A Cycle by Any Other Name…


The Krebs cycle was named after the German/British biochemist, Sir Hans Adolf Krebs who later received a Nobel Prize for his discovery. His research on metabolic processes led to his discovery of both the Krebs and urea cycles.


giphy (4)

The Krebs cycle is a set of reactions which living cells carry out in order to make energy. Most aerobic organisms gain energy through the Krebs cycle via the breakdown of glucose and other simple sugars in the presence of O2. However, this isn’t a one man show… the Krebs cycle is only the second of four stages in ATP (energy) formation, occurring between glycolysis and oxidative phosphorylation (Chemistrylearning.com n.d.).

Fig 1: The Citric Acid Cycle

Fig 1: The Citric Acid Cycle

Also, the first product that is formed from the reactions of the cycle is citric acid, which is another name for the Krebs cycle. Incidentally, it’s also one of the final reactants in the cycle due to its being regenerated to actually complete the cycle. This doesn’t end there, actually citric acid is also called tricarboxylic acid because it contains 3 carboxylic groups (COOH) hence citric acid is also called tricarboxylic (TCA) cycle.

You Should Know…

  1. The Krebs cycle starts with the condensation reaction involving one oxaloacetic acid molecule and one acetyl COA (coenzyme A) molecule -(acetyl COA  is a coenzyme A derivative)
  2. Acetyl COA is actually formed from pyruvic acid.
  3. Citric acid is formed by a reaction between acetyl COA and oxaloacetic acid; in enzyme-catalyzed reactions citric acid acts as a substrate and 7 intermediate compounds are formed ( mainly succinic, fumaric and malic acid)
  4. Malic acid is converted to oxaloacetic acid which then combines with acetyl COA to produce citric acid.
  5. 2 molecules of CO2 and 8 atoms of Hydrogen are formed as byproducts with every reaction of the cycle. While it’s other products for every molecule of Acetyl CoA include: 3 NADH’s,1 FADH2, &1 ATP.
  6. CO2 is removed from the blood by the end product of the reaction
  7. H2 atoms are converted to hydride ions to the electron transport systems needed for oxidative phosphorylation process.

    Krebs cycle steps….

  • Condensation
  • Isomerisation
  • Dehydrogenation
  • Decarboxylation
  • Oxidative Decarboxylation
  • Substrate level ATP/GTP Synthesis
  • Dehydration (Oxidation) of succinic acid
  • Dehydration (Oxidation) of malic acid


You bet your life it’s important…

  • The intermediate compounds such as succinic acid, fumaric acid and malic acid are used to make nucleotides, amino acids, fats, chlorophyll and cytochromes.
    • succinyl COA, an intermediate, is involved chlorophyll formation
    • a-ketogluteric acid, oxaloacetric acid and pyruvic acid and form Amino  acids.

Energy in the form of ATP is made during the Krebs cycle (Infoplease.com 2012).



Chemistrylearning.com. “Krebs Cycle | Chemistry Learning.” 2014. http://www.chemistrylearning.com/krebs-cycle/ (accessed 16 Mar 2014).

DISNEY ANIMATED GIF. 2014. [image online] Available at: http://giphy.com/gifs/YwTXexxX2yEVy [Accessed: 15 March 2014].

Infoplease.com. “Krebs cycle | Infoplease.com.” 2014. http://www.infoplease.com/encyclopedia/science/krebs-cycle.html#ixzz2w3IplEEb (accessed 16 Mar 2014).

Frey, R. 2007. A diagram of cellular respiration including glycolysis, Krebs cycle (AKA citric acid cycle), and the electron transport chain. [image online] Available at: http://en.wikipedia.org/wiki/File:CellRespiration.svg#filelinks [Accessed: 15 March 2014].

Uic.edu. n.d. Glycolysis. [online] Available at: http://www.uic.edu/classes/bios/bios100/lectures/respiration.htm [Accessed: 16 March 2014].

Contributors:  Thalia, India, Roi (editor)


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