Simply put, biology is the study of life and living organisms. Biologists study how organisms originated, how they are classified and how they behave, as well as how they evolve and interact with other organisms. Biology can further be broken down into a number of more specified disciplines and coupled with other sciences for experimental purposes. A major in biology will certainly help a pre-med student on their MCAT, which has an entire section geared specifically on the biological sciences.
Chemistry is the study of the structure and behavior or atoms, the properties of compounds, the reactions of substances to energy exchange and scientific law as a whole. Like biology, chemistry can be broken down into more specified disciplines, such as genetics, physical chemistry, and cell and molecular biology. If your goal is to become a doctor, you will have to know, among many other things, how each drug reacts to various other drugs, diseases and compounds. This learning has its basis in chemistry.
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In microbiology, you study a large group of microorganisms, with everything from bacteria and fungi to viruses and algae. These are all types of microorganisms that doctors must deal with on a regular basis. As a medical professional, you will need to know how to recognize and treat these, among other, microorganisms, so studying them closely before medical school is another opportunity to drill that knowledge into your brain so that it becomes second nature.
There is an entire section of the MCAT that is scored based on your knowledge of the physical sciences, so choosing a path that educates you in the physical sciences is an obvious choice. Still, choosing a major in any of the sciences will be helpful in the long run, as you learn about the scientific process, important terms that will come up again and again, and the proper way to study and test in the scientific field.