It's possible (and likely) that your raw scores won't be as good in a higher-level science course.
However, there are two things to consider:
1) If you're reading the material and/or you have a solid professor, you'll get a lot of reinforcement of the basic concepts. For example, some of the ideas you learn in, say, genetics, might already be covered in your cellular biology class; and a lot of the cell bio concepts might be touched on in general biology. Basically, the sciences build on each other (far more so than the humanities from what I've seen/studied).
2) You're not alone. Sure, you might not pick up the minutae in Figure 15.6 (c), but chances are a good portion of the class didn't, either. That's no excuse to slack off, but remember that, as the complexity and difficulty of a subject rises, the average scores on exams, papers, etc. will drop.
Trust me on this... coming from a non-science background, the first few classes are going to seem daunting; but once you get your feet wet, get a solid understanding of the fundamentals, and find out what study methods work best for you, that huge pile of information won't seem quite as daunting. It's hard work, but 90% of it is just putting your nose to the grindstone and getting it done.