Microbiology, part 22: Metabolism - Fermentation

Hi. I'm Cathy with Level Up RN. In this video, I'm going to continue my coverage of cellular respiration. In my last video in this video playlist, I covered glycolysis and the transition reaction. In this video, we are going to talk about the citric acid cycle as well as oxidative phosphorylation. At the end of the video, I'm going to give you guys a little quiz to test your understanding of some of the key points I'll be covering, so definitely stay tuned for that. And if you have our Level Up RN microbiology flashcards, go ahead and pull out your flashcards on the citric acid cycle and oxidative phosphorylation so you can follow along with me, and pay close attention to the bold red text on the back of the cards because those are the things that you are likely to get tested on.

As a reminder, cellular respiration is the process that cells use to break down glucose to produce ATP. Glycolysis takes place first, where one glucose molecule is converted into two pyruvate molecules. These two pyruvate molecules then enter the transition reaction where they are converted into two acetyl-CoA molecules. Then these two acetyl-CoA molecules will enter the citric acid cycle, which is also referred to as the Krebs cycle. The citric acid cycle is a sequence of biochemical reactions that occurs in the cytoplasm for prokaryotic cells and in the mitochondria for eukaryotic cells. During this cycle, the acetyl group from acetyl-CoA is attached to a four-carbon molecule called oxaloacetate to form a six-carbon citrate molecule. That citrate molecule is then oxidized in a series of steps.

So my understanding is that most microbiology classes do not expect you to memorize every step of the cycle, which is hopefully the case for you as well. However, chances are you do need to know what enters the cycle and what comes out of the cycle. So for each acetyl-CoA molecule that goes through the citric acid cycle, we get one ATP, three NADH, one FADH2, and two carbon dioxide molecules. But because one glucose molecule produces two pyruvate molecules, which are converted into two acetyl-CoA molecules, two turns of the citric acid cycle are required to process all the carbon from one glucose molecule. So the net output after two cycles is two ATP, six NADH, two FADH2, and two carbon dioxide molecules. And of note, the NADH and FADH2 molecules will go on to the electron transport chain, which we will be talking about next.

The last part of cellular respiration is oxidative phosphorylation, which is composed of two processes, the electron transport chain and chemiosmosis. Oxidative phosphorylation occurs along the inner membrane of the mitochondrion for eukaryotic cells and along the plasma membrane for prokaryotic cells. The input to this process is 10 NADH molecules, 2 FADH2 molecules, and a final electron acceptor. In aerobic respiration, the final electron acceptor will be oxygen. In anaerobic respiration, the final electron acceptor will be a substance other than oxygen, such as nitrate or sulfate. And as a reminder, the NADH and FADH2 molecules that are entering the electron transport chain were outputs from glycolysis, the transition reaction, and the citric acid cycle.

In the electron transport chain, high-energy electrons from NADH and FADH2 are passed from one protein complex in the membrane to another in a series of redox reactions. So reduction and oxidation reactions. And the energy that is released from these redox reactions is used to move protons, which are hydrogen ions, written as H+, from one side of the membrane to the other, forming a proton gradient. After being passed from one protein complex to another, the electrons are ultimately delivered to the final electron acceptor, which is oxygen in aerobic respiration. Oxygen combines with electrons and hydrogen ions to form water molecules. So we start with six oxygen molecules, and we end up with six water molecules. With anaerobic respiration, since the final electron acceptor is a substance other than oxygen, the final product will be something other than water.

The next process after the electron transport chain is chemiosmosis. During chemiosmosis, we use the energy from the proton gradient that was formed by the electron transport chain to drive the production of ATP. During chemiosmosis, hydrogen ions flow down the electrochemical gradient through ATP synthase, which is a protein complex in the membrane. In prokaryotic cells, hydrogen ions flow from the extracellular space into the cytoplasm. Whereas in eukaryotic cells, hydrogen ions flow from the intermembrane space of the mitochondria into the mitochondrial matrix. The amount of ATP that can be generated during oxidative phosphorylation will vary. During aerobic respiration in a prokaryotic cell, up to 34 ATP can be generated.

So if you're keeping track, we generated 2 ATP during glycolysis, 2 ATP during the citric acid cycle, and now 34 ATP during oxidative phosphorylation. So the total number of ATP that can be generated during aerobic respiration in a prokaryotic cell is 38 ATP. It's important to note that less ATP is generated in eukaryotic cells because additional energy is required to move substances involved in oxidative phosphorylation from the cytoplasm into the mitochondria of the cell. It is also important to note that less ATP is generated during anaerobic respiration as compared to aerobic respiration.

All right. It's quiz time, and I have five questions for you. Question number one, what molecule produced in the transition reaction is the input for the citric acid cycle? The answer is, acetyl-CoA. Question number two, how many ATP are generated from two turns of the citric acid cycle? The answer is, 2. Question number three, blank is the flow of hydrogen ions down an electrochemical gradient through ATP synthase. The answer is, chemiosmosis. Number four, what is the maximum number of ATP that can be generated during oxidative phosphorylation in a prokaryotic cell? The answer is, 34. And number five, where does the electron transport chain take place in a prokaryotic cell? The answer is, along the plasma membrane.

All right. That's it for this video. I hope it was helpful. Take care, and good luck with studying.