In the first part of this lecture on cellular respiration, we talked about aerobic respiration and in the second part we're going to talk about the anaerobic respiration and fermentation. Okay, let's begin. Now if you remember from cellular respiration, it was broken down into glycolysis, citric acid cycle, and oxidative phosphorylation.
And this is aerobic respiration. And most of the ATP was generated here in this oxidative phosphorylation, right? Most ATP was generated here. And we know that oxidative phosphorylation requires oxygen.
And here, let me draw this out. Here's free energy. And this is the electron transport chain.
Right, electron transport chain. And when electrons come in, and successfully go down the electron transport chain, it loses energy. And the final acceptor of these electrons is oxygen.
And it's converted to water. So we know that oxidative phosphorylation requires oxygen. And without the electronegative oxygen to pull down electrons, right, to pull down this electron at the last step, the electron transport chain will stop.
Oxidative phosphorylation will stop. So without electronegative oxygen, this would stop. But there are two general mechanisms for cells to oxidize organic fuels without oxygen. Okay, two mechanisms for cells to oxidize organic fuels without. oxygen.
One is called anaerobic respiration and two is called fermentation. So what's the difference between anaerobic respiration and fermentation? Well, the anaerobic respiration uses the electron transport chain.
Fermentation doesn't use. electron transport chain. Remember anaerobic respiration is respiration without the presence of oxygen. So what is the final electron acceptor? What is the final electron acceptor if not oxygen?
What? Well, there are organisms that use anaerobic respiration, use another electronegative molecule to draw the electrons down. Uses another electronegative compound as a final.
electron acceptor. An example of that is the sulfate reducing marine bacteria. These, instead of using oxygen to draw the electrons down at the final step of the ETC, it uses sulfate ions.
And instead of water being the last waste product, the last waste product here will be hydrogen sulfide. This gas smells a little bit like rotten eggs. So if you've ever walked through a salt marsh or a mud flat and you've smelled this rotten egg smell, it's these bacteria that are present that's doing anaerobic respiration. And fermentation doesn't do any of this. It does not use electron transport chain.
And the only thing that it uses to oxidize glucose is NAD+, like in glycolysis. So let's talk about fermentation in a little bit more detail. So fermentation is glycolysis plus reactions that regenerate NAD plus by transferring the electrons from NADH to pyruvate or derivatives of pyruvate.
And we'll go through that in a little bit more detail. Let's just put that down. Fermentation is glycolysis plus reactions to regenerate NAD+, by transferring Electrons from NADH, the reduced form of NAD+, to pyruvate or derivatives of pyruvate. There are many different types of fermentation. But here are the two common ones.
We're going to talk about alcohol fermentation. And the second one is this, lactic acid fermentation. Okay, let's talk about alcohol fermentation.
So pyruvate, this is glycolysis, right? Glucose converted into pyruvate. And then pyruvate is actually converted to ethanol in two steps. First, it's converted to... acetaldehyde while releasing a molecule of carbon dioxide going from a three carbon molecule to a two carbon molecule and then acetaldehyde is reduced to form ethanol And this is where NAD plus is regenerated.
The electrons are given two ethanol, and now NAD is oxidized and ready to go back into doing glycolysis again and enter the cycle. So let's write this down. So pyruvate is converted to ethanol, and this is going to be my shorthand for ethanol, E-T-O-H, in two steps.
Okay, step one releases CO2. from pyruvate to form acetylaldehyde. And two, acetylaldehyde is reduced by NADH to make ethanol.
Again, this is to regenerate NAD+, so that it can participate in glycolysis again. And many bacteria do this when there's no oxygen present. Yeasts do this as well. For example, yeast, a type of a fungi.
We humans have taken advantage of this in brewing, in winemaking, in baking. Brewing, winemaking, baking. How so in baking?
Well this carbon dioxide being released creates these little bubbles and it raises the dough. And in brewing and winemaking, the final ethanol product is what people consume. Ethanol for consumption and CO2 bubbles for raising dough. And as you can see in this process, only two ATP molecules are made.
And we'll talk about ATP production later. The second way that fermentation can occur, amongst many other ways, is that the pyruvate gets directly converted to lactate. a three carbon molecule. Right? Ethanol is a two carbon because acetaldehyde was two carbon.
