Biology affects the study of psychology in many ways:
1. Genetics-you'll gain most of your understanding of genetics from your bio courses. Genetics can have an influence on psychological disorders such as schizophrenia, among other things. And your DNA effects, well...you in your entirety.
2. Chemistry-you don't really do chemistry in bio, you're just introduced to the concept of it. Biochemistry effects your behavior and such.
3. Nervous system-as you know, this includes your brain, spinal cord, neurons, etc...I don't know about you, but I love reading about behavior of people who have nervous system injuries. And you certainly need understanding of neurons, neurotransmitters, etc (neurotransmitters are a different section I guess, but anyway).
4. Endocrine system-this is a big one. Your hormones control tons of stuff that can affect how you feel and your behavior. I was blown off and told I was depressed or 'its just anxiety' for years and I had endocrine system problems.
5. Ethology-in most bio courses there will be a section on animal behavior. This is another discipline of psychology, comparative animal behavior; comparing their behavior to humans.
6. Evolution-even if you don't believe in it, you should understand it. I was taught it before, but I learned more about it this year. Many psychologists have theories about our behavior that they base on evolution.
That okay?