A chain has three links: a cause, a process and an effect. Read the diagram to find each one, then join them with “so” or “because”.
This lesson belongs to from biological recall to causal explanation. It is the base for the other three skills in that set.
How do I move from label to chain?
Follow four steps on any labelled diagram. They keep you from describing the picture and push you towards explaining it.
- Read the question and find the outcome asked for.
- Pick the two or three labels that matter for that outcome.
- For each label, write what it does, then what that causes.
- Join the statements in order, cause first.
Worked example: an alveolus
Diagram. An alveolus with a wall one cell thick, surrounded by a capillary network, with moist inner lining. Labels: alveolus wall, capillary, moisture, air space.
Question. Explain how the alveolus is suited for gas exchange. (3 marks)
Describe-only answer: “The alveolus has a thin wall and many capillaries and is moist.”
This only lists features. It earns one mark at most, because no feature is linked to an effect.
Chain answer: “The wall is one cell thick, so the distance for oxygen to diffuse into the blood is short. The capillaries carry the oxygen away in the blood, so the concentration gradient stays steep. The lining is moist, so oxygen can dissolve before it diffuses across.”
Each sentence has a cause (a feature), a process (diffusion or dissolving) and an effect (faster exchange).
The mistake that costs marks
The slip is to repeat the feature as its own reason: “The wall is thin because it is thin for gas exchange.” That loops back on itself.
Instead ask, “What does thin allow to happen?” The answer is a short diffusion distance. That is the process, and it carries the mark.
Check yourself
A diagram of a root hair cell shows a long extension into the soil. Write a two-link chain explaining how this helps water uptake. (2 marks)
Answer
The long extension gives the cell a large surface area in contact with the soil, so more water can enter by osmosis in a given time.
Cause: the extension. Process: a larger surface area for osmosis. Effect: faster water uptake.
What to study next
Continue with separating an adaptation from the advantage it can provide, then try the integrated practice. For a teacher to question your sentences, see online one-to-one Biology tuition.