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

The Citric Acid Cycle, also known as the Krebs Cycle, consists of three phases: citrate formation, succinyl CoA formation, and oxaloacetate regeneration. It involves a series of biochemical reactions that convert Acetyl CoA into high-energy molecules like NADH, FADH₂, and ATP, while releasing carbon dioxide. Key enzymes include citrate synthase, aconitase, and various dehydrogenases. The cycle is crucial for cellular respiration, facilitating energy production through substrate-level phosphorylation and oxidative phosphorylation, ultimately driving metabolic processes essential for life.

Phase A - Citrate Formation Concept 1

Now, the Citric Acid Cycle is a sequence of 8 biochemical reactions. For phase a, we have citrate formation. Here we're going to say phase a consists of the first reaction of the pathway. Here, we have Oxaloacetate, which was created in phase C. It then reacts or interacts with Acetyl CoA that comes into the Krebs Cycle or Citric Acid Cycle in order to form our citrate molecule. So again, we're creating this as a product in the form of Oxaloacetate. It then becomes interactive with Acetyl CoA for the first reaction in order to form our Citrate molecule. Alright. So let's click on the next video and see what exactly does this entail in the formation of Citrate.

Phase A - Citrate Formation Concept 2

So in step 1, we have citrate formation. Here, the Acetyl group from the Acetyl CoA combines with Oxaloacetate in order to produce citrate. Remember, we're going to say here that Acetyl CoA is basically this structure here, and it is this portion of Acetyl CoA that gets added here because the rest of the structure was this part right here. So we can see here that we have Oxaloacetate being changed into citrate. And then, in order to do this, we have water being involved, which means that we're going to have some type of hydrolysis taking place. We have Acetyl CoA just becoming our CoA enzyme with our thiol group fully formed because the Acetyl group has been transferred over in order to make citrate. All of this is accomplished with our enzyme Citrate Synthase. Now, remember here that Synthase basically catalyzes the synthesis of new compounds within the cell. And we're going to say here the energy that drives this reaction comes from the hydrolysis of this Acetyl CoA. The carbon-sulfur bond is high energy. So, we're using the water in order to cut or cleave that bond. That energy that we release is being used to drive the formation of citrate from Oxaloacetate. So keep this in mind, we're talking about step 1 of citrate formation.

Phase A - Citrate Formation Example 1

How many carbon atoms are added to oxaloacetate to produce citrate? Remember, we are undergoing hydrolysis of our Acetyl CoA, and it is this portion, this acetyl group that gets added to my oxaloacetate in order to form citrate. This Acetyl CoA which I've just put within this black box has how many carbons? It has 1, 2 carbons. So 2 carbon atoms are added to oxaloacetate in order to produce citrate. That means that option b would be our correct answer.

Phase B - Succinyl CoA Formation Concept 3

This video we're going to take an overview of Phase B of the Citric Acid Cycle, which deals with succinyl CoA formation. Here, we're going to say Phase B, which is highlighted in this yellow portion. It consists of pathways reactions 2, 3, and 4. In reaction 2, we have citrate becoming isocitrate. In step 3 or reaction 3, we're going from isocitrate to alpha-ketoglutarate. And to do that we also have the releasing of NADH, our electron carrier, as well as carbon dioxide. And then for reaction 4, we're going from alpha-ketoglutarate to succinyl CoA. This also has the releasing of another mole of NADH and carbon dioxide. So, we can see that within Phase B, we have 2 moles of NADH and CO₂ being produced, 1 of each happening in reaction 3, and again in reaction 4. Now that we've looked at the overview of Phase B, let's start going one by one for each one of these reactions and delve a little bit deeper into what causes the transformation of the starting material to the next step, the next reaction.

