One to four mark questions will start with command words such as 'describe' or 'explain'. Some command words are easy to understand such as:
The command words 'describe' and 'explain' can be confusing. If you are asked to describe a graph, you will be expected to write about its overall shape, whether it is linear or curved, the slope of gradients etc. If you are asked to explain why a pattern or trend is seen in a graph, you will be expected to use your science knowledge, not just say what you see (which is a description), eg the graph shows a steep linear increase for the first three hours because…
Explain how and why questions often have the word 'because' in their answer. Describe questions don't.
The number of marks per question part is given in this form '[2 marks]'. It is essential that you give two different answers if a question is worth two marks. Sometimes you can gain a second mark by giving the units in a calculation or stating specific data points, eg during the first two years the number of lions deceased by seven.
You will be expected to write in more depth for three and four mark questions. They might ask you about a process such as the carbon cycle or the method you would use in an experiment.
Edexcel questions courtesy of Pearson Education Ltd.
A person has emphysema. This reduces the number of alveoli in the lungs.
Explain how emphysema would affect the amount of oxygen carried in the bloodstream. [2 marks]
Give one reason why it is important that human lungs have a high surface area to volume ratio. [1 mark]
To maximise gas exchange/maximise oxygen uptake. 
This figure shows the structures of an artery and a vein.
Explain how the structure of the artery is related to its function. [2 marks]
This figure shows a diagram of the circulatory system of a fish.
Compare the differences between the structure of the circulatory system of a fish and the human circulatory system. [4 marks]
Give one reason why the wall of the left ventricle is thicker than the right. [1 mark]
To pump blood around the body under higher pressure. 
Valves in the human heart may become damaged and no longer function.
Describe what would happen to the flow of blood in the left side of the heart if the valve between the left atrium and ventricle did not function effectively. [2 marks]