Question: Why does the body need oxygen to breathe? Why do we give out carbon dioxide? Could we swap roles with plants? Can we breathe in carbon dioxide and give out oxygen while plants do the opposite?

Sean Murphy answered on 14 Mar 2012:

The main use of oxygen in humans and aerobic organisms is to allow the body to make energy. Energy is needed a host of processes in the body e.g. growth, repair, movement, nerve conduction… Energy can be gained from carbohydrates fats and amino acids however they to be oxidised. The oxidation of these are done by the reduction of a series of molecules the (known as the electron transport chain) the last of these molecules is oxygen (which is reduced to water). This process is termed respiration. Carbon dioxide is a waste product of this process. We can’t change to respirate like plants, but in some situations plants can change to use oxygen like us.