| | Re: it's been 15 years since i've taken organic chem or zoology classes. now i
The concepts that you learn in general chem are applied in organic, biochemistry, and to some degree microbiology. If you don't have a solid foundation in the basic principles of chemistry, these courses may seem more difficult. Perhaps you could run to your school's library, find a general chem textbook, and browse through it to see how much you remember from 15 years ago. If it seems elementary to you, you may be okay. If you find you don't remember any of the material, you may benefit from retaking the course.
Another thing to consider is the MCAT which has a fairly brutal chemistry section. You will want to have a solid chemistry knowledge base for this.