And this because it's going from a three carbon to three carbon molecule no CO2 is made. There's no loss of carbon. Lactic acid fermentation. Pyruvate is directly reduced by NADH to form lactate. An important characteristic is no CO2 produced, because lactate is a three carbon, one, two, three carbon compound.
And we as humans have also used this type of fermentation in processing milk products. So for example, bacteria, yeast, or other fungi. This type of fermentation has been used to make cheese and yogurt. Also, the human muscles can also undergrow lactic acid fermentation when oxygen is scarce.
When O2 is scarce. It will undergo lactic acid fermentation to produce ATP from sugars. So when the oxygen is scarce, e.g. strenuous exercise, sugar catabolism outpaces oxygen supply.
And then cells switch to lactic acid fermentation to make ATP. And what is really cool is that lactate... goes into the blood and gets to the liver where it is converted back to pyruvate and then this pyruvate enters the citric acid cycle in the liver. and more energy is derived from this. And it used to be a common misconception that when you worked out hard and the next day your muscles hurt it's because of the lactic acid buildup in your muscles.
That is a misconception that people have. It's not the lactic acid from it. is just probably just general muscle damage. So now let's compare fermentation with anaerobic respiration to aerobic respiration.
Aerobic respiration versus anaerobic respiration. Compare that to fermentation. Well, let's list those similarities. Well, all three use glycolysis, making pyruvate from glucose.
And they will net. 2 ATP. It costs 2 ATPs and you get out 4 ATP, so you get a net 2 ATP gain. And all three of them use NAD plus as the oxidizing agent. All three use NAD plus in glycolysis.
as oxidizing agent that accepts electrons. Now let's talk about the differences. The difference is the mechanism of oxidizing NADH back to NAD plus to sustain glycolysis.
Okay. Fermentation, the final electron acceptor, is an organic molecule. Right?
In pyruvate is the electron acceptor in the lactic acid fermentation, and acetaldehyde is the electron acceptor in... Alcohol fermentation. In cellular respiration, let's talk about NADH's transfer to the electron transport chain.
electrons to the electron transport chain. And then electrons move stepwise down a series of redox reaction. Right? Every time the electrons are transported down the electron transport chain, it is a redox reaction. Something is getting reduced, something is getting oxidized to the final electron acceptor that is electronegative.
because we need something that wants to accept electrons. And in the case of aerobic respiration, that final acceptor is oxygen. In anaerobic respiration, that's another electronegative. molecule like sulfate.
And in the case of aerobic respiration, we said it could make up to 32 ATP molecules, right? Fermentation only makes two ATP molecules. per glucose. So when you compare aerobic versus fermentation, 16 times more ATP is made. Let's talk a little bit more about organisms that use fermentation or aerobic respiration, or both.
So, there are organisms that can carry out only fermentation or anaerobic respiration. And they're called obligate anaerobes. can only carry out fermentation or anaerobic respiration. And in fact, oxygen is toxic to them. They'll die in the presence of oxygen.
An example of such an organism is a bacteria that causes gingivitis. gum disease. Another example is a bacteria that causes botulism.
When you went out shopping people say do not buy dented cans because dented cans can have botulism in it and if you eat it it produces a poison that will kill you and the cosmetic industry has used this toxin for beauty purposes right? Botox injections that's a poison that's made by bacteria that wants to kill you so think about that next time you are tempted to do injections into your face Some cells can only do aerobic respirations. These are obligate anaerobes.
So only aerobic respiration. Those are, an example of it is our neurons, or cells in the brain, that can only do aerobic respiration. No fermentation. And some can do both. They can switch between one or the other.
So some can survive on ATP from either fermentation or respiration. And these organisms are called facultative anaerobes. A cell type that exists in us that acts kind of like a facultative anaerobe is our muscles. We just talked about that. E.g. muscles.
They can take up the glucose and perform glycolysis and when they have pyruvate, they have a choice to make. If no oxygen is present, they'll undergo fermentation and take out 2 ATP. from a glucose molecule. But when oxygen is present, it will allow pyruvate to enter the rest of the cellular respiration, and you can get up to 32 ATPs generated from that step. So the sugar consumption during fermentation is much faster, right?
