in contrast to what other people are saying, http://www.swarthmor...gradschool.html (i highly recommend you read that. it's geared towards a humanity majors audience, but parts of it are 100% applicable to you in this situation). especially mjj58, i don't think s/he has had self-doubt, which makes his/her opinion much less valuable to you. namely, "trying it out" could be very psychologically costly for you. i am doing a MA right now and looking at PhD offers, and the line "Independently evaluating academic life from within its confines is a near-impossibility" speaks volumes of truth to me right now.
i'm going through the same thing as you (if in slightly different fields), and honestly i'm beginning to feel if i have doubts now, i'm either going to collapse in the midst of comps OR i'm going to have PTSD when I try to leave OR i'm going to hate to my life.
of course, again the audience of that original article was humanities majors. so potentially there's less of an opportunity cost if you're doing biophysics. myself, i'm thinking of going into a social science, but my work and passion has been computer engineering/programming, so there's a _very_ significant opportunity cost to a phd in social science if i want to change paths afterwards.
also: if you don't have passion now, you may never see the other side. you have to go in with a passion, or so tells me every tenured professor i've ever spoken with, because you have to be willing to do what you are studying through thick and thin.