Let's review a theme which was a major focus in the first half of the course. The primary function of the respiratory system is to enable gas exchange with the environment. Oxygen is needed by the cells to carry out the transfer of stored internal energy from nutrient molecules to ATP. (Bear in mind the roll of oxygen's high reduction potential in driving cellular metabolism. Oxygen atoms are very electronegative and electron greedy, a low energy (held tight) destination for the electrons held by carbon (and hydrogen) in nutrient molecules. Electrons held in weaker carbon-hydrogen bonds, are called 'high energy electrons' but 'high energy' is with reference to the lower energy position of the electrons after metabolism within covalent bonds to oxygen. It takes less energy to break carbon-hydrogen bonds than is released when oxygen-hydrogen bonds are formed. Oxidative metabolism transforms the free energy decrease of the carbon - hydrogen - oxygen system into ATP formation.