Question

Why do we breathe oxygen? how do we get energy out of our food?

Answer 1

Out body breaks down food with enzymes and the glucose produced is burned to form energy measured in calories.

we breathe in oxygen to help our muscles function 

Answer 2

We breath oxygen because it is needed to burn fuel [ sugars and fatty acids] in our cells to produce energy. Oxygen is also brought into the lungs via breathing; where it is transported by red blood cells to the entire body to be used to produce energy. The food we eat gives us protein, sugars, vitamins etc. for energy