Hello. To understand cellular respiration, we have to understand two simple ideas, oxidation and reduction, and I'm going to break down this concept for you today. And if you find this video to be helpful, please do subscribe to my channel. The more subscribers the channel has, the more likely the video will be suggested to other people and might have a chance of helping them. So thank you for subscribing.
Now, oxidation and reduction reactions are at the heart of generating ATP. Well, what does that mean? Oxidation and reduction reactions transfer electrons. Okay, so electrons become transferred from one molecule to another. Now, the removal of an electron is called oxidation, and the addition of an electron is called reduction.
Now, these two always occur together, and it makes sense because the removal of an electron is oxidation. So, if a molecule is oxidated, those electrons have to go somewhere, and those electrons are transferred to another molecule. and that other molecule is reduced because it has gain electrons.
Okay? So oxidation reduction reactions always occur together. And so you'll hear them termed redox reactions. The R-E-D part is from the reduction part of the reduction oxidation reaction, and the O-X is from the oxidation part of the reduction and oxidation reaction. So keep this in mind, reduction and oxidation, they're just transferring electrons, and the molecule that loses the electrons is oxidized, and the molecule that gains those electrons is reduced.
Now one way that we can remember which is which is by using this mnemonic, oil rig, which the letters in oil rig help us remember that oxidation is loss of electrons, and reduction is gain of electrons. I don't know about you, but I was always a visual learner. So Let's take a look at a visual way to explain oxidation and reduction. Okay, so here we have molecule A, and you can see molecule A right now has its electrons. And during this reaction, molecule A loses its electrons.
So loss of electrons is oxidation. Substance A has been oxidized. Now remember, at the same time, those electrons have to go somewhere.
So reduction happens as well. And in this particular situation, we have substance B. Substance B picks up those electrons that were lost from substance A.
And so substance B, having those electrons, was reduced. OK, so substance A was oxidized when it lost the electrons. Substance B was reduced when it gained those electrons from substance A. Here's another way to look at it. Right.
We have two molecules, molecule A and molecule B. Molecule A loses its electrons and molecule B picks up those electrons. Okay, so substance A has been oxidized. Substance B has been reduced. And it's simple as that.
It's like playing catch, right? If you toss a ball to somebody, you're losing the ball. You're being oxidized.
You're losing the electrons. But somebody else hopefully is catching that ball. And so when they catch the ball or the electrons, they are reduced. So oxidation and reduction are really just about transferring electrons. In cellular respiration, there's an electron carrier molecule called NAD that is reduced or oxidized.
So there are reactions of cellular respiration where the food that we eat, the molecules that we eat, are going to be broken down and the electrons are going to be stripped from those food molecules. Well, as the food molecules lose their electrons, be oxidized, this molecule called NAD plus is going to pick up those electrons. OK, so NAD stands for nicotinamide adenine dinucleotide, which is kind of a mouthful.
And you see why we abbreviate it NAD instead. But really, it's just two nucleotides bound together. And this particular substance, NAD, can pick up electrons or it can pass along electrons.
So it can be. reduced or oxidized. NAD is actually made from niacin, which is vitamin B3.
And there's two forms of NAD, okay? NAD plus is the oxidized form. So NAD plus is ready to accept electrons. And after it does accept electrons, it becomes NADH, okay?
And that's the reduced form. Remember, gain of electrons is reduction. So NADH is the reduced form that has electrons. Here's another graphical way to look at this. Now, here's the molecule NAD+, and it still has room for these two electrons.
So when it is reduced, right, when it gains these electrons, NAD+, becomes reduced. to NADH. You want to see that again?
Here's the oxidized form NAD+. It has room for electrons, so it becomes reduced when it picks up these two electrons. And we call it NADH when it is reduced.
Now on the other hand, if NADH loses its electrons, well that's what oxidation is. So when NADH loses those two electrons it's picked up, we call it NAD+. During the reactions of cellular respiration, that glucose molecule gets broken down and rearranged.
And as that happens, it releases those electrons. The NAD plus picks up. So glucose is oxidized and NAD plus is reduced to NADH.
And that's what's going to happen. That's the whole point of cellular respiration is the food molecules we eat are going to be broken down and rearranged. During all these reactions of cellular respiration, the electrons are going to be harvested from our food molecules, and NAD plus is going to become reduced to NADH when it picks up all of those electrons.
So remember, oil rig, oxidation is loss of electrons, reduction is gain of electrons. So the food molecules are going to be oxidized. They're going to lose their electrons, and NAD plus is going to be reduced. to NADH.
Again, NADH is the one that's going to have those electrons. Now during the final stages of cellular respiration, NADH is going to finally become oxidized itself. It is going to lose those electrons to what we call the electron transport chain. And that's the last stage of cellular respiration. Now I want to specify something here.
