I'll tell you about the curves ;O
In lower div gen science classes (Chem 6ABC, BILD 123, PHYS 1/2ABC) you need to be within the top > 15% of your class to get an A- or higher. A's are usually awarded to top > 10% and A+'s are given to the top 1% of the class. General Science classes are all curved to a C/C+ depending on your professor, and so a significant portion of the class will have B- -- C's.
My BILD 1 class's distribution is posted here:
Home Page
It seems that the lower div bio classes are a bit more lenient than chem, but you still have to be in the top of your class.
Class Grade, total of 600 points (+40 EC); class average = 411 (high 595!)
A 502 - 571 (640) 21.3% of class
B 432 - 501 32.7% of class
C 362 - 431 20.0% of class
D 292 - 361 12.7% of class
F ≤ 291 13.3% of class