Metabolism of Fats
Home Table of Contents Preliminary Information Chemistry of Living Things Water Lipids Metabolism of Fats Wrap-Up


Hydrolysis of Fats
Glycerol to Acetyl CoA
Fatty Acid Spiral
Citric Acid Cycle
Electron Transport System

Metabolism of Fats

In this section we will consider one of the primary aspects of the metabolism of fats--their use as a source of energy.

Oxidation of Fats

The overall reaction can be summarized by this equation.

O2 + fat rtarrow.gif (850 bytes) CO2 + H2O + energy

In it oxygen combines with the fats to make carbon dioxide and water and release the energy that is needed by our bodies or any living thing to do the things that it needs to do. However, the process is much more involved than that.
Consider that a particular fat molecule may have 58 carbons, 112 hydrogens and 6 oxygen atoms in it.

O2 + C58H112O6 rtarrow.gif (850 bytes) CO2 + H2O + energy

Also consider what has to happen in order to convert that fat molecule into carbon dioxide and water. You must separate those 58 carbons from one another. You must separate the hydrogens from the carbon atoms. You must break the oxygen molecule apart. Then you must to recombine all those atoms to make CO2 and H2O. It shouldn't be a surprise that a process like that might take hundreds of smaller individual reaction steps.


Even though you do now know quite a bit of chemistry, you will not be held responsible for knowing all of those steps. However, you should be able to make sense of them and be able to identify the kinds of reactions that are involved in each of those steps, when you are shown what those steps are.


Let me give you an overview of the process that occurs in taking fats to carbon dioxide and water. (This is also shown in Example 8 in your workbook.)

The fats are broken down by hydrolysis reaction to form glycerol and fatty acids. Glycerol is oxidized to form pyruvic acid (both have three carbon atoms). The pyruvic acid is converted to a compound called acetyl CoA, which only has two carbon atoms in the acetyl part thus giving off carbon dioxide. In the process of losing a carbon atom, the pyruvic acid also combines with something called coenzyme A.

Diagram of overall process for oxidation of fats. [66rxn04.JPG]

Meanwhile, the fatty acids are also converted into acetyl CoA by means of a series of chemical reactions. The acetyl CoA is then combined with another chemical called oxaloacetic acid to form citric acid. Then a sequence of reactions occurs in which the oxaloacetic acid is regenerated and then converted back in to citric acid by more acetyl CoA. This cyclic process is known by several names: the citric acid cycle, the Krebs cycle, the tricarboxylic acid (TCA) cycle.


At a couple different steps in this process, carbon dioxide is generated. Also, at several different steps, hydrogen atoms are removed from the molecules and those hydrogen atoms are then subsequently allowed to react with oxygen to form water. So in the process of the oxidation of fats, the oxygen actually comes into the process quite late.

Next, let's consider each of the portions of the overall process in turn: hydrolysis of fats, glycerol to acetyl CoA, fatty acid spiral (to acetyl CoA), citric acid cycle, and electron transport system (oxygen finally gets the electrons).


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