Enzymes catalyze the many reactions in a cell. there are hundreds of different enzymes in a cell—each with a unique three-dimensional shape. why do cells have so many different enzymes?

To ensure that they can only catalyse specific reactions. The unique 3D shape of each enzyme means that only a specific substrate that is complementary (fits into the active site of the enzyme) will bind to the enzyme to be hydrolysed. In this way, the metabolic reactions of the body can be controlled.

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.