Command words are important and you should check carefully exactly what the question is actually asking for. If a question asks you to 'compare', all you need to do is point out similarities and differences, which means you don't need to go into the detail of an 'explain' question. The similarities and differences you give must be clear so to do this make sure your statements are comparitive, which means you address both things you are comparing - this one does this, but this one does not. It is possible to get all the marks by listing just similarities or just differences, but always try to give at least one of each. For example, for the question 'Compare anaerobic respiration in a yeast cell with anaerobic respiration in a muscle cell', I would say:
Anaerobic respiration in yeast cell produces ethanol but anaerobic respiration in a muscle cell produces lactic acid.
Anaerobic respiration in yeast cell produces CO2 but in a muscle cell it does not.
Both release small amounts of energy.
