fix: 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.
Why it happens: Students may omit one or more steps when explaining the mass flow hypothesis, leading to incomplete answers.
Fix: 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.
Why it happens: Students may not fully understand how tracer experiments provide evidence for the mass flow hypothesis.
Fix: 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.
Why it happens: Students may not understand why swelling occurs above the cut in a ringing experiment.
Fix: 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.
Why it happens: Students may struggle to identify how factors like transpiration rate or temperature affect water movement in xylem or phloem transport.
Fix: 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

The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.

Question type	What you’re asked to do	Marks
Explain Water Transport	Explain how cohesion, adhesion and transpiration-driven tension move water up the xylem.	4
Describe Mass Flow	Sequence how sugars are loaded, moved and unloaded to drive phloem transport by pressure.	6
Evaluate Experimental Evidence	Explain how radioactive tracer experiments support the mass flow hypothesis for phloem transport.	4
Ringing Experiment Evidence	Describe ringing's effect on plant growth and explain how it demonstrates phloem transports organic substances.	6
Total across these question types		20

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.