What are some of the principles of human biology?

A couple of examples of these would be the principle of the conservation of energy - the body tries to do everything as efficiently as possible so as to not waste valuable energy; can you tell me what the form of energy is in the human body and give me an example of a family of proteins that increase energy efficiency during chemical reactions? Another principle that can be applied to many systems is negative feedback (draw diagram), which is the basis of homeostasis - can you define homeostasis and give me an example of something that has its level controlled by negative feedback in the human body?