How do I become a cat keeper in a zoo?

I am currently in school for a degree in Zoology. Is this degree necessary to become a keeper? Can you do this by just working your way up or getting into a program at a zoo? I'm having a lot of trouble with the chemisry and physics (most of which I do even see why its necessary for zoology) and want to make sure of the path I need to take so I'm spending all my time and money with something I don't necessarily need. Please help and be as detailed as possible. Thanks.