Glycolysis is a series of enzyme catalyzed reactions that break down a single molecule of glucose into two molecules of pyruvate. Have you ever wondered how your body makes energy? Well if you haven't, you should start, because otherwise this is going to be a really boring video.
It uses a process called the Krebs cycle, which produces a molecule called ATP, which is a high energy, 3-phosphate molecule. But for the Krebs cycle to work, we need a 3-carbon molecule called pyruvate. And I'm guessing you've never heard of that because it doesn't exactly come in soda cans.
So to get pyruvate, the body goes through a process called glycolysis, which starts with one molecule of glucose. Glucose is a simple 6-carbon sugar that results from the breakdown of starchy crap that you eat in your stomach. Now, for the glucose...
To be transformed into pyruvate, it has to go through a long process, the first step of which involves the investment of an ATP molecule. The ATP loses one phosphate group to transform the glucose into glucose 6-phosphate. Next, the glucose has to be transformed into fructose 6-phosphate.
There are enzymes involved in this, but no energy is invested. Next, another ATP loses a phosphate group to turn the fructose 6-phosphate 6-phosphate into fructose 1,6-biphosphate. Fructose 1,6-biphosphate breaks into two different molecules, DHAP and glyceraldehyde 3-phosphate, which, for my sake, we're going to call G3P for the rest of this exercise. DHAP is similar in chemical composition to G3P, but is structurally different.
For it to go through the process, it has to turn into a molecule of G3P, which involves moving a hydrogen molecule from one end of the... particle to another. From this point forward, two G3Ps go through the rest of the cycle parallel to one another.
For the sake of simplicity, we're just going to follow one of these molecules. First, the G3P loses a hydrogen and two electrons to energy carrier NAD+, turning it into higher energy NADH. This NADH will go to the electron transport chain, which we'll talk about later.
Then, it gains an inorganic phosphate turned into 1,3-biphosphoglycerate. If you can't pronounce biphosphoglycerate, don't feel bad, because it will immediately lose a phosphate group to an ADP, turning it into an ATP molecule, used for energy. This losing molecule will phosphoglycerate in its place. If you're wondering what the point of adding and then immediately removing an inorganic phosphate group was, it's because of the structural changes that occur due to the bonds.
that are changed for that group to attach. The phosphoglycerate is then rearranged into phosphenolpyruvate, which is another mouthful that doesn't stick around for very long. The phosphate is then removed from phosphenolpyruvate and added to an ADP molecule to create ATP. What is left is called pyruvate.
The pyruvate then moves through the membrane of the mitochondria, where some changes have to take place before the Krebs cycle can begin. First, an enzyme called coenzyme A usually just called CoA, is added to the pyruvate. It then loses a hydrogen and two electrons to NAD+, turning it into NADH. It also loses a carbon dioxide group. What results is something called acetyl-CoA, which is a complex molecule that basically is two carbons bonded to the CoA molecule.
We didn't diagram acetyl-CoA because it's extremely complicated, molecularly speaking, and because it doesn't exactly stick around for very long. This is the beginning of the Krebs cycle. Immediately after being formed, it donates its acetyl group to an oxaloacetate, which is already in the mitochondrial matrix from a previous Krebs cycle.
Then, water is added and CoA is released in order to form citrate. Citrate is then rearranged to form isocitrate. Like DHAP and G3P earlier, isocitrate and citrate are chemically similar, but structurally different.
Isocitrate then loses a hydrogen and two electrons to NAD+, to form another NADH. We know that show already. Then it loses another CO2 group to form alpha-ketoglutarate.
Alpha-ketoglutarate then forms another NADH by, once again, losing a hydrogen and two electrons, and it loses another CO2 group. The extra energy from these losses is stored in ATP, which is formed from an ADP and inorganic phosphate found outside of the molecule. Water is then added to form succinate. which is then transformed into fumarate, losing two hydrogen atoms in the process.
These are transferred to the energy carrier FAD, which is then transformed into FADH2. Fumarate is then added to fumarate to form malate. Malate then loses, surprise surprise, another hydrogen molecule and two electrons to form another NADH. This transforms it into oxaloacetate, which we remember from the first step.
You do remember the first step, right? This brings us back to the beginning of the Krebs cycle, which will continue to go in a circle as long as you remain living. You may be wondering what the point of all this was. Well, if you remember all of the NADH and the FADH2 that we produced over the course of this cycle, all of those now move to the electron transport chain, which is essentially a magical device in the inner mitochondrial membrane, which inputs NADH and FADH2 and outputs 34 ATP.