Some hints on writing essay-type answers:
(i) you do not have to write a great literary work; the logic of the science is the important thing,
(ii) the answer should not be a historical essay; the correct logical sequence is what matters,
(iii) use diagrams, which must be clear but not necessarily beautiful, to illustrate your arguments,
(iv) use examples to illustrate patterns of behaviour,
(v) always quote the appropriate equations,
(vi) try to express complex ideas in your own words rather than a paraphrase of the textbook (the test of whether you understand something is whether you can express it in your own words).
The main purpose of the exercise is that you should understand the topic fully and that you should retain a written record for your own future reference.