Explain, describe, suggest: what each command word in the exam wants

You revised enzymes. You know the active site, denaturation and the optimum temperature. The question says describe the graph, you write two lines about denaturation, and you get one mark out of three.

That is not a knowledge problem. The question asked for one thing, and the answer gave another. The word that starts each question, the command word, tells you what kind of answer earns marks. Mark schemes give marks for separate points of the right kind, not for showing that you know the topic in general.

The meanings below are the ones science teachers commonly use for O Level and International GCSE. The exact wording can differ between boards, so your own syllabus has the final word.

These want a short answer: a word, a phrase or a value. No reasons are needed, and reasons earn nothing extra.

• Name the gas that turns limewater milky. Carbon dioxide.
• State the unit of power. Watt, or W.

The trap is writing a paragraph. It eats time you need later, and one wrong detail in it can cost you the mark you had already earned.

Describe asks what happens, or what something is like. It does not ask why, so reasons in a describe answer do not add marks, however correct they are.

When the question gives you a graph or a table, a good description has three parts:

1. The overall trend. "As the temperature increases, the rate increases."
2. Any change in the pattern. Where it peaks, levels off or starts to fall, and at what value.
3. Figures from the data, with units. "The rate is highest at 40 °C." Read the actual values off the graph.

When the question asks you to describe a method, such as how to carry out a test, write the steps in the order someone would do them.

Explain asks why or how. Each point needs a reason, and a reason needs a link: because, so, which means, therefore.

It helps to write a chain: this happens, so that happens, so this is the result. "The temperature increases, so the particles have more kinetic energy, so they collide more often and more of the collisions have enough energy to react, so the rate increases." Each link is a separate idea, and separate ideas are what mark schemes look for.

Two common problems:

• Restating the question. If the question tells you the rate increases and asks why, "because the rate goes up" says nothing new.
• Vague words. "Amount", "it" and "things" hide what you mean. Say which amount: the concentration, the number of particles, the volume of gas. Use the terms your syllabus uses: an enzyme is denatured, not killed; energy is dissipated to the surroundings, not simply lost.

Suggest usually means the situation is new to you. It might be an organism you have not studied, an unfamiliar experiment, or data you have never seen. You are not expected to know the answer already. You are expected to apply what you do know to this case.

Because the situation is unfamiliar, more than one answer can often score, as long as it is sound science and fits the context. The two usual mistakes are freezing because it is not in your notes, and writing a general fact that ignores the details in the question.

What works: find the syllabus idea the question is really about, then say how it applies to the thing in front of you. Use a detail from the question in your answer, so it is clear you are writing about this case.

Compare asks for similarities or differences between two things, and each point must mention both. A paragraph about one thing followed by a paragraph about the other leaves the reader to do the comparing, and that often earns little.

Use comparative words: faster than, higher than, more, fewer, both.

• Weak: "Arteries have thick walls. Veins have valves."
• Stronger: "Arteries have thicker walls than veins. Veins have valves, but arteries do not."

If the question gives data, compare the figures directly: "The rate at 30 °C is twice the rate at 20 °C", not just "it is higher".

The number in brackets at the end of a question is your best guide to how much to write. As a rough rule, a question worth three marks needs three separate, correct points. It is not exact, because a mark can go to a figure or a unit, but it stops you writing one sentence for four marks or a page for one.

Do not hedge. Writing two different answers in the hope that one is right is risky. If one of them is wrong, you may lose the mark for the right one too. Choose one.

Other command words follow the same logic. Calculate wants the working and the unit as well as the number. Evaluate wants points on both sides and then a judgement. Your syllabus lists the words your board uses.

Here is an invented question, written to show the difference. A graph shows the rate of an enzyme-controlled reaction from 10 °C to 60 °C. The rate rises to a peak at 40 °C and falls to zero at 60 °C.

(a) Describe the results. [3]

• As the temperature increases from 10 °C to 40 °C, the rate increases.
• The rate is highest at 40 °C.
• Above 40 °C the rate decreases, and it is zero at 60 °C.

No reasons, and every point uses figures from the graph.

(b) Explain the results. [3]

• Up to 40 °C, the enzyme and substrate molecules have more kinetic energy as the temperature rises, so they collide more often and more enzyme-substrate complexes form.
• Above 40 °C, the shape of the active site changes: the enzyme is denatured.
• So the substrate no longer fits the active site, and the rate falls.

Every point gives a reason, linked with so.

(c) Suggest why an enzyme from bacteria that live in hot springs might behave differently. [2]

• Its optimum temperature is likely to be higher than 40 °C.
• Its active site may keep its shape at higher temperatures, so it is not denatured as easily.

You have not studied hot-spring bacteria, and you do not need to. The answer applies what you know about denaturation to a new case.

Reading about command words helps a little. Practising them helps much more. Answer written questions on one topic, then compare your answer with the mark scheme, point by point. For each lost mark, ask which kind of loss it was: did you not know it, or did you know it and answer a different command word?

On Eudika Learn, written questions can be practised by topic. You type your answer or photograph your handwritten work, then the real mark scheme is revealed and you tick each point you earned. Cambridge O Level Biology and Chemistry papers are a good place to see these words in use, and the full method is in how to practise past papers.