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A key to achieving the most marks possible in exams rests not just with your knowledge, though this is of course important. What is equally (if not more!) important is your ability to show the examiner that knowledge and understanding. I have met many students, who could verbally explain a process to me, in physics, or chemistry or biology. However, when it came to writing that down it was clear that they were not going to get the grade they deserved because they were not evidencing their understanding properly.
Evidencing your understanding is fancy way of 'saying what you mean'. Which in science is very important. If asked,
"Plant A was given 100mls of tap water with 1g Phosphorous diluted in it each monday for a year, and Plant B was given 100 mls of plain tap water. After 1 year plant A was 35cm high and plant B was 16.5cm high.
a) What is the size difference between plant A and plant B? (1 mark)
b) Can you explain this difference? (2 marks)"
Not a difficult question. The answers are straightforward.
a) What is the size difference between plant A and plant B? (1 mark)
You could write - "double". Some people would have done just this. One word answers are rarely good enough. What is "double"? I am sure we can all see that "Plant A is double the HEIGHT of Plant B" is a more complete answer - and guarantees the mark.
b) Can you explain this difference? (2 marks)
You could write "Plant A absorbed the phosphorous through its roots and so grew bigger". Again, this is true, but it is not complete. One way to think of this is - 'without the question, would my answer make sense?'. Also, important to note, is that this question is worth 2 marks. So examiners will probably want 2 independent points.
A better answer would be: "Plant A grew bigger than plant B because phosphorous was added to its water. This is because phosphorous is a key nutrient in plant nutrition, and a limiting factor on the growth of these plants."
This is assuming that you had learnt or studied plant physiology in your course of course! Aside the technical details. The reason answer 2 is better is it recognises the TWO points that the examiner is looking for and so provides the answer as an OBSERVATION-EXPLANATION/HYPOTHESIS couplet. A mark is given for recognising the influence of phosphorous. But only the second answer gets both marks as it contains a SECOND POINT when it suggests why phosphorous caused the difference.
This may seem pretty obvious. But as a tutor, too often I see bright students who know their subjects lose points because of lazy or unclear writing. It's a shame as the hard part was learning the subject - why lose marks because you don't write what you know effectively?
