In this lesson we look at the final three steps in the citric acid cycle with three goals in mind: 1) we need to invest another oxygen atom to allow the continued release of carbon dioxide in future turns of the cycle, 2) we need to reconstitute the alpha-keto group we lost during the formation of succinyl CoA to allow continued exchange with amino acids, and 3) we need to create an electron-deficient portion of the molecule that is able to accept the incoming acetyl group in the next turn of the cycle. In doing so, we look at the broader concept that water is the source of oxygen whenever we lack the oxygen we need to release carbons as carbon dioxide. This concept is critical to master to understand one of the most important differences between fat and carbohydrate: because of their different oxygen contents, they consume different amounts of water and generate different amounts of carbon dioxide in their metabolism. This impacts the functions of vitamin K and biotin, the stress we place on our lungs, and the delivery of oxygen to our muscles during exercise. We also look at the role of phosphate in “borrowing” oxygen from water, which will help us understand later why glycolysis is a source of water in the cell.