Why do you need to breathe in oxygen? a. Oxygen is needed to split glucose.
b. Oxygen provides energy that is to be put into ATP.
c. Oxygen pulls electrons through the process of cell respiration.
d. Oxygen reduces the NAD+ and FADH to produce NADH and FADH2

C. O2 pulls electrons through the process of cell respiration

Atp is is our little "pocket" of energy. of course, if you have too little, then it means you have too little energy, and you'll pass out. the way to not have enough, is to have no energy sources (glucose, fats, proteins). you can't have too many atp molecules. cycles like the krebs cycle produce it, but if you have too much of any substrate (or too little for that matter), the cycle stops and atp is no longer made (it stops until substrate levels are such that the process can go on). in any case, this is unlikely since atp is constantly hydrolyzed and remade, even while you're asleep. you won't have too much. if you have zero, you are dead or dying.