Phase B - Succinyl CoA Formation Concept 4

So in this video, we're going to take a look at reaction 2 of phase B of the citric acid cycle. Here, reaction 2 deals with isomerization. We're going to say that the tertiary alcohol in citrate must be isomerized to a secondary alcohol so that it can be oxidized. This is for oxidation. If we take a look here, we have citrate. Here is our carbon with the OH. That carbon is connected to three other carbons, and that's why it's a tertiary alcohol. Remember, tertiary alcohols cannot be oxidized. That's why we have to use our enzyme aconitase to change citrate into isocitrate. So the enzyme here, aconitase, isomerizes citrate to isocitrate. Now, all that happens is we had the OH here and the H here, and they're going to basically switch places. So now our OH is here and our H is over here. And if we're looking at where the OH is connected, we'd say that this carbon, which is our alcohol carbon, is only connected to two carbons. So, it is now a secondary alcohol. Secondary alcohols can be oxidized. So, reaction 2 is just changing our alcohol into a new alcohol that can undergo oxidation within the next reaction. Right. So keep that in mind for reaction 2 of phase B of the citric acid cycle.

Phase B - Succinyl CoA Formation Concept 5

Now, in reaction 3, we have isocitrate which represents a secondary alcohol that can be oxidized. So here we have oxidation and then decarboxylation. Remember, decarboxylation just means the loss of CO₂. Now, here we're going to say the enzyme that's going to be used is isocitrate dehydrogenase. It oxidizes isocitrate to alpha-ketoglutarate. For all oxidation reactions, we're going to be using a dehydrogenase enzyme. It's easy to remember the name because all it is is the substrate followed by dehydrogenase. The substrate here is isocitrate, so then add isocitrate dehydrogenase as the enzyme required to change it from isocitrate to alpha-ketoglutarate. Here in this process, we have NAD⁺ which is our oxidized form of our coenzyme. It's gonna be reduced to 1 NADH. So here it has become NADH, and also one carbon atom is lost as CO₂. Remember, decarboxylation means that we have the loss of CO₂. And this is the first location where we're going to have the creation of 1 mole of NADH and 1 mole of CO₂. Right. So, if we take a look here, isocitrate is a secondary alcohol. Because this is an oxidation, that secondary alcohol is transformed into a carbonyl group. Remember, the carbonyl group here is gonna be oxidized, and it changes into a carbonyl group which is C=O. This gives us our alpha-ketoglutarate.

Phase B - Succinyl CoA Formation Concept 6

Now, here we have reaction 4 of phase b of the citric acid cycle. Here we also undergo oxidation and decarboxylation. This is going to be the 2nd location where we lose yet another mole of NADH and also CO₂. And by losing means we're going to release it or produce it. Now, here we're going to say that our Alpha Keto Glutarate, what's the enzyme we use here? Remember for all these oxidation reactions, it's just going to be the substrate followed by dehydrogenase. So this would be Alpha Keto Glutarate dehydrogenase. It oxidizes our Alpha Keto Glutarate to succinyl CoA. So if we take a look here, we have our succinyl, our alpha ketoglutarate, and here we have our carboxyl group.

We're going to say in this process we use our alpha ketoglutarate dehydrogenase enzyme, and, what this is going to do, it's going to oxidize this and help us connect and form the highly energetic carbon sulfur bond. Remember, this highly energetic bond, if we can hydrolyze and break it, we can have the releasing of ATP. So we know that ATP will be formed at some point if we can successfully break this carbon sulfur bond. Now, in this process, we say that the oxidized form of our coenzyme, NAD⁺ is reduced to 1 mole of NADH. And then also we're going to say one carbon atom is lost as CO₂. Remember decarboxylation means the loss of CO₂.

So here we'd have our NAD⁺ become NADH. And then here we have our coenzyme a with its thiol group, and we have the loss of CO₂ as a result. So we lose the CO₂ but we have the coenzyme a being connected to this structure in order to make our succinyl CoA. So this will represent reaction 4 of phase b of the citric acid cycle.

Phase B - Succinyl CoA Formation Example 2

Each of the following reactions described below identifies a corresponding step of the citric acid cycle. The first one states that the oxidation of alpha-ketoglutarate produces succinyl CoA. From our previous discussions, we know that this is reaction 4 that occurs in Phase B of the citric acid cycle. Next, oxaloacetate is converted to citrate. This takes place in reaction 1, Phase A of the citric acid cycle because oxaloacetate interacts with acetyl CoA to form citrate.

