I think you should try to get at least one letter from your supervisors, as they know you well, and can therefore write a very personalised letter. I wouldn't use a letter from a chem/bio professor unless they knew you at least as well as your supervisors (which is unlikely).
However, I would try to also obtain a letter from an MD. Simply put, using your application materials, the admissions committee is trying to decode whether you have the qualities to be a good doctor. A physician is therefore the perfect person to write a letter confirming that you possess those attributes.