Citric acid cycle

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Citric acid cycle



Citric acid cycle


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USMLE® Step 1 questions

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High Yield Notes

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Citric acid cycle

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USMLE® Step 1 style questions USMLE

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During normal respiration, oxygen is absorbed into the bloodstream and carbon dioxide is released. The latter is generated during each turn of the citric acid cycle. Which of the following best describes the location of the citric acid cycle in the cell?  

External References

First Aid








Adenosine triphosphate (ATP)

in TCA cycle p. 75

α-ketoglutarate dehydrogenase

TCA cycle p. 75


TCA cycle p. 75

Metabolism p. 70-92

TCA cycle p. 75

NADH (nicotinamide adenine dinucleotide)

TCA cycle p. 75


TCA cycle p. 75

TCA cycle p. 75

diagram p. 72, 75

hyperammonemia p. 80

metabolic site p. 70

pyruvate metabolism p. 75

rate-determining enzyme for p. 71


For cells to perform any function, any work, they must have energy.

You can’t go jogging or lifting weights if you’re tired, because a cell won’t work without the help of chemical energy.

The main energy currency in the cells is adenosine triphosphate, or ATP, but any nucleoside triphosphate, like guanosine triphosphate, GTP, will do.

For cells to make ATP, a process generating electricity has to take place in our mitochondria.

Electricity is power!

And thanks to this electricity, ATP is made.

Now to create electricity, electron rich molecules must deliver electrons to a chain of complexes, the electron transport chain, which move them to a final acceptor, a molecule of oxygen.

And there are two electron donor molecules: nicotinamide adenine dinucleotide, or NADH, and flavin adenine dinucleotide, or FADH2.

But of course, the cell has to produce NADH and FADH2 in the first place, and they’re produced by critical enzymes called dehydrogenases.

Dehydrogenases are the main enzymes found in the citric acid cycle or Kreb’s cycle.

In fact, the citric acid cycle is a set of 8 enzymatic reactions that start with a molecule called acetyl-CoA, and four of the enzymes, half of them, are dehydrogenases.

And in this process, AcetylCoA gets converted into carbon dioxide.

Acetyl-CoA comes from various sources depending on whether you’ve just eaten or are starving.

Let’s say that you’re hungry and a bit angry - so you’re feeling hangry.

That’s when stress hormones like glucagon, epinephrine, and cortisol start to rise.

In this hangry state, fatty acids from triglycerides become the primary source of acetyl-CoA.

Now, let’s say you have a bowl of delicious French onion soup, everything changes - insulin is plentiful and you have plenty of acetyl-CoA from breaking down glucose, fructose, and galactose -with glucose playing the biggest role.

Now, alcohol is also a source of Acetyl-CoA in the liver where it’s metabolized.

In addition, proteins can also help contribute to acetyl-CoA production.

But in the case of glucose, after a meal, one glucose, a 6-carbon molecule, splits into two 3 carbon pyruvate molecules through glycolysis, which occurs in the cytoplasm of the cell.


The citric acid cycle, also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle, is a series of chemical reactions in aerobic organisms' cells. The TCA cycle generates energy in the form of ATP from nutrients able to give acetyl-CoA molecules. These include carbohydrates, lipids, alcohol, and ketogenic amino acids. Molecules.

The citric acid cycle begins with pyruvate oxidation to acetyl-CoA by the enzyme pyruvate dehydrogenase. Acetyl-CoA is then transported into the mitochondrial matrix and enters the TCA cycle, where it is oxidized by succinyl-CoA synthase to succinyl-CoA, which is then oxidized by Succinate dehydrogenase to fumarate. Fumarate is then hydrated by fumarase to malate, and malate is oxidized by malate dehydrogenase to oxaloacetate. Oxaloacetate is then reduced by phosphoenolpyruvate carboxykinase to phosphoenolpyruvate. Phosphoenolpyruvate is then dehydrogenated by pyruvate kinase to pyruvate.


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