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 decreased 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.
Explain the effect of increasing air temperature on the rate of transpiration in a plant. [2 marks]
 for an explanation that combines identification - understanding and  for reasoning/justification - understanding, eg:
Define both translocation and transpiration. Include the names of vessels involved in your answer. [2 marks]
Transpiration is the movement of water through xylem vessels from plant roots to their leaves for photosynthesis . Translocation is the movement of sugar (dissolved in water) made in photosynthesis through phloem vessels to all cells of a plant .
Describe the conditions that increase the rate of transpiration. [4 marks]
A class was asked to measure the volume of oxygen produced by a water plant called Cabomba in one minute.
One student decided to count the number of bubbles.
Explain the student could improve the method in order to measure the volume of oxygen released more accurately. [2 marks]
 for an explanation that combines identification - improvement of the experimental procedure and  for justification/reasoning, which must be linked to the improvement, eg:
Will accept alternative gas collection method with measuring cylinder and beehive shelf and will accept leave the apparatus for a longer amount of time.