Hi Shalom12345,
Just adding to this. I would say that, in general, you can achieve an A/A- with hard work in any course. Now, realistically speaking, when preparing for a course like biochemistry, much of your final grade can be contingent on other factors—some of which may not typically be in your control, such as how other students in the course perform. In some courses, you may have to deal with a curve, in which only a certain proportion of the highest scorers in the class are able to receive an A or an A-. In this case, it is possible that you can work quite hard but still fall short of an A- if others in your class perform better.
Of course, your professor typically has final say on what your grade is. Many professors will appreciate seeing you work hard and enjoy a class. For this reason, if you find yourself struggling, be sure to show interest and attend that professor's office hours. This will convey interest an effort—both of which bode well for your end of the semester grade.
In terms of preparation, feel free to ask your professor how best to prepare for the exams, as he or she is likely creating them. Practice exams are a great method. When you're struggling with a general concept, it is also often helpful to go to the textbook to look for a clear explanation (especially if your professor's explanation is less than optimal).
One method that is particularly systematic is to write down all the questions you come across while studying or doing homework. Then, bring them to the professor during an office hour, or after class. Again, this conveys genuine effort—not to mention, the fact that you care about learning the material.
Hope this helps!