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

The Citric Acid Cycle, also known as the Krebs Cycle, is essential for energy production from food. It oxidizes the Acetyl group of Acetyl CoA through three phases: citrate formation, Succinyl CoA formation, and oxaloacetate regeneration. This process generates high-energy molecules like NADH, FADH₂, and ATP, which are crucial for ATP synthesis in the electron transport chain. Understanding this cycle is vital for grasping metabolic pathways and energy conversion in aerobic organisms.

Intro to Citric Acid Cycle Concept 1

Now, the Citric Acid Cycle is also known as the Krebs Cycle or the TCA Cycle, and it's a central stage in energy generation from food. Here, it oxidizes the Acetyl group of Acetyl CoA. This represents our Acetyl CoA. When we say the Acetyl group, it is this portion that is in orange. And it does this to produce high energy molecules such as ATP, NADH, and FADH₂. We're talking about this. Now remember, we say that this is broken down into 4 stages. We have our proteins, carbohydrates, and lipids undergoing digestion in the first stage, and they are broken down into amino acids, monosaccharides, and fatty acids in the second stage. We then form our Acetyl CoA, which then goes into our Citric Acid Cycle or Krebs Cycle for stage 3 to create NADH and FADH₂. Now the NADH and FADH₂ are utilized in the ETC or electronic transport chain to produce energy required for ATP synthesis. So remember, in stage 3, these are generated and they go directly into the electron transport food catabolism. And now we're going to pay a little bit more attention to the food catabolism and the Krebs Cycle or 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 coenzymes NAD⁺ and FAD. This is not true. We oxidize our Acetyl CoA in order to reduce these two to create NADH plus 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 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.

Phases of the Citric Acid Cycle Concept 2

Now, the citric acid cycle consists of multiple steps which can be grouped into three phases. Now, here we're going to say the first phase is our citrate formation. Here, the Acetyl group from the Acetyl CoA, so we're starting with Acetyl CoA here, reacts with our Oxaloacetate, so reacts with this, in order to form our citrate. 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. The second phase deals with Succinyl CoA formation. Here, we have the isomerization and oxidation reactions that convert our citrate into succinyl CoA. Here's succinyl CoA. As a result of this, in this stage, we have also the production of NADH+CO2. Going into the third stage, we have oxaloacetate regeneration. Here, we are going to say we have the hydrolysis and oxidation reactions convert our succinyl CoA back into oxaloacetate. Basically, we're going from Acetyl CoA and going towards Oxaloacetate. As a result of this, we're also creating NADH, FADH2, and some ATP in the form of energy. Remember, this NADH and FADH2, later on, will go into the electron transport chain to create even more energy in the form of ATP.

Phases of the 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

Here’s what students ask on this topic:

What is the Citric Acid Cycle and why is it important?

The Citric Acid Cycle, also known as the Krebs Cycle or TCA Cycle, is a crucial metabolic pathway that generates energy from food. It oxidizes the Acetyl group of Acetyl CoA to produce high-energy molecules like NADH, FADH₂, and ATP. These molecules are essential for ATP synthesis in the electron transport chain (ETC). The cycle consists of three phases: citrate formation, Succinyl CoA formation, and oxaloacetate regeneration. Understanding this cycle is vital for grasping how aerobic organisms convert food into usable energy.

What are the main steps of the Citric Acid Cycle?

The Citric Acid Cycle consists of three main phases: 1) Citrate Formation: Acetyl CoA reacts with oxaloacetate to form citrate. 2) Succinyl CoA Formation: Citrate undergoes isomerization and oxidation reactions to form Succinyl CoA, producing NADH and CO₂. 3) Oxaloacetate Regeneration: Succinyl CoA is converted back into oxaloacetate through hydrolysis and oxidation reactions, generating NADH, FADH₂, and ATP. These high-energy molecules are then used in the electron transport chain to produce more ATP.

How does the Citric Acid Cycle contribute to ATP production?

The Citric Acid Cycle contributes to ATP production by generating high-energy molecules like NADH and FADH₂. These molecules carry electrons to the electron transport chain (ETC), where they undergo a series of redox reactions. This process creates a proton gradient across the mitochondrial membrane, driving the synthesis of ATP through oxidative phosphorylation. Additionally, the cycle directly produces a small amount of ATP (or GTP) during the conversion of Succinyl CoA to succinate.

What are the products of the Citric Acid Cycle?

The primary products of the Citric Acid Cycle are NADH, FADH₂, ATP (or GTP), and CO₂. For each Acetyl CoA molecule that enters the cycle, three NADH, one FADH₂, one ATP (or GTP), and two CO₂ molecules are produced. NADH and FADH₂ are crucial for the electron transport chain, where they help generate additional ATP through oxidative phosphorylation.

What role does Acetyl CoA play in the Citric Acid Cycle?

Acetyl CoA plays a central role in the Citric Acid Cycle by providing the Acetyl group that initiates the cycle. It reacts with oxaloacetate to form citrate, the first step in the cycle. This Acetyl group is then oxidized through a series of reactions, leading to the production of NADH, FADH₂, ATP, and CO₂. The energy stored in NADH and FADH₂ is later used in the electron transport chain to produce ATP.