Then, an oxidation reaction is catalyzed by isocitrate dehydrogenase. Isocitrate dehydrogenase is the enzyme used on isocitrate to convert it into alpha-ketoglutarate. This occurs in reaction 3 of Phase B of the citric acid cycle. Finally, the enzyme that catalyzes the isomerization of citrate to isocitrate is discussed. This reaction changes a tertiary alcohol in citrate to a secondary alcohol in isocitrate, preparing that secondary alcohol to be oxidized later on. This occurs in reaction 2 of Phase B of the citric acid cycle. These are the reactions or steps in which they would occur for the citric acid cycle.

Which enzyme oxidizes α-ketoglutarate in step 4 of the citric acid cycle?

• A. isocitrate dehydrogenase
• B. α-ketoglutarate oxidase
• C. succinyl CoA synthase
• D. α-ketoglutarate dehydrogenase

Which two steps of the citric acid cycle produce CO₂?

• A. 1 and 3
• B. 3 and 4
• C. 2 and 4
• D. 2 and 3

Phase C - Oxaloacetate Regeneration Concept 7

So in this video, we're going to take an overview of our Citric Acid Cycle and pay closer attention to Phase C. In Phase C, we have Oxaloacetate regeneration. Remember we can recreate oxaloacetate and then it can combine with our Acetyl CoA to make citrate yet again to go through the whole cycle again. Now here we're going to say Phase C consists of the pathway reactions of reactions 5 to 8. And here we've already talked about steps or reactions 1 to 4. 5 to 8 here, we're just continuing from succinyl CoA. We'd have the production of GTP, which is guanosine triphosphate. All it is there is used to generate ATP through substrate level phosphorylation. So here ADP can become ATP. So we have the generation of energy and then doing this changes succinyl CoA to succinate. Succinate can then undergo a change to fumarate and in the process release FADH₂. Yet another electron carrier. Now, here we can have the introduction of water, which changes fumarate into malate. And then finally going from malate to oxaloacetate, which we want to regenerate, we have the releasing of another NADH. Right. So here, we're going to go in greater depth when it comes to reactions 5 to 8. Here, we're just looking at a general overview of Phase C of the citric acid cycle.

Phase C - Oxaloacetate Regeneration Concept 8

Now, in step 5 or reaction 5, we have hydrolysis. Here, the enzyme succinyl CoA synthase hydrolyzes succinyl CoA to succinate. Now, here remember, hydrolysis means that we're going to cut or break or cleave a highly energetic bond and as a result energy is released. So energy will be released from the hydrolysis reaction and that produces GTP. Now, GTP, we're going to do hydrolysis of that in order to free up an inorganic phosphate group, which provides energy to produce ATP. Now, we have our succinyl CoA and our high energy bond is this carbon sulfur bond. Through hydrolysis, we're going to recreate our coenzyme A with its thiol group that's reattached and we're also going to produce GTP, which remember, that's going to undergo hydrolysis as well, freeing up an inorganic phosphate giving a phosphate group to ADP to create ATP, creating energy. This helps drive the reaction forward and that creates succinate here as our product. And remember, what's being used to do this is substrate followed by synthase. Since the substrate is succinyl CoA, the enzyme utilized is succinyl CoA synthase.

Phase C - Oxaloacetate Regeneration Concept 9

In step 6 or reaction 6, we have oxidation. The enzyme that we're going to use is the substrate name followed by dehydrogenase. Remember, these oxidation reactions that we're going to cover utilize the enzyme dehydrogenase. Here, the enzyme succinate dehydrogenase oxidizes our succinate to fumarate. Now, as a result of this, 1 FAD is reduced to 1 FADH₂. So here we have our succinate molecule, FAD is reduced to FADH₂ as a result through the utilization of our enzyme Succinate dehydrogenase. This creates our fumarate here. Now, remember, carbon must make 4 bonds. So, what's happening here, to make 4 bonds, these 2 carbons have to double bond. So, we can see here that dehydrogenases utilize FAD to convert carbon single bond carbon bonds to double-bonded carbon bonds. So that's what we do in terms of this particular step.

Phase C - Oxaloacetate Regeneration Example 3

Of the following statements, which is incorrect about the citric acid cycle?

