There are eight steps to the citric acid cycle, which is a closed loop. This breaks down glucose further into CO2 and generates some important energy-producing molecules. It starts with acetyl CoA and oxaloacetate and forms citric acid or citrate (and thus the name). This is a six-carbon molecule. Through a series of reactions, 2 molecules of NAD become three NADH molecules and one FAD becomes an FADH2 molecule. There is a series of reactions inside the mitochondrial matrix that take the oxaloacetate and acetyl-CoA, removes two carbon dioxide molecules to turn the entire thing back into oxaloacetate, which is recycled for the next loop. Another energy molecule (GTP or ATP, depending on the organism) gets made in this cycle. Because there are two acetyl CoA molecules per glucose molecule made, the net end is that there are six NADH molecules, 2 FADH2 molecules, and 2 ATP molecules (or GTP molecules) made. Because GTP can be used to make ADP without difficulty, most sources consider that ATP is essentially made in this cycle, at least in higher animals.
OXIDATIVE PHOSPHORYLATION This is a highly important part of cellular respiration, taking place in the mitochondrial cristae. Remember that there are all of these high-energy molecules that aren’t ATP, which can be used to make ATP in the process. It involves a “proton gradient” or a positively-charged gradient (also referred to as a chemiosmotic potential) across the inner membrane of the mitochondria. ATP synthase is heavily involved in this process as many ATP molecules are made using this gradient to drive the biochemical process. So far, there has been a lot of “reduction” or reductive chemical reactions going on. This is a time for oxidation to take place. This is why it’s called oxidative phosphorylation. It takes oxidative processes to “phosphorylate” the ADP molecule to make ATP. This is where oxygen is necessary because something has to be the final electron acceptor in this chain of electrons being transferred from one molecule to the next. Oxygen is a perfect electron acceptor because it can bind with hydrogen atoms to make water. Figure 22 describes what oxidative phosphorylation looks like:
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