depends on the sixth form/college, usually they are for B and above in said subjects for:
-English
-Biology
-Physics
-Maths
-Chemistry
This is they are the hard academic subjects, so they want to know that you can take the work, that is to come.
THAT BEING SAID (DON'T SKIP THIS AND GET DISHEARTED)
If they do not allow you to do Biology, do it privately! that is what I'm doing. I'm taking Biology and Chemistry privately, so I can do Chemistry at Uni. OR teach yourself/distance learn and pay to do the exam. However, talk to your college/sixth form first before you jump the gun.