• Reaction 5 of the cycle converts succinyl CoA to succinate. Yes, that is true. It's done through hydrolysis and the utilization of the enzyme succinyl CoA synthetase. So, this is true.
• Oxidation of succinate in reaction 6 produces fumarate. Yes, that is also true. We utilize FAD in order to do this.
• Phase c of the citric acid cycle does not result in the loss of any carbon atoms. Alright. So this is also true. Now, we may have instances where carbons that are single-bonded to each other become double-bonded where that's a loss of hydrogens, but we don't have the loss of carbon atoms.
• Energy released from the hydrolysis of succinyl CoA comes from the GTP. No. We hydrolyze our energetic Carbon-Sulfur Bond. This released energy helps us to create GTP. GTP later on is hydrolyzed, and its inorganic phosphate is given to ADP to make ATP. So, the energy doesn't come from GTP, the energy is used to create GTP. So this is what's incorrect.

Therefore, our answer is option d.

Phase C - Oxaloacetate Regeneration Concept 10

Now, step 7 or reaction 7, the enzyme Fumarate Hydratase, also called fumerase, converts fumarate to malate by adding water. Remember, hydration means the addition of water. In fumarate, we have our carbon double bonded carbon. Remember this pi bond can be broken so that things can be added to each one of these carbons. Here we use fumarate in order to add the water molecule. The water molecule will add an OH here and an H here. Here we've added the water to our fumarate and this transforms it into malate. Alright. So, step 7 or reaction 7 deals with hydration. The addition of water to our pi bond of fumarate in order to create malate.

Phase C - Oxaloacetate Regeneration Concept 11

Reaction 8 or step 8 is yet another oxidation reaction. Here, the enzyme malate dehydrogenase oxidizes malate to oxaloacetate. Here, we're going to say that 1 NAD⁺ is reduced to 1 NADH. If we take a look here, here is our malate. And what we have here is a secondary alcohol. Remember, secondary alcohols can be oxidized. As it's being oxidized, NAD⁺ becomes reduced. This is all facilitated through the use of the enzyme, malate dehydrogenase. Here, our -OH group becomes a double bond O. So, we've gone from an alcohol to a carbonyl group.

Phase C - Oxaloacetate Regeneration Example 4

The oxidation of malate to oxaloacetate. Alright, so, malate dehydrogenase catalyzes the oxidation of Malate to Oxaloacetate. Remember, the regeneration of Oxaloacetate happens in step or reaction 8, so that the whole cycle can go one more time if necessary. Next, Succinate loses 2 hydrogen atoms to yield Fumarate. In this process, 1 FAD becomes 1 FADH₂. This happens in step 6 of the Citric Acid Cycle. Succinyl CoA undergoes hydrolysis to produce succinate. So this here is the hydrolysis of succinyl CoA. This happens in step 5. And then malate is produced from the hydration of fumarate. This happens in step 7. So these will be the steps for each of the Citric Acid Cycle reactions.

Which one of the following enzymes catalyzes the addition of water to the C=C bond in fumarate?

• A. Fumarate reductase
• B. Malate synthase
• C. Fumarase (Fumarate hydratase)
• D. Malate dehydrogenase

How many final high-energy molecules are produced in phase C of the citric acid cycle?

• A. 2
• B. 4
• C. 1
• D. 3

Citric Acid Cycle Summary Concept 12

Hey everyone. So let's take a look at the citric acid summary. Here we're going to say the citric acid cycle degrades acetyl groups to produce carbon dioxide and high-energy molecules. Now, when we say high-energy molecules, we're referring to NADH, FADH₂, and ATP.

Now, here we have some memory tools to help us remember how many of these will be produced, where they're produced. So let's take a look at memory tool number 1. Here it says, the Krebs cycle is a big crab. Remember when it comes to the Krebs cycle or the citric acid cycle, we have phases A, B, and C.

Next, we have memory tool 2. Here we say there are 4 owls and 1 sea hawk in a circus ring. So here, what does exactly mean? Well, 4 owls, there are 4 oxidation reactions that occur within the citric acid cycle. And these oxidation reactions involve NADH and FADH₂. Here we say 1 CHAWC, 1 C, C here is referring to the letter C, which is phase C of the citric acid cycle. In phase C, this is where we have a hydrolysis reaction occurring. Hydrolysis helps to create ATP. And when we talk about a circus ring, we're talking about the cyclic nature of the citric acid cycle.