Because you're only getting 2 ATP out of a molecule. This is per glucose, right? Only getting 2 ATPs from glucose.
You have to burn a lot of glucose to have enough energy to survive. As opposed to if you had oxygen present, you sip glucose and you can... get a lot of ATP out of it.
So why is there this difference? Let's talk about the evolutionary significance of glycolysis and how cellular respiration is thought to have evolved. So ancient prokaryotes are thought to have used glycolysis to make ATP long before oxygen was present in our atmosphere.
Prokaryotes about 3.5 billion years ago, used glycolysis to obtain ATP from food. Oxygen levels only built up appreciable levels in our atmosphere about 2.7 billion years ago. Oxygen buildup in the atmosphere was thought to be about 2.7 billion years ago, almost a billion years after the oldest known prokaryote.
And those were made by the cyanobacteria, photosynthetic bacteria which produce oxygen as a byproduct. So this is why Glycolysis occurs in the cytoplasm because it's a remnant of when our earliest ancestors were prokaryotes. only when the endosymbiotic relationship happened when the earliest eukaryote ate the first prokaryote around when oxygen was present, only then when the rest of the respiration takes place in the organelle. So rest and respiration takes place. in organelles about one billion years after prokaryotes.
Okay, just to put this whole thing into context. Now, we've been focused on and talking exclusively about glucose and how we all derive energy from it, from glycolysis to citric acid cycle. But glycolysis and citric acid cycle... actually connect many many other different types of metabolic pathways.
And here is an overview picture of the complex nature and interrelatedness of these metabolic pathways. So here is a citric acid cycle. And here is the glycolysis.
You can see glucose here, pyruvate here, right? But there are other pathways that feed into the citric acid cycle. Instead of looking at this picture, we will just focus on the simplified version. And this goes to show the versatility of catabolism.
the ability to generate energy from food. We eat and obtain energy from fats, complex sugars like sucrose, proteins, different form of carbohydrates, starches, etc. Right? And all of these can be used by cellular respiration.
Let's look at carbohydrates. We talked about glucose, but there are other sugars that can feed into this pathway. If it's sucrose, it can be broken down into glucose. If it's glycogen, it can be broken down into glucose.
Carbs. Let's say glycogen. The polysaccharide that we use to store glucose in our muscles and liver. Stored in liver and muscle can be converted to glucose. Sucrose, again in the digestive tract, can be broken down into glucose.
Proteins. In order for them to be able to be used to generate energy, they must be digested into single amino acids. And Before they're used, the amino group must be removed. And the amino group becomes a waste product.
And the waste product can be urea and ammonia you find in our urine, etc. And these amino acids enter, as you can see, amino acids can enter glycolysis at many different steps as intermediaries. Converted to intermediary molecules. glycolysis and citric acid cycle. Fats!
Once they're broken down into glycerol and individual fatty acids, the glycerol can be, again, converted to become an intermediary in the glycolysis step. So glycerol converted for glycolysis, and the fatty acids They are broken down by a process called beta oxidation, which breaks down fatty acids into two carbon molecules, which then enters citric acid cycle, as shown here. Broken into two carbons. and enters the citric acid cycle.
And it enters it as acetyl CoA. Enters citric acid cycle as acetyl CoA. So glycolysis and citric acid cycle have many, many pathways feeding into it, not just the glucose that we've been talking about. And of course, every system has a mechanism to be regulated by.
And we're going to talk about that just briefly. Okay, if you have enough energy, right, you want to be able to shut down cellular respiration, slow it down, so you don't waste energy. So if enough ATP As shown here, there's a lot of ATP being built up. It's going to inhibit one of the enzymes in the glycolysis pathway. Phospho-fructokinase inhibited, thus inhibiting glycolysis.
Also, if enough energy citrate builds up via the citric acid cycle, as shown here and it also inhibits the same enzyme also inhibits phosphofructokinase i think that should be a c right there If not enough ATP, when you want to ramp up the system, you'll have a MP monophosphate, the spent version of ATP, and it's going to stimulate the same enzyme, phospho... fructokinase to turn on glycolysis. Okay, so that was cellular respiration, both aerobic and anaerobic, as well as fermentation in a nutshell.