And that's that we follow the electrons because it's the electrons that are going to be used in the electron transport chain during this last stage of cellular respiration. But when the oxidation of the food molecules occurs, it's actually two hydrogen atoms that are removed from the molecule. OK, and so I know you're thinking, well, if it's the hydrogen atoms that are removed, why are we focusing on the electrons and where do those come from?
Well, remember a hydrogen atom is made up of one proton and one electron. So we're following the electrons from the hydrogen atoms because it's those electrons that are going to become part of the electron transport chain that's going to generate ATP. So depending upon where you read about this, you may hear about the two hydrogen atoms being removed from the molecule that was once glucose. But it's...
Often the electrons that we follow, because those electrons are finally going to be given up by NADH during the electron transport chain. And that's finally when NADH is going to become oxidized back to NAD+. Okay, so it's sort of this circle.
NAD+, is available. It has room for electrons. So NAD+, is reduced to NADH as it gains those electrons. Now, NADH has the electrons and it can drop them off to something else. So NADH becomes oxidized back to NAD+.
So it's the cycle between NAD+, which can accept the electrons, that's the oxidized version, and NADH, which now has the electrons, that's the reduced version. So let's go over a couple practice questions just to use this information. Okay, here's question one.
During glycolysis, those are the first reactions of cellular respiration, the molecule G3P is blank to 1,3-bisphosphate glycerate, while NAD+, is blank to NADH. Now, we need to fill in the blanks with either oxidized or reduced. I've also drawn out a little illustration here that shows us that during glycolysis, This G3P goes through a reaction to become 1,3-bisphosphoglycerate.
Okay, so G3P becomes 1,3-bisphosphoglycerate. During that reaction, NAD plus becomes blank to NADH. So stop the video if you can.
See if you can fill in the blanks with either oxidized or reduced. And remember,... Oxidation is loss of electrons, reduction is gain of electrons.
Okay, so let's use oil rig to remind ourselves. Oxidation is loss, reduction is gain. Well, in this case, we know, maybe we start here, right, with NAD+.
NAD plus is the oxidized form, so it has room for those two electrons. So NAD plus picks up the electrons to become NADH. Now, it's gaining electrons, right? So reduction is gain of electrons. NAD plus becomes reduced to NADH.
So that's the answer for the second part of this question. Now, what then happens to G3P and 1,3-bisphosphoglycerate? Okay, G3P, if NAD+, is picking up those electrons, they must, it must be getting it from somewhere. So G3P must be losing electrons in the process of becoming 1,3-bisphosphoglycerate. Thus, G3P, if it's losing electrons, G3P is being oxidized.
Okay, so G3P is oxidized to 1,3-bisphosphoglycerate and NAD+, is being reduced to NADH. It picks up those electrons to become NADH. So now, NADH has the electrons that were released as part of the process. I want to point out a few things that might just help you in the future.
The first tip is that enzymes that remove hydrogens from molecules are called dehydrogenases. We know it's an enzyme because it ends in A-S-E. So dehydrogenases mean that they're removing hydrogens from a molecule. So when you see a dehydrogenase enzyme like we see here, that means that hydrogens are being removed from the molecule. So that's another hint that G3P is being oxidized.
It's losing electrons. The other thing I want to point out is that there's this H plus that always occurs when NAD plus is reduced to NADH. And that H plus means a hydrogen ion, okay, which is the same thing as a proton actually.
I want to mention this because you'll often see in the reactions of cellular respiration that whenever NAD plus is reduced to NADH, there's always an extra hydrogen ion that occurs at that time, okay? So keep that in mind that during the reduction of NAD plus to NADH, there's an extra hydrogen ion that's given off as well. Let's do another question. Here's question two. As the final step of cellular respiration, oxygen is blank to form water.
So is oxygen reduced or is oxygen oxidized? Pause the video, see if you can reason through this. And I put the little equation here as well so that you can sort of see what's maybe happening in this process of oxygen being converted into water. So let's take a look at this. Remember, oil rig, so oxidation is loss of electrons.
Reduction is gain of electrons. So during the final step of cellular respiration, the oxygen that we inhale is going to be combined with four hydrogen ions and four electrons to form water, H2O. And it's actually two molecules of water that are formed. So reasoning through this, is oxygen oxidized or is it reduced? Well, oxygen is gaining electrons.
right? It's gaining four protons, four hydrogen ions, and four electrons. So it's gaining electrons.
And reduction is gain of electrons. So oxygen becomes reduced to water molecules. So it's sort of interesting that during cellular respiration, we inhale oxygen, and that oxygen actually picks up electrons and hydrogen ions and becomes water.
Okay, so that's what redox reactions are. Reduction and oxidation. Oxidation is loss of electrons and the molecule that picks up those electrons is reduced.