At this point, we are pretty darn familiar with the cellular respiration equation. It's C6H12O6 plus O2, and the products are carbon dioxide, water, heat, ATP, and heat. All right, so this chemical reaction, we know that with chemical reactions, the things lifted to the left are reactants. and those are things that are needed for the reaction to occur, and then things listed to the right are products, what you have after the reaction is done. So based on that information, it stands to reason that we need both glucose and oxygen in order to do cellular respiration.
We've talked about the role that both of these play in the last video, the summary. We said the glucose is the energy-rich molecule that's being broken down, and we have mentioned before that you can use things other than glucose to harvest energy. but that's the one we've been focusing on.
And you need oxygen because oxygen is the final acceptor of the electron transport chain and without it no oxidative phosphorylation. Okay now here's where things are a little bit different than what we've portrayed so far. It's true that you need oxygen to do oxidative phosphorylation of cellular respiration, but there's another form of respiration that can occur even when oxygen is not available.
If oxygen is not available, an alternative form of respiration can occur called fermentation, or also known as anaerobic respiration. Those mean the same thing. You need oxygen for the cellular respiration we've been talking about, which is called aerobic respiration, but you do not need oxygen for anaerobic respiration.
Now there's a big downfall to not having oxygen and that is the amount of energy that's produced. We said on the previous slide that the ATP output for aerobic respiration, what we've been talking about up till now, is about 38 ATP per one glucose molecule. Whereas the overall output roughly for fermentation, aka anaerobic respiration, it hails in comparison.
It's only around 2. So your energy output is vastly different when oxygen is not available. And a good question is why? Why is there such a big difference, a difference of 36 ATP per glucose molecule just because oxygen is not around? Well, let's remember what oxygen is used for.
right? Oxygen is the final acceptor of the electron transport chain, right? Now, we've talked about four sort of steps to cellular respiration. I'll put those in blue. We got glycolysis.
I'm gonna have to run out of room here one minute. All right, we've got glycolysis. We've got And actually, sorry, one more thing.
I'm going to put it down here. All right, we've got glycolysis. We've got pyruvate oxidation.
We've got the Krebs cycle, a.k.a. the citric acid cycle. And we have oxidative phosphorylation. All right, the oxygen is needed for this step right here, right?
It's got to be the final acceptor of the electron transport chain, which is what this step is all about. Incidentally, that's also the step that makes the vast majority of the energy, right? We said about 34 ATP are made in oxidative phosphorylation, and you only got about two in glycolysis and about two in the Krebs cycle.
When oxygen's not available, oxidative phosphorylation... doesn't occur and neither does the Krebs cycle because the Krebs cycle is more or less build up to the oxidative phosphorylation. So when you don't have oxygen you are restricted to glycolysis only. This is what you got when oxygen is lacking and that means that your energy output is a lot less because you're only undergoing the first step of cellular respiration that would be in aerobic respiration and the output there is only a net of 2 ATP. Not only is there a difference in energy output, there's a difference in what the end product is.
The electrons eventually end up with oxygen and then form water in the end. aerobic cellular respiration, which we saw kind of as we walked through the steps. But again, this water is made during oxidative phosphorylation. If oxidative phosphorylation isn't happening, you're not going to have water as your end product.
Instead, you're going to end up with a different end product. What the end product is depends on what organism type this is happening in. So for instance, for animals, the end product is known as lactic acid.
Animals are able to undergo some anaerobic respiration and some aerobic respiration. When they undergo anaerobic respiration, they can produce this lactic acid as their final product. Yeast, another type of organism that's able to undergo anaerobic respiration, their final product is ethanol. And actually, ethanol is what gives alcoholic beverages their alcohol content.
For instance, if you've ever known anybody who's brewed beer before, yeast is a primary ingredient in beer. What you do is you add the yeast, and you make it so that it doesn't have access to oxygen, and causes the yeast to undergo anaerobic respiration. And when it does that, it produces ethanol, which gives the alcoholic content to the beer.
And it's also, different other organisms are used anaerobically to produce other kinds of alcoholic beverages. Now, the fact that animals... can do anaerobic respiration, but we know that we also have to be able to do aerobic respiration, kind of leads to this thought that, like, can all organisms do both?
Is it kind of just, if oxygen's there, yay, do aerobic. If oxygen isn't there, boo, do anaerobic. Not exactly. There's different kinds of organisms. So facultative anaerobes can do either aerobic or anaerobic.
And for them, what they do is going to be contingent on whether or not oxygen is available. So for instance, animals are facultative anaerobes. If oxygen's available, they're going to do aerobic respiration. If oxygen becomes depleted, they're going to do anaerobic respiration.
Now, just to kind of distill, like to get to dismiss a... A road you might be going down, you might be thinking, oh, we can do respiration even without oxygen? We don't need oxygen.
No, you need oxygen. You can do anaerobic respiration, but the energy output is so much less that it's not enough to maintain life in most organisms. So if oxygen is gone for a long enough period of time, which for most organisms is only a couple of minutes, your cells are not going to be able to do...
produce enough ATP to keep up and you will die. But it's a good way of supplementing. So for instance, if you will start doing really heavy exercise and you're working out heavily, your breath, your breathing might not be able to keep up with the oxygen that you need to bring in for cellular respiration.
And at some point in that heavy exercise, some of your cells might start doing anaerobic respiration and the others that have access to the oxygen you are inhaling can continue to do aerobic. That's often why people who are working out start to feel kind of like a burn in their muscles when they're really pushing themselves. It's because when they're really, really pushing themselves, some of their cells start undergoing anaerobic respiration because their body can't bring in oxygen fast enough for every cell to undergo cellular respiration as quickly as it's trying to.
And when it starts undergoing anaerobic respiration, lactic acid is produced in those cells, usually in muscle cells that are working hard. And if enough of that lactic acid builds up, that can cause a burning sensation in the cell. So that's kind of the burn you feel when you're really pushing it at the gym.
Yeast cells are also facultative anaerobes, so they could switch back and forth between aerobic and anaerobic respiration, depending on if oxygen was available. Now, there are some organisms called obligate anaerobes. They can only do anaerobic respiration. Now, you're probably thinking to yourself, this does not sound advantageous, right? You can only do the mechanism of respiration that has a lower energy output.
Sucks to be you. Well, here's the thing. Obligate anaerobes are usually, they usually have low energy requirements as is.
I mean, we're talking about, a lot of times, single-celled organisms that don't require a lot of energy. So it's not really a big deal that they aren't freeing up as much. And a lot of them live in places where oxygen isn't even available, so it wouldn't matter if they could do aerobic. They wouldn't be able to because oxygen would be limited.
Overall, the amount of energy in the glucose isn't changing whether aerobic or anaerobic respiration is happening. The glucose still possesses the same amount of potential energy regardless. What's different is how much of that energy is being freed up.
In aerobic respiration, the maximum amount of the energy stored in the glucose is being harvested as ATP. Whereas with