In this video, we're going to dive a little bit deeper into the chemical reactions that are important for cellular respiration. Keep in mind the whole point of cellular respiration. The goal is to trap the chemical energy that was found in biomolecules like sugars. Trap that chemical energy into the chemical bond of ATP. Now we can store even more energy in larger molecules since they have more chemical bonds. So polysaccharides like glycogen or fats. Uh and we go through a series of reactions called oxidation reactions which is sort of a stepbystep removal of hydrogen atoms and their associated electrons from the substrates. And the result is CO2 as a waste product. U and the energy is coming from the breaking of chemical bonds. So this is going to be found in three main processes that we're going to focus on. Glycolysis, the KB cycle, which is also known as the citric acid cycle or the caroxyic acid cycle. unfortunately has three different names, but we're just going to use KB cycle here just for consistency. And the last process is oxidative phosphorilation. And so these are multiplestep pathways. Glycolysis alone has over 10 chemical reactions. So there's a lot of intermediate molecules that are found in these reactions. So we're not going to highlight each chemical step. We're going to give you sort of a big picture view. But once we have through our digestion things like amino acids, simple sugars, glycerol, and fatty acids, we can actually shuttle all of these molecules in at various stages of these three chemical reaction, three main types of chemical reactions into glycolysis. uh we're using glucose for the most part, but some amino acids and glycerol can be shunted into the pathway to ultimately produce ATP. We can break down fatty acids into acetal COA and then shuttle that acetal COA into the KB cycle. So all of these are going to be catabolic reactions. That's not to say that we can't do the reverse, do the anabolic reactions and use those intermediates as substrates to build up our own versions of fatty acids or our own types of uh amino acids. But the main point of this module is going to be understanding this type of unique chemical reaction. So earlier we talked about exroonic and endergonic and how we couple these reactions. Well, this coupling of reactions are known as exchange reactions because you're exchanging energy. A very special type of exchange reaction is known as an oxidation reduction reaction. So we're basically oxidizing a molecule and removing hydrogen and electrons. So that molecule is losing energy. So we say it is becoming oxidized. So in this particular chemical reaction here again there are multiple steps but we're just treating it as a single reaction. Glucose is going to be broken down ultimately into carbon dioxide. So bonds are being broken. So glucose is losing energy. So glucose is becoming oxidized. Well, you can't have one without the other. So if one molecule is losing high energy electrons, then the other is gaining them. So in this case oxygen is going to be gaining those high energy electrons. So we say the oxygen is being reduced. So a lot of the times this this is going to be incrementally done. So you have small amounts of energy that are going to be transferred. It's not just one whopping big wave of energy that is transferred to these molecules. So this is going to be catalyzed by a number of uh enzymes uh and we'll focus primarily on the first one the dehydrogenases. So, as their name implies, they are removing hydrogen atoms. And as you remove hydrogen atoms from a molecule, we're also removing the associated electrons that were part of a chemical bond. So, we're going to highlight the importance of that on the next slide. Okay. Oxidases are going to transfer oxygen from one molecule to another. Now the action of these enzymes is going to be facilitated by co-enzymes. We talked about how vitamins can be used as very important building blocks to make co-enzymes. So we'll talk about these two co-enzymes NAD+ and FAD because they're going to be carriers of these high energy electrons and hydrogen. So let's look at sort of a stepbystep chemical reaction. So let's say I have this organic molecule where the gray circles are carbons. Along comes my dehydrogenase and it's going to break that chemical bond and it's going to break that chemical bond. So what do I get as a result? Well, I'll have that carbon carbon carbon oxygen and the electrons that were part of that first chemical bond. The carbon will just have one and the oxygen will just have one. the hydrogen that was part of that chemical bond, it gets its electron back as well. So I'm drawing these electrons in red just to show you that these are high energy electrons because the energy that was found initially in the chemical bond goes back to the hydrogen and goes back to that carbon. And over here it goes to the hydrogen and goes to the oxygen. But the problem we face here is now both that oxygen and that carbon are unstable. They don't have a complete shell. And so what happens is we repurpose the energy that is found in this high energy electron and we pair up and share these electrons, which is why you have a new bond formed in this molecule. So, we've repurposed that energy. So, this organic molecule has lost some energy. That's why it's oxidized. Well, what about the hydrogens? Well, recall hydrogens have very low electro negativity. they don't really hold on well on their own to electrons. So they would lose that energy fairly quickly and this is where the co-enzyme comes in. Think of the co-enzyme as having a higher electro negativity and it will basically strip away the electrons from the hydrogen. So if I have my co-enzyme NAD+, it has two binding sites for electrons. So let's say here's my hydrogen, here's my hydrogen, and each of those hydrogens has a high energy electron. So basically what's going to happen is it's going to strip away the high energy electrons. So NAD+ gets reduced. Well by picking up two electrons now technically this is not NAD+ but it's NAD minus and the two hydrogens are now an H+ and an H+ right? They haven't lost their proton. They've only lost an electron. Well, one of the hydrogens actually gets attracted to the NAD carrying the two electrons. So, this is why we say NAD+ gets reduced and becomes NADH and it's carrying two high energy electrons. So these electrons again I can't stress enough are not just regular electrons they're high energergy electrons. So we've transferred the potential energy that is found right at the end of the day with these oxidation reactions. What we have done is we've transferred energy the chemical excuse me the the potential energy from a chemical bond into a highly excited electron. So that has potential energy as well. Now, if we didn't do anything with this, eventually the energy from that electron would be lost as heat or as light. But biological organisms will repurpose that energy to do work, right? The three types of work that we talked about earlier. So, in the next video, we'll talk about some of these chemical reactions that ultimately allow us to produce ATP.