So the only science classes you've taken are gchem1/2, orgo1, and gen. bio 1? When are your lors due to the committee? You could always get them from your upcoming professors if you do well in their classes and explain your situation to them at the beginning of the semester. It would be a little time crunched depending on when you need to submit them, but I'd rather have meaningful letters than generic ones. Also, you can still get one from your gchem professor as long as you sit down with them and really discuss your future with them. I didn't have stellar relationships with some of my writers and they were still more than willing to write me outstanding letters after having an in-depth conversation with them.