Now here we're going to say that there are 4 oxidation reactions total between phases B and C, each one has 2. So we have 2 oxidation reactions each in phases B and C, and then we're gonna say oxidation reactions yield NADH and FADH ₂. Next, we're going to say, hydrolysis reactions. Again, we said earlier, yield ATP. So there are just a couple of memory tools that'll help us remember what's going on in terms of the citric acid cycle or Krebs cycle. We have phases A, B, and C. We have 4 oxidation reactions. 2 of them happening in phase B. 2 of them happening in phase C. We have one hydrolysis reaction happening within phase C of the citric acid cycle. Now, remember, oxidation reactions help to yield NADH and FADH₂, and hydrolysis helps to yield ATP.

Citric Acid Cycle Summary Concept 13

So remember that one glucose molecule helps to create 2 acetyl CoA's. So that means we can go through the citric acid cycle twice. Now here, as we're going from the citric acid cycle, we're going from acetyl CoA in conjunction with oxaloacetate, we create citrate. Now, going from citrate to succinyl CoA, we have reactions 3 & 4. Each one makes 1 NADH and carbon dioxide. So through one cycle, we're making 1 NADH in reaction 3 and one NADH in reaction 4. Since we're going over this twice, that'd be 2 times 2, so we have 4 NADH's being created. Then we're gonna say here, going through the citric acid cycle once, we're gonna make 1 CO₂ in reaction 3 and 1 CO₂ in reaction 4. One cycle creates 2 CO₂s. But since we're doing this twice, it'd be 2 times 2, so we also make 4 CO₂s. Over here, ATP. Going through the citric acid cycle once, gives us one ATP. Going through it again, gives us our second ATP. So we make a total of 2 ATP. FADH₂, we make 1 FADH₂ in reaction 6, and since we're going through the citric acid cycle twice we make our second FADH₂. And then NADH, we create 1 NADH in reaction 8, and we're gonna say here, since we're doing this twice, we're gonna make another NADH. So that's 2 over here. So coming over here, our starting material again is 2 Acetyl CoA's. How many carbon dioxide do we make through the 2 rounds of the citric acid cycle? 4. How many ATP do we make in the 2 rounds? 2. FADH also 2. And then finally we make a total of 4 NADH's here, plus another 2 over here. So that would be 6 NADH's that are created as we go through the citric acid cycle twice. And then our end molecule from reaction 8 would be oxaloacetate. Right. So this is how we look at a summary when it comes to the citric acid cycle.

Citric Acid Cycle Summary Concept 14

Now remember, we classify our high energy molecules as being NADH, FADH₂, and ATP. A way for us to remember where they're made within the Citric acid cycle is with memory tool 3. And it says that under trees in a forest, there lived 5 ants and 6 flies. So 'under' stands for NADH. 'Trees' we say 'threes'. In a forest, 'forest' for 4, 8. There lived 5 ants and 6 flies. Alright. So 'NAND' is NADH. NADH is created in reactions 3, 4, and 8 of the Citric Acid Cycle. And we're going to say here, that gives us what? In one round of TCA, that's 3 NADHs, but we go through it twice, so that's how we come up with 6 NADHs. There lived 5 ants, so it is in reaction 5 that we make ATP. And then here, 'F' for flies is FADH₂. Which we make in reaction 6. So this is how we're able to remember what the high energy molecules created in the electron transport called the citric acid cycle are, and in which reactions they are made. So NADH is reactions 3, 4, and 8. ATP is reaction 5, and FADH₂ is reaction 6.

Now, what is memory tool 4? Well, beside these 3 we also have the generation of CO₂, carbon dioxide. Well, one CO₂ is made in reactions 3 and 4. So just remember 1234. So 1 CO₂ is created in reactions 3 and 4, giving us a total of 2 CO₂s per round of the Citric Acid Cycle, which is a total of 4 CO₂s when we go through the citric acid cycle twice. Right? So just keep these memory tools on hand. They'll help you remember what the high energy molecules are and which reactions create them.

