8. Respiration

Krebs Cycle

Where does the citric acid cycle occur in eukaryotes?
a. in the cytosol of cells
b. in the intermembrane space of mitochondria
c. in the inner membrane of mitochondria
d. in the matrix of mitochondria

Animation: The Citric Acid Cycle

Cellular Respiration 3 - TCA Cycle (Krebs Cycle)

Consider the following:

Plasmodium has an unusual organelle called an apicoplast. Recent research has shown that apicoplasts are derived from chloroplasts via secondary endosymbiosis and have a large number of genes related to chloroplast DNA.

Glyphosate is one of the most widely used herbicides. It works by poisoning an enzyme located in chloroplasts.

Biologists are testing the hypothesis that glyphosate could be used as an antimalarial drug in humans.

How are these observations connected?

Four ATP, six NADH, two FADH2

 No. This would be the answer if we considered the production of both glycolysis and the citric acid cycle. 

Three ATP, three NADH, one FADH2

 No, this misstates the numbers for all three energy carriers. 

 No. This would be the production from the entire process of glycolysis and cellular respiration including electron transport and oxidative phosphorylation. 

One ATP, three NADH, one FADH2

 No. Remember that for each glucose molecule that enters glycolysis, there are two molecules of pyruvate that enter the citric acid cycle. 

Two ATP, six NADH, two FADH2

Consider how glycolysis and the citric acid cycle are connected.

In the citric acid cycle, for each pyruvate that enters the cycle, one ATP, three NADH, and one FADH2 are produced. For each glucose molecule that enters glycolysis, how many ATP, NADH, and FADH2 are produced in the citric acid cycle?

Krebs Cycle Example 1

Cellular Respiration - Link Reaction and Krebs - Post 16 Biology (A Level, Pre-U, IB, AP Bio)

 The acetyl CoA that enters the cycle is regenerated in the last step of the pathway. 

 No. Acetyl CoA is oxidized by the citric acid cycle, but it is not regenerated by the cycle. 

 No. They are recycled by the process of oxidative phosphorylation, but that recycling is not part of the citric acid cycle. 

 All of the carbon from glucose is cycled back into the atmosphere as carbon dioxide. 

 No. This is true, but it is not why the pathway is called the citric acid cycle. 

 NADH is cycled down the electron transport chain. 

 No. NADH gives up electrons, which flow one way down the electron transport chain to oxygen. This is separate from the citric acid cycle. 

 The four-carbon acid that accepts the acetyl CoA in the first step of the cycle is regenerated by the last step of the cycle. 

Consider what aspects of this pathway make it a cycle.

Why is the citric acid cycle called a "cycle"?

Phases of The Krebs Cycle

6 CO2, 8 NADH, 2 FADH2 and 4 ATP.

6 CO2, 8 NADH, 1 FADH2 and 2 ATP.

6 CO2, 10 NADH, 2 FADH2 and 4 ATP.

6 CO2, 10 NADH, 2 FADH2 and 2 ATP.

Taking one molecule of glucose through glycolysis, pyruvate oxidation, and the Krebs cycle generates:a) 6 CO2, 8 NADH, 2 FADH2 and 4 ATP.b) 6 CO2, 8 NADH, 1 FADH2 and 2 ATP.&nbsp;c) &nbsp;6 CO2, 10 NADH, 2 FADH2 and 4 ATP. &nbsp;d) 6 CO2, 10 NADH, 2 FADH2 and 2 ATP.

Krebs! (Mr. W's Krebs Cycle Song)

 No. This is input into the citric acid cycle. 

 No. The citric acid cycle produces only a few ATP. 

No. However, CO2 is a by-product of the citric acid cycle.

No. The citric acid cycle produces only one FADH2 per turn of the cycle.

Where is most of the energy conserved in the citric acid cycle?

After completion of the citric acid cycle, most of the usable energy from the original glucose molecule is in the form of __________.

Which product of the Krebs cycle is also used as a reactant in the Krebs cycle?a) Citrate.b) ATP.c) Acetyl-CoA.d) Oxaloacetate.

BioFlix: The Citric Acid Cycle

 No. Oxidative phosphorylation produces ATP, not NADH. 

 No. Substrate-level phosphorylation involves the synthesis of ATP, not of NADH. 

 No. Although a small quantity of NADH is produced during glycolysis, this is not the correct choice. 

 No. Anabolic pathways are synthetic processes and are energy intensive; that is, they use energy rather than contribute it. 

This stage produces NADH and FADH2.

Which of the following processes generates most of the NADH that delivers electrons to the electron transport chain?

 No. If carbon is diverted from the oxidative reactions in the citric acid cycle, less ATP can be made, but is this the only correct answer? 

Less CO2 will be produced by the cell.

No. If carbon is diverted from the oxidative reactions in the citric acid cycle, less CO2 will be produced, but is this the only correct answer?

 The four-carbon compound that combines with acetyl CoA will have to be made by some other process. 

 No. To keep the citric acid cycle going to produce energy and carbon skeletons for amino acid synthesis, some other way to make the four-carbon acceptor compound of the cycle must be found, but is this the only correct answer? 

Consider the consequences of removing compounds from the middle of the citric acid cycle pathway for amino acid synthesis.

If significant amounts of materials are removed from the citric acid cycle to produce amino acids for protein synthesis, which of the following will result?

 No. However, the citric acid cycle forms ATP. 

No. O2 is neither a reactant nor a product of the citric acid cycle.

 Completion of substrate-level phosphorylation 

 No. This results in the production of only a few ATP. 

Think about the products of the citric acid cycle.

Which of the following represents the major energy-related accomplishment of the citric acid cycle, even though it is not the only energy-related accomplishment?

 No. This is the location of glycolysis. 

 No. The cristae of the inner mitochondrial membrane are the location of the electron transport chain. 

 The intermembrane space of the mitochondrion 

 No. Protons are pumped into this space. 

 Across the inner membrane of the mitochondrion 

 No. Chemiosmotic synthesis of ATP occurs across the inner mitochondrial membrane. 

What is the fate of pyruvate produced by glycolysis under aerobic conditions?

Where do the reactions of the citric acid cycle occur in eukaryotic cells?

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