Cellular respiration. is essentially the reverse of photosynthesis

In terms of the net reactants and products, yes, cellular respiration is the reverse of photosynthesis. 
Cellular respiration: Glucose and oxygen are used to make ATP, carbon dioxide, and water.
Photosynthesis: ATP, carbon dioxide, and water are used to make glucose and oxygen.

However, their pathways are NOT the reverse of each other. The enzymes involved are vastly different for each, so only the net reaction is the "reverse."

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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.