Citric Acid Cycle Summary Example 5

Under this example question, it asks how many reactions in the citric acid cycle produce high energy molecules. Remember, when we say high energy molecules, we're referring to NADH, FADH₂, and ATP. What we have to do here is utilize our memory tool. Recall, "under threes in a forest, so 48, there lived 5 ants and 6 flies." Here, we're talking about NADH in steps 3, 4, and 8, where NADH is produced. "There lived 5 ants" refers to ATP, produced in step 5. This happens because GTP is used for substrate-level phosphorylation of ADP to convert it into ATP. Then, step 6, "f for flies," stands for FADH₂. Thus, steps 3, 4, 8, 5, and 6 contribute to the production of these high energy molecules. That means there are 5 total reactions in the citric acid cycle that produce high energy molecules.

Complete the following net equation for one turn of the citric acid cycle.

Acetyl CoA + __H₂O + 3 NAD+ + FAD + GDP + ____ ⭢  CoA + ___NADH + ___H⁺ + FADH₂ + GTP + ___CO₂

• A. 2 H₂O; ATP ; 3 NADH; 3 H⁺; 1 CO₂
• B. 3 H₂O; P_i ; 3 NADH; 3 H⁺; 1 CO₂
• C. 2 H₂O; ATP ; 3 NADH; 3 H⁺; 2 CO₂
• D. 1 H₂O; P_i ; 3 NADH; 3 H⁺; 2 CO₂

Which reaction of the citric acid cycle produces ATP directly?

• A. Reaction 3
• B. Reaction 5
• C. Reaction 4
• D. Reaction 6
• E. Reaction 8
• F. None of these

Remembering Citric Acid Cycle Concept 15

Now when it comes to remembering the citric acid cycle, just recall that each reaction of the citric acid cycle can be remembered by memorizing the intermediate metabolite names. Here our memory tool is: Oxford, city, ice crates, kegs, and such contain succulent foamy milk. Alright. So how does this work? Okay. So Oxford. So we're talking about oxaloacetate here. Remember that's produced in reaction 8. We're going to say acetylcholine is coming in and together they're going to help to make our citrate. Citrate, city. Then we're going to go to reaction 2 where citrate is going to become isocitrate, ice crates. Then going from isocitrate, that's reaction 3, we're going to kegs, which is alpha-ketoglutarate. Then we're going to say, going to 4, Alpha-ketoglutarate is going to become succinyl CoA. So such contain so succinyl CoA. Then in reaction 5 succinyl CoA becomes succinate. So succulent, succinate. Then we're going to say that it's going to undergo an oxidation, where succinate is going to become fumarate, foamy. Then we're going to say here, in reaction 7, we're going to say that fumarate becomes malate, milk. And then finally, reaction 8 is where we go from malate to oxaloacetate. So remembering this memory tool is just helping us remember the order of each one of these compounds, and the reactions for the Citric Acid Cycle.

Remembering Citric Acid Cycle Concept 16

Now when it comes to the citric acid cycle, the name of the enzyme can be predicted by knowing the substrate and the type of reaction. So here if we take a look at hint 1, hint 1 talks about reaction 1. We have citrate formation is catalyzed by citrate synthase. Remember, when we're synthesizing a new compound, we use a synthase. So this would be citrate synthase. Reaction 2, citrate isomerizes or undergoes isomerization and is catalyzed by aconitase. Now, next, hint 3, reactions 3, 4, 6, and 8. We're going to say all of these are oxidation reactions, and when it comes to oxidation reactions we say that the enzymes we're using are dehydrogenases. Hint 4, reaction 5 is a hydrolysis, where we can use that energy, harness that energy in order to create, to help push the reaction forward. So reaction 5 is hydrolysis, reaction 7 is a hydration, for adding water. Now, here we have an exception. Reaction 5 is catalyzed by a synthase though instead of a hydrolase. Right? So, these are just hints and shortcuts to help us remember the types of enzymes that are involved within the different steps of the Citric Acid Cycle.

