Microbiology, part 20: Metabolism - Cellular Respiration - Glycolysis & Transition Reaction

Hi. I'm Cathy with Level Up RN. In this video, I will begin my coverage of cellular respiration. In this specific video, I will discuss the differences between aerobic and anaerobic respiration, and I will go through the steps involved in glycolysis and the transition or bridge reaction. And then in my next video, we will 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 cellular respiration 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.

Cellular respiration is the process that cells use to break down glucose to produce ATP. Cellular respiration includes the following steps: glycolysis, the transition reaction, which is also called the bridge reaction, the citric acid cycle, which is also referred to as the Krebs cycle, and oxidative phosphorylation, which includes the electron transport chain, as well as chemiosmosis. Aerobic respiration is the type of cellular respiration used by aerobes, which are organisms that can live and grow in the presence of oxygen. During aerobic respiration, glucose and oxygen are converted into water and carbon dioxide. The final electron acceptor during aerobic respiration is oxygen.

Anaerobic respiration is the type of cellular respiration used by anaerobes, which are organisms that live and grow in environments without oxygen. Anaerobic respiration includes the same steps as aerobic respiration, but the final electron acceptor is a substance other than oxygen, such as sulfate or nitrate. And while aerobic and anaerobic respiration include the same steps, anaerobic respiration produces less ATP because the electron acceptors have a lower reduction potential compared to oxygen, meaning they are not as easily reduced, which ultimately decreases the amount of ATP that can be generated.

Fermentation, which we will be talking about in detail in another video, is an alternative way for glucose to be broken down in the absence of oxygen. Fermentation begins with glycolysis but does not include the citric acid cycle or oxidative phosphorylation. So some sources or professors may consider glycolysis followed by fermentation to be anaerobic respiration. But the experts and professors that I worked with in the development of our microbiology flashcard deck consider fermentation to be a separate process and not the same as anaerobic respiration. But definitely check with your professor or book to see how they define anaerobic respiration.

So whether we are talking about aerobic or anaerobic respiration, the first step in cellular respiration is glycolysis. Glycolysis occurs in the cytoplasm of the cell, and it is anaerobic, meaning it does not require oxygen. The input to glycolysis includes one molecule of glucose and two NAD+ molecules, which are electron carriers. Glycolysis is broken down into two phases. The first phase is the energy investment phase, and the second phase is the energy payoff phase. During the energy investment phase, glucose is split into two three-carbon molecules called glyceraldehyde 3-phosphate, or G3P. The energy used to split glucose into these two G3P molecules comes from two ATP molecules. During the energy payoff phase, these two G3P molecules are oxidized to produce two pyruvate molecules. And because every oxidation reaction is paired with a reduction reaction, we also have the reduction of the two NAD+ molecules during this phase. These two redux reactions produce a total of four ATP molecules.

So the net output for glycolysis is two pyruvate molecules, two ATP molecules, and two NADH molecules. The reason why we have two instead of four ATP is that even though we produced four ATP during the energy payoff phase, we had to put in two ATP molecules during the energy investment phase. The two pyruvate molecules produced during glycolysis will go on to the transition reaction, which we are going to talk about next. The two NADH molecules will go on to the electron transport chain, which is part of oxidative phosphorylation, which we'll be talking about in the next video in this video playlist.

The next step in cellular respiration after glycolysis is the transition reaction or bridge reaction. So the pyruvate that was produced during glycolysis cannot enter the citric acid cycle directly. It must first go through this transition reaction, which is essentially a bridge between glycolysis and the citric acid cycle. The transition reaction occurs in the cytoplasm in prokaryotic cells and in the mitochondrial matrix in eukaryotic cells. The input to the transition reaction is the two pyruvate molecules that we produce during glycolysis and two NAD+ molecules.

During this reaction, the carboxyl group is removed from each of the pyruvate molecules, which releases carbon dioxide. Then the remaining two-carbon acetyl group is attached to coenzyme A, forming acetyl-CoA. Also, during this process, the two molecules of NAD+ are reduced to two molecules of NADH. So the net output from the transition reaction is two acetyl-CoA molecules, two NADH molecules, and two carbon dioxide molecules. The two acetyl-CoA molecules will go on to the citric acid cycle, and the two NADH molecules will go on to the electron transport chain. One important thing to note is that ATP is not generated during the transition reaction.

All right. It's quiz time, and I have five questions for you. Question number one, what is the final electron acceptor in aerobic respiration? The answer is, oxygen. Question number two, what is the net ATP generated during glycolysis? The answer is, two. Number three, what is the net ATP generated during the transition reaction? The answer is, zero. Number four, what product from glycolysis becomes an input to the transition reaction? The answer is, pyruvate. And number five, where does glycolysis occur in the cell? The answer is, in the cytoplasm.

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

[BLOOPERS]

And then the two NADH molecules that were produced under two NAD+ molecules, which are-- oh, shoot. Oh no. I guess I held up the fingers too long.