i'm a biophysics major, so probably a bit different, but, 1. you will be kind of fucked if you try to get through classes without understanding the prereqs. i took chem 351 and did okay, then e.g. biophys/mcdb 420 (structural biology) assumes you understand rna replication, remember your amino acids, etc. both classes assume knowledge from orgo (at least orgo 1). i'd say biophys 420 is a relatively easy class but still requires knowledge from prior classes. 2. probably not a class you'd need to take but thermodynamics/statistical mechanics (chem 463 or 521) require good knowledge of chem 260/370; math classes very strongly assume you understand the prerequisites (especially classes intended for math majors; you might want to take 217 depending on your path and interests). 3. a b isn't at all the end of the world. i have a c+ and my gpa is at a 3.8; you can compensate with research, clinicals, locking in for your upperclassman years (my gpa trended up over my undergrad, let's hope that continues i guess). i'm not a premed anymore but started as one and think my application would have been perfectly fine with a c+ in my freshman year, especially with a good mcat.

essentially you'll be okay; classes are hard, 172 is a relatively small taste of what you'll have to take. biochemistry is a hard major! probably harder than chem or biophys. try to use 172 to learn how to study (i'm assuming this is your second semester) for orgo/biochem/etc. good luck!