Summer courses could be more difficult too, especially for classes that are pretty demanding even in the academic year. Trying to cram so much information into a few weeks is not a recipe for long-term retention or understanding, although it's not impossible. Often, teachers skip things that they would have elaborated on during the academic year. This, I think, is reason enough to avoid taking the core courses during the summer. Peripheral sorts of courses, though, are manageable, and it won't be the end of the world if you don't retain so much. Biochemistry is one of those classes that I would not take over the summer.