Remembering Citric Acid Cycle Example 6

Write the name for the enzyme that catalyzes the first reaction of the citric acid cycle. Remember, in the first reaction or step 1, we have Acetyl CoA interacting with Oxaloacetate. Together they help to make citrate. Now, here we are synthesizing citrate and because we're synthesizing a new compound, that means we're going to use the enzyme synthase. It is citrate that we're synthesizing, so we'd have to use citrate synthase as our enzyme. This would mean that option d would be our answer.

Write the name for the substrate, enzyme, and product of reaction 6 of the citric acid cycle.

• A. Succinyl CoA, succinyl CoA synthase, succinate
• B. Succinate, succinate dehydrogenase, and fumarate
• C. Succinyl CoA, succinate dehydrogenase, and fumarate
• D. Fumarate, fumarase, and malate

Identify the two reactions that involve oxidation and decarboxylation.

• A. Reactions 1 and 5
• B. Reactions 3 and 6
• C. Reactions 6 and 8
• D. Reactions 2 and 7
• E. Reactions 3 and 4

Write the name of the substrate and the enzyme of reaction 8 of the citric acid cycle.

• A. Oxalate, oxalate oxidase
• B. Malate, malate dehydrogenase
• C. Succinate, succinate dehydrogenase
• D. Fumarate, fumarase

What is the name of the enzyme for the reaction that produces a GTP molecule in the citric acid cycle?

• A. Succinyl CoA synthase
• B. Succinate dehydrogenase
• C. α-ketoglutarate dehydrogenase
• D. Fumarate hydratase

Which reactions of the citric acid cycle produce NADH?

• A. Reactions 1, 2, and 8
• B. Reactions 2, 4, and 6
• C. Reactions 2, 5, and 7
• D. Reactions 3, 4, and 8
• E. Reactions 3, 4, and 6

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, is a series of eight biochemical reactions that occur in the mitochondria. It plays a crucial role in cellular respiration by converting Acetyl CoA into high-energy molecules like NADH, FADH₂, and ATP, while releasing carbon dioxide. These high-energy molecules are essential for producing ATP through oxidative phosphorylation, which powers various cellular processes. The cycle also regenerates oxaloacetate, allowing the process to continue. Overall, the Citric Acid Cycle is vital for energy production and metabolic functions necessary for life.

What are the main phases of the Citric Acid Cycle?

The Citric Acid Cycle consists of three main phases: citrate formation, succinyl CoA formation, and oxaloacetate regeneration. In the first phase, citrate is formed from Acetyl CoA and oxaloacetate. The second phase involves the conversion of citrate to succinyl CoA through a series of reactions, producing NADH and CO₂. The final phase regenerates oxaloacetate from succinyl CoA, producing ATP, FADH₂, and NADH. These phases ensure the continuous production of high-energy molecules and the regeneration of cycle intermediates.

What enzymes are involved in the Citric Acid Cycle?

Several key enzymes facilitate the reactions in the Citric Acid Cycle. Citrate synthase catalyzes the formation of citrate from Acetyl CoA and oxaloacetate. Aconitase is responsible for the isomerization of citrate to isocitrate. Various dehydrogenases, such as isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase, catalyze oxidation reactions, producing NADH and FADH₂. Succinyl CoA synthase hydrolyzes succinyl CoA to succinate, generating GTP. Finally, malate dehydrogenase oxidizes malate to oxaloacetate, completing the cycle.

How does the Citric Acid Cycle produce ATP?

The Citric Acid Cycle produces ATP through substrate-level phosphorylation and oxidative phosphorylation. In reaction 5, succinyl CoA is hydrolyzed to succinate by succinyl CoA synthase, generating GTP, which is then converted to ATP. Additionally, the high-energy molecules NADH and FADH₂ produced during the cycle donate electrons to the electron transport chain, driving the production of ATP through oxidative phosphorylation. This dual mechanism ensures efficient energy production for cellular processes.

What are the high-energy molecules produced in the Citric Acid Cycle?

The Citric Acid Cycle produces several high-energy molecules essential for cellular respiration. These include NADH, FADH₂, and ATP. NADH is generated in reactions 3, 4, and 8, while FADH₂ is produced in reaction 6. ATP is synthesized in reaction 5 through substrate-level phosphorylation. These molecules are crucial for the electron transport chain, where they facilitate the production of additional ATP, providing energy for various cellular functions.