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Video transcript
Normally when we talk about the production of energy in a cell, glucose and ATP are the main characters of the story. But in this video, we're going to talk about a behind-the-scene player called electron-carrier molecules that really do play a vital role in this energy-production process as well. But in order to talk about electron-carrier molecules, we need to first review the main character of our story. So let's start off with glucose. Glucose, which has the chemical formula of 6 carbons, 12 hydrogens, and 6 oxygens, we know is broken down in our body to produce energy. And specifically, this breakdown process is an oxidation process-- a process of losing electrons. And glucose is oxidized to carbon dioxide. And if the flow of electrons isn't very clear here to you, definitely review one of the previous videos on oxidation reduction from a biological viewpoint. But the key idea is to notice that this carbon is far more oxidized, or electron poor, when it is attached to highly electronegative oxygen molecule, than when it's attached to hydrogen. So that's the key idea here. Now in our bodies, this reaction does not take place in a single step. Instead, it takes place in many steps. And so I'm going to actually literally draw a staircase with many steps to indicate this. And I just want to note that this collective process of breaking down glucose in several steps is called cellular respiration, and that there are videos just devoted to the details of this process. Now we're ready to talk about our electron-carrier molecules. And we can think about our electron-carrier molecules a bit like a molecular shuttle. I'm drawing a picture of an actual shuttle here just to remind us of this analogy. At each step of glucose's breakdown or oxidation, a new breakdown product of glucose is formed that is given a fancy name of a metabolite. And just for short, I'm going to abbreviate the rest of these as "M." Now the key point to realize here is that as glucose is broken down, the metabolites are more and more oxidized. That is to say, they have less electron density. And this is where our electron carriers come in. So recall that energy is extracted from the oxidation process by harnessing the flow of electrons. And it is the job of the electron carriers to harness the electrons that are lost at each step of the breakdown process. And so I'm going to go ahead and draw a metaphorical shuttle for our electron-carrier molecule at each stage of glucose's breakdown. And these electron-carrier molecules are of course carrying the electrons that are lost from the oxidation process. Ultimately, all of these electron-carrier molecules are going to shuttle the electrons that are harnessed from the breakdown of glucose to something called the electron transport chain, which is in the mitochondria. And at this point, there are enzymes that can facilitate the transfer of these electrons to the final electron acceptor in our body, which is oxygen. And remember, it is this flow of electrons in the electron transport chain that allows the production of energy to produce ATP-- the cell's currency of energy. So another question that you might have is, why does the body such a complex mechanism that involves many steps and all of these electron-carrier molecules to fuel the production of ATP? Well, let's reconsider the chemical equation that describes the overall breakdown of glucose. So glucose combines with oxygen to produce carbon dioxide and water. And just so that we conserve our mass, let's put in our stoichiometry. This describes the overall breakdown of glucose. And we know that this reaction releases energy to fuel the production of ATP. Now another way to view this process is as a simple combustion process. And combustion is exactly what it sounds like. It's what you think of when you think of something burning. And a combustion process just requires a fuel. And in this case, our carbon rich or electron rich molecule of glucose can be considered a fuel. And it also requires molecular oxygen, which we also have here. So to reword the question that I just posed, why doesn't glucose, the fuel, spontaneously combust in one step, instead of being broken down into multiple steps inside our body? Let's first think about the consequences of combustion in our body if glucose spontaneously combusted in our body to produce a large burst of energy, likely in the form of heat. All of that would be unusable energy. It would be unusable because remember, the only energy that our body really is able to use is in the form of ATP. And luckily for us, this combustion processes does not take place in one step inside of our body. Because even though it is energetically favorable-- that is to say, it releases energy-- it is actually kinetically unfavorable. That is to say, it has a very high activation energy. And you know this actually intuitively, because the combustion or burning of sugar in food, only happens when you overcook something at a high temperature. And a high temperature is able to overcome the activation energy to allow this combustion process to occur. Now our bodies are, of course, at a temperature much lower than that needed for a combustion process, which is thankful. Because otherwise, we would be producing all of this unusable energy. Instead, our body overcomes the high activation energy of this reaction by using enzymes at each step of the reaction. So I'm going to abbreviate this here as "E." And there are two benefits of using a multitude of enzymes to break down glucose. The first benefit-- I'm going to write benefits of enzymes. The first benefit is that we're able to produce a large number of metabolites. And this is important, because it allows our body to essentially reshuffle all of the metabolic products into many different pathways, so that we can reuse and recycle these products as much as possible. And the second benefit of using many enzymes to break down glucose is that we have a slow and controlled oxidation of glucose, which allows us to harness all that energy, which is in the form of the electrons that are being oxidized in a very controlled manner. As opposed to the big burst of heat that we would get in a single-step combustion process. And electron-carrier molecules play a big role in this controlled oxidation process that is facilitated by enzymes, as you can see in our diagram above, because they are essentially serving as a temporary storage for all of the electrons that are being lost by glucose. And in fact, because of they're close association with the enzymes that facilitate the breakdown at each step, electron carriers are also called coenzymes. And coenzyme is exactly what it sounds like. It's a molecule or it's a chemical functional group that helps enzymes perform their function. Now the enzymes involved in the breakdown of glucose, for the most part, are in the class of enzymes that have a special name called dehydrogenases. And these dehydrogenase enzymes do exactly what their name implies-- they dehydrogenate the glucose. That is to say that they take away hydrogen. And when they're taking away hydrogens, they also take away electrons. And most often they do this by taking away two electron along with a proton, which is what chemists call a hydride. And this hydride has a negative charge, so it's often referred to the hydride ion. And just another way to think of this is as one proton plus two electrons. Now the two electron-carrier molecules are coenzymes that are most commonly discussed in the breakdown of glucose, are two molecules that go by the name of NAD and FAD. And I want to write down the reaction that occurs between these electron-carrier molecules and the electrons that they're accepting from the glucose molecule through this dehydrogenase enzyme. So first, it's notable that NAD has a positive charge in its most oxidized form, while FAD does not. And each co-enzyme reacts as two protons and two electrons each. In other words, reacting with a hydride ion plus an extra proton. And once the electron-carrier molecules accept the electrons in this way, they become reduced. And the reduction products are slightly different for each. In the case of NAD, it's reduced into NAD H. So notice that we only have one hydrogen. And the reason for that is NAD can only except to one hydride ion. On the other hand, FAD can accept both hydrogens, so it's reduced to FAD H2. The extra proton is just donated to solution in the case of NAD. And it is these molecules here-- these reduced form of our electron-carrier molecules-- that shuttle the electrons to the electron transport chain to allow for the production of ATP.