 There's one more thing about cell respiration we haven't looked at. I told you before that cell respiration uses this equation, which is the reverse of photosynthesis. Glucose plus oxygen goes to carbon oxide and water. And it's exothermic, so we get energy out of it. And that energy, the cell packages into little rechargeable batteries of ATP. And I did tell you this equation is pretty loose, because it doesn't just happen like that, it happens over many, many steps. The other thing that makes it kind of loose is that the cell doesn't need to have exactly glucose. It can use some other molecules that it finds if it needs to. So other sugars, amino acids, and fats can even be broken down in cell respiration. What's even more loose here is the oxygen part. Do cells always need oxygen to harness the energy from their nutrients? And the answer is no. And when they don't have oxygen at all, they undergo a slightly different process called fermentation.