First and foremost, remember the purpose of the essay: to prove to your grader that you have mastered introductory biological science. Every part of your essay should be devoted to fulfilling this purpose. Graders—usually high school or lower-level college teachers—sit in a room and grade essay after essay after essay. They tire very quickly, and essays that get to the point and stay on the point are easier to score and thus often get a higher grade.
Always write in complete sentences with your best spelling and grammar. According to the rules, notes and outlines cannot be graded.
Avoid flowery, complex, literary essays. Do not include long introductory paragraphs that do not answer parts of the question. Do not include complex conclusions
Always answer the question asked, no more or no less.
Organize your answer the same way the question is organized, so that your grader does not have to hunt for points as they score it