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Oxidation and reduction in metabolism

Video transcript
I really want to go ahead and integrate some of the topics that you've learned in general chemistry with topics in metabolism. Specifically, from a general chemistry perspective, I want to review electrochemistry. And then I'm going to connect it to one of the biggest topics in metabolism-- the breakdown of glucose in a process called cellular respiration. And if you recall, cellular respiration describes the body's way of being able to efficiently produce the energy currency of the cell, which is ATP. Now admittedly, this is kind of an unconventional way to first present the topic of cellular respiration. But what I think is really neat and what I think you'll realize is that we're really killing two birds with one stone here. Because if you understand the concepts of electrochemistry, you also understand cellular respiration and vice versa. So let's go ahead and get started by returning to the topic of electrochemistry from general chemistry class. And let's go ahead and review a simple oxidation and reduction reaction between solid zinc and copper ions floating around in solutions. So the aq just stands for aqueous solution. And the products of this reaction are zinc ion and solid copper. Remember that whenever we're talking about reduction oxidation reactions, or as they're fondly referred to as to redox reactions, we're talking about the flow of electrons. We must ask the question, what is gaining electrons and what's losing electrons? So copper here, which is positively charged, is gaining electrons and therefore being turned into solid copper, which has no charge. So we say it's gaining electrons, which means it's being reduced. On the other hand, solid zinc is turning into a positive charge. So it's losing negative charge. So we say here that it's losing electrons. It's being oxidized. And another way to really see this simultaneous gain and loss of electrons is to note here that there is a flow of electrons from the zinc, which is losing electrons, to the copper ions, which are then reduced to solid copper. And this, of course, is where electrochemistry comes in, because electrochemistry allows us to isolate this flow of electrons by building what is called an electrochemical cell. So let's go ahead and build a electrochemical cell for this particular reduction oxidation reaction. So I'm going to go ahead and draw two containers, which, if you recall, are referred to as half cells, because they're each half of the entire electrochemical cell. And remember that they're connected to one another through a wire that we can isolate the flow of electrons. And there's also another component called a salt bridge, which connects these two half cells. Just as a brief review, remember that this allows the flow of ions between each cell so that there's not a buildup of charge from this movement of electrons. Now, let's go ahead and remind ourselves where these electrons are coming from. So remember, the electrons are flowing from the zinc, which is losing electrons, to the copper, which is gaining electrons. And this is really the ingenious part of the electrochemical cell. It allows us to separate what's losing electrons-- in this case, solid zinc-- from what's gaining electrons, which is the copper ions, and therefore allows us to isolate this flow of electrons through a wire. And of course, we also have zinc ions in solution on this side of the cell and solid copper. The take-home message is we're really separating what's getting oxidized from what's getting reduced. And ultimately, this flow of electrons through the wire, which is also referred to as current, allows us to perform energy requiring processes such as in lighting up a light bulb. Now, here's the most important point that I want to make in this video. In our body, we also isolate a flow of electrons during cellular respiration. But instead of lighting up a light bulb, we harness this flow of electrons to produce chemical energy in the form of ATP. So let's go ahead and take a look at how this works by looking at the overall reaction for cellular respiration. Specifically here, we're looking at the breakdown of glucose. So glucose is the chemical formula of 6 carbons, 12 hydrogens, and 6 oxygens. And even though it's broken down in many multiple steps, the overall reaction for the breakdown of glucose is glucose combining with oxygen to produce water and carbon dioxide. And I'm going to go ahead and put in our stoichiometry here. This overall reaction is also a reduction oxidation process that involves a flow of electrons, just like our zinc and copper example. The flow of electrons however may not be as clear here. So I just want to take a minute and review the oxidation and reduction from more of a biological perspective, by looking at this reaction. So in the zinc and copper example, it was clear what was gaining and losing negative charge. But here, we have to really think about electron density. And so when we look at this molecule of glucose, it is oxidized-- that is, it loses electron density to form carbon dioxide. And the way we can rationalize it-- so I'm going to go ahead and say that this is oxidized. If we look at the carbon, when it's attached to two atoms of oxygen, the oxygen atoms essentially steal the show, because they are much more electronegative than the carbon. So carbon has very little electron density. It lost a lot of it from before, when it was surrounded by hydrogens as well, which didn't steal the electrons as much. Now remember, if something's being oxidized, something must be reduced. In this case, our oxygen molecule, which is also referred to as the final electronic acceptor, is being reduced. It's gaining electron density. It's essentially gaining the electrons and the hydrogens that were lost from our glucose molecule. So here, we have a gain of hydrogen, and we form water. So the water is the reduced product of the oxygen. Now once again, we can summarize this flow of electrons by saying that the electrons flow from the glucose to the oxygen. So with this written out, I really want to go back to our electrochemical cell up here and point out that the solid zinc is really analogous to our glucose, because both of these are losing electrons. And the copper ion here is really analogous to our oxygen, because both of these are getting reduced. So I'm actually going to go ahead and erase the zinc and the copper and replace it with the glucose and the oxygen to essentially show you that there's an analogy here between the electrochemical cell and cellular respiration. Now clearly, I'm not being very technical about the creation of this electrochemical cell, mostly because, as you can guess, this really isn't what's happening in our body. But it's really this flow of electrons that our body harnesses. And instead of lighting up a light bulb here, which I'm going to erase here, our body uses this flow of electrons. It harnesses it to allow the body to convert ADP back into ATP, which is, of course, the entire goal of cellular respiration. And ATP can fuel all of the energy-requiring processes in our body. We can continue to extend this analogy between cellular respiration and the electrochemical cell we've drawn by pointing out that even though we don't have a wire or separate containers in our body, it turns out that our body has actually functional equivalents of both of these two things. So let's start off with a wire. So in our body, instead of a wire, we have something called the electron transport chain, which I'm going to abbreviate here as ETC. And this electron transport chain contains an array of proteins that readily accept and lose electrons. Now when it comes to having a functional equivalent for having separate containers, our body is able to compartmentalize many of the reactions involved in cellular respiration. And specifically, I want to point out, you'll run into the fact that cellular respiration, in its final step, which is called oxidated phosphorylation, occurs in the mitochondria. So I'm going to just draw an organelle that looks like a mitochondria. And the mitochondria has two kind of spaces, because it has two membranes. It has the inside space as well as the outside space. And by having the oxidation and reduction reactions of cellular respiration occurring at different locations, at the interface between these two spaces and regulated by different proteins, allows the body to efficiently isolate the flow of electrons. So to summarize and to make one final point, I want to reiterate that glucose is broken down in a series of step. And along the way, when it's oxidized, it forms various metabolites or bi-products that are more and more oxidized. And these metabolites essentially donate electrons at each step to molecules that are referred to as electron carrier molecules. And as a technicality, I want to point out that it's really these electron carrier molecules that ultimately donate electrons to the electron transport chain and ultimately to oxygens. So I'm going to go ahead and indicate this with an asterisk here and note that it's really the electron carrier molecules which directly donate electrons into this circuit.