Integrated SequencePhysics Chemistry Organic Biology

Web Resources

Kimball's Biology Pages - The Citric Acid Cycle

Online Biology Book - Aerobic Respiration
Basic introduction to aerobic respiration. Good place to start if you are rusty with the rudimentary biochemistry taught in 1st year biology.

  click if a link is broken

Special points of emphasis

Abnormal Psychology



Chemical Thermodynamics and the Equilibrium State



Biological Membranes

The Eukaryotic Cell


Oxidation-Reduction in Organic Chemistry


Bioenergetics and Cellular Respiration

The Citric Acid Cycle including the initial pyruvate mobilization is a series of reactions oxidizing the pyruvate from glycolysis to form three molecules of CO2. Through the process NAD+ and FAD act as electron acceptors. These electrons are eventually passed to their final acceptor O2 by means of the electron transport chain. Read that again if you did not completely understand.

While direct substrate phosphorylation does occur to an extent in the citric acid cycle, the processes of the electron transport chain yield a great deal of energy, forming most of the ATPs per glucose in aerobic respiration. Oxidative metabolism yields far more energy than glycolysis and fermentation because the ultimate electron acceptor is oxygen, rather than organic carbon. While glycolysis and fermentation alone yields only two ATP, the entirety of oxidative metabolism, including glycolysis, oxidation of pyruvate, and the citric acid cycle will yield thirty-six molecules of ATP per molecule of glucose (38 in aerobic bacteria).

All fuel molecules, amino acids, fatty acids, and carbohydrates, are ultimately oxidized in aerobic metabolism by means of the citric acid cycle. Most enter the cycle as acetyl CoA, although there are other points of entry for some exceptions. In addition to serving as the primary pathway in oxidative metabolism for the degradation of nutrient molecules, ultimately to give rise to ATP through electron transport, the citric acid cycle is a major source of precursors for the synthesis of biological molecules.

Although glycolysis takes place in the cytoplasm of eukaryotic cells, the oxidative decarboxylization of pyruvate to form acetyl CoA and the citric acid cycle take place within the mitochondria.

The WikiPremed MCAT Course is a free comprehensive course in the undergraduate level general sciences. Undergraduate level physics, chemistry, organic chemistry and biology are presented by this course as a unified whole within a spiraling curriculum.

Please read our policies on privacy and shipping & returns.  Contact Us.
MCAT is a registered trademark of the Association of American Medical Colleges, which does not endorse the WikiPremed Course.

Creative Commons License
The work of WikiPremed is published under a Creative Commons Attribution Share Alike 3.0 License. There are elements of work here, such as a subset of the images in the archive from WikiPedia, that originated as GNU General Public License works, so take care to follow the unique stipulations of that license in printed reproductions. You can use the resources here for commercial or non-commercial purposes, but please give attribution and a link to the production credits and edit history of the resource. For the works here which began as my individual work, please attribute "John Wetzel, an author at".