Intro to Citric Acid Cycle: Study with Video Lessons, Practice Problems & Examples

The Citric Acid Cycle, also known as the Krebs Cycle or TCA Cycle, is essential for energy production from macromolecules like proteins, carbohydrates, and lipids. It begins with Acetyl CoA combining with oxaloacetate to form citrate. The cycle progresses through stages, producing NADH, FADH₂, and ATP, while regenerating oxaloacetate. These high-energy molecules are crucial for the electron transport chain, leading to significant ATP synthesis. Understanding this cyclical process is vital for grasping cellular respiration and energy metabolism.

Intro to Citric Acid Cycle Concept 1

Everyone, let's take a look at an intro to the Citric Acid Cycle. Now, the Citric Acid Cycle, AKA the Krebs Cycle or the TCA Cycle, is a central stage in energy generation from food. Here, it oxidizes the Acetyl group of Acetyl CoA to produce high energy molecules, ATP, NADH, and FADH₂. So, if we take a look here, this is an overview of catabolism where we have our macromolecules initially, which are proteins, carbohydrates, and lipids. In stage 1, they undergo hydrolysis to create these monomers here, which are our amino acids, glucose, and fatty acids.

They can then undergo Acetyl CoA formation to make Acetyl CoA. So we think of this as stage 2. In stage 3, that Acetyl CoA can enter the Citric Acid Cycle. It will produce our high energy molecules of NADH and FADH₂, ATP, as well as some CO₂. Now, NADH and FADH₂, we sometimes call them in biology, electron carriers.

They will be shuttled into stages 34 of Catabolism, which deals with the electron transport chain and oxidative phosphorylation. Going through all of that would help us to get a boon—a large amount of ATP being created at the end. Now, here we're going to say that NADH and FADH₂ are utilized in the electron transport chain to produce energy required for ATP synthesis. Right? So, this is just a very big overview of catabolism, but remember, it's this part here.

Once we have the creation of Acetyl CoA, where we can go into the Citric Acid Cycle. Within the Citric Acid Cycle, we have this cyclical reaction that's occurring. We're going from one compound to another compound as a result of this, creating these high energy molecules as well as carbon dioxide as well. Alright.

So as we talk about the intro to Citric Acid Cycle, just think about catabolism as an overview and we're focusing in on this portion of the Citric Acid Cycle.

Intro to Citric Acid Cycle Example 1

Which of the following statements about the Citric acid cycle is incorrect? The Carbon dioxide produced from the Citric acid cycle is a product of oxidation. That is true. Remember, we have Acetyl CoA going into the Citric acid cycle. We make NADH, FADH₂, but we also create energy and Carbon dioxide by oxidation.

The Citric Acid Cycle oxidizes the Acetyl group of the Acetyl CoA to produce energy. This is true. Oxidation reactions in the Citric Acid Cycle produce Coenzyme NAD⁺ and FAD. This is not true. We oxidize our Acetyl CoA in order to reduce these two to create NADH and FADH₂.

So this is incorrect. Then finally, the Citric Acid Cycle is part of the common metabolic pathway. Remember, the common metabolic pathway is stages 3 and stage 4 of our food catabolism. The Citric Acid Cycle being part of stage 3 means that it is part of the common metabolic pathway. So this is true.

Right. So here the only statement that is incorrect would be option C.

Intro to Citric Acid Cycle Concept 2

Now, the citric acid cycle consists of multiple steps, which can be grouped into 3 phases. Now, here we're going to say the first phase is our citrate formation. Here, the acetyl group from Acetyl CoA, so we're starting with Acetyl CoA here, reacts with our oxaloacetate, so reacts with this, in order to form our citrate. Here, this is a cyclic metabolic pathway. So, this here is interacting with what's coming into the Citric Acid Cycle in the form of Acetyl CoA.

Phase B deals with succinyl CoA formation. Here we're going to have the isomerization and oxidation reaction that converts our citrate into succinyl CoA. So here's succinyl CoA. Now, as a result of this, in this stage, we have also the production of NADH plus carbon dioxide. Now, going into stage C or the 3rd stage here, we have oxaloacetate regeneration.

So, here we're going to say we have the hydrolysis and oxidation reactions convert our succinyl CoA back into oxaloacetate. Here, so basically we're going from succinyl CoA and we're going towards oxaloacetate. As a result of this, we're also creating NADH, FADH₂ and some ATP in the form of energy. Now remember, this NADH and FADH₂, later on, will go into the ETC or electron transport chain to create even more energy in the form of ATP.

Intro to Citric Acid Cycle Example 2

Identify each of the following statements about the Citric Acid Cycle as true or false. So for A, it says, Phase C of the Citric Acid Cycle includes reactions that regenerate oxaloacetate. That is true. Because then it reacts with Acetyl CoA at the beginning of the Citric Acid Cycle to create Citrate. Which leads us to the next statement.

The first phase of the Citric Acid Cycle, yes, uses Acetyl CoA and Oxaloacetate to produce Citrate. This is true. The Citric Acid Cycle relies on reduction reactions to produce high energy molecules. That is not true. It relies on oxidation reactions to produce high energy molecules in the form of NADH plus FADH₂.

And then finally, oxidation reactions in phase c produce CO₂. Now in phase c, we're producing NADH, FADH₂, as well as ATP. So this would be false. Alright. So this is how we could describe each of the statements within this given example question.

Which one of the following substances is a part of both phases A and C of the citric acid cycle?

• A. Succinyl CoA
• B. Oxaloacetate
• C. Acetyl CoA
• D. Citrate