http://www.ck12.org Chapter 26. Biochemistry
Lipids and Biochemical Energy
Triglycerides in the body function as a long-term method for storing biochemical potential energy. Changes in the
levels of the hormones insulin and epinephrine in the blood lead to the release of triglycerides from adipose tissue.
The triglycerides are then broken down by lipase enzymes into glycerol and fatty acids. After being transported into
the mitochondria, fatty acids are broken down further into two-carbon fragments, called acetyl CoA. Finally, acetyl
CoA molecules can be used to generate ATP, which functions as a kind of universal biochemical energy source
within the cell. A normal fatty acid might contain approximately 16 carbons, which would result in the formation of
8 molecules of acetyl CoA. Each acetyl CoA can then be used to generate 12 molecules of ATP. Thus, a single fatty
acid might be used to produce nearly 100 ATP molecules.
Regular exercise will increase the demand for ATP and help lower the amount of triglycerides (stored fat) in the
body, as will a decrease in the consumption of materials that contain triglycerides or other metabolic energy sources
(such as carbohydrates).
Overview of Metabolism
The metabolism of fatty acids is highly interconnected with the breakdown of other dietary energy sources, such
as carbohydrates and protein. Recall that glucose and amino acids can both be broken down to form the pyruvate
molecule. Pyruvate can then be converted into acetyl CoA, which is then further metabolized to produce ATP.
Eventually, all sources of biochemical energy are funneled into the same overall reaction pathways.
FIGURE 26.13
Overview of energy metabolism.Additionally, all of these processes are reversible. The breakdown or rebuilding of biomolecules is directed by
specific environmental cues in various parts of the body. Because all of these types of molecules can be broken down
to the same common intermediates, they are all interconvertible using our biochemical machinery. For example, say
the body needs to produce more steroids, but only complex carbohydrates are available as a source of material.
The carbohydrates can be successively broken down into disaccharides, monosaccharides, pyruvate, and finally
molecules of acetyl CoA. Then, the acetyl CoA can be assembled into cholesterol, which is then modified to produce
the necessary steroid. Fatty acids, triglycerides, and other lipids can be generated in a similar way.
Cholesterol and Heart Disease
Because of the link between high levels of cholesterol and heart disease, it is commonly seen by the general public
as a "bad" molecule. However, cholesterol plays a vital role in the body, as both a structural component of cell