The Krebs Cycle


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My AP Biology Thoughts

Unit 3 Episode #74 Krebs Cycle

Welcome to My AP Biology Thoughts podcast, my name is Corrinna and I am your host for episode 74 called Unit 3 cell energetics: the Krebs Cycle. Today we will be discussing the Krebs Cycle.

Segment 1: Defining The Krebs Cycle

The Krebs cycle, also known as the citric acid cycle, is the third step in cellular respiration, the process by which organisms combine oxygen and other molecules into energy that is used in life-sustaining activities. Before the Krebs cycle, glycolysis and pyruvate oxidation occur.

The Krebs cycle occurs in the mitochondrial matrix in eukaryotes. The matrix of the mitochondria is the part of the mitochondria inside the inner membrane. This process occurs twice for every glucose molecule that goes through glycolysis. The Krebs cycle is a very detailed process.

First, acetyl coenzyme A, which was produced in the previous step of cellular respiration, combines with oxaloacetate to form citrate. This molecule is converted to its isomer, which is then oxidized and releases carbon dioxide. During this process, NAD+ is reduced to form NADH. Next, another molecule is oxidized and NAD+ is again reduced to NADH, and a molecule of carbon dioxide is released. THe coenzyme A of succinyl coensyme A is replaced by a phosphate group which is transferred to ADP to produce ATP, or in some cases, GDP. A four carbon molecule called succinate is also produced. Next, succinate is oxidized and FAD is reduced to FADH2. Water is then added to the resulting molecule, and another molecule of NAD+ is reduced to NADH. 2 Oxaloacetate is also produced which allows the cycle to start again.

Those are the very detailed steps of the Krebs cycle, but the most important part to remember is the energy transfers that occur and what the krebs cycle produces. In the Krebs cycle NAD+ is reduced to NADH, and FAD is reduced to FADH2. ADP and phosphate are combined to produce ATP. Citrate is oxidized, and heat is lost in the process. In the end, the krebs cycle produces 4 CO2, 2 ATP, 6 NADH, and 2 FADH2. Carbon dioxide is the waste product and is moved into the blood, and acetyl coa is used to convey the carbon atoms to the cycle.

Segment 2: Examples of the Krebs Cycle

The Krebs cycle is important because it produces molecules that are required for cellular respiration, which enables organisms to create energy that they need to function. The Krebs cycle occurs in all organisms that undergo cellular respiration. It happens in an aerobic environment.

Segment 3: Digging Deeper into the Krebs Cycle

The Krebs cycle also supports the endosymbiotic theory. Prokaryotes go through the Krebs cycle in the cytoplasm. One main aspect of the endosymbiotic theory is that mitochondria used to be prokaryotic cells, but were absorbed by larger cells to form eukaryotic cells with membrane bound organelles. Since eukaryotic cells go through the krebs cycle in the mitochondria, this supports the endosymbiotic theory.

Thank you for listening to this episode of My AP Biology Thoughts. For more student-run podcasts, make sure that you visit See you next time on My AP Biology thoughts podcast!

Music Credits:

  • "Ice Flow" Kevin MacLeod (
  • Licensed under Creative Commons: By Attribution 4.0 License

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