Lipid Metabolism

Glycolysis &
Citric Acid Cycle
Protein Metabolism  Elmhurst College
Fatty Acid Spiral Reactions Acetyl CoA Fates Energy Summary  Chemistry Department
Overview, Summary Lipids Diabetes Reveiw Metabolism  Virtual ChemBook

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Overview of Lipid Function

Introduction - Function of Lipids:

Lipids as an Energy Reserve:

Nearly all of the energy needed by the human body is provided by the oxidation of carbohydrates and lipids. Whereas carbohydrates provide a readily available source of energy, lipids function primarily as an energy reserve. The amount of lipids stored as an energy reserve far exceeds the energy stored as glycogen since the human body is simply not capable of storing as much glycogen compared to lipids. Lipids yield 9 kcal of energy per gram while carbohydrates and proteins yield only 4 kcal of energy per gram.

It is interesting to compare the relative amounts of energy provided by various biochemicals in a typical 154 lb male. The free glucose in the blood provides only a 40 kcal energy reserve -- only enough to maintain body functions for a few minutes. Glycogen remaining stored in the liver and muscles after an overnight fast, amounts to about 600 kcal energy. Glycogen reserves can maintain body functions for about one day without new inputs of food. Protein (mostly in muscle) contains a substantial energy reserve of about 25,000 kcal.

Finally, lipid reserves containing 100,000 kcal of energy can maintain human body functions without food for 30-40 days with sufficient water. Lipids or fats represent about 24 pounds of the body weight in a 154 pound male. Lipids provide the sole source of energy in hibernating animals and migrating birds. Fortunately, lipids are more compact and contain more energy per gram than glycogen, otherwise body weight would increase approximately 110 pounds if glycogen were to replace fat as the energy reserve.

Link to: Rodney Boyer Animation of Lipids in Body


Functions of Lipids:

Lipids or fats are stored in cells throughout the body principle in special kinds of connective tissue called adipose tissue or depot fat. Whereas many cells contain phospholipids in the bilayer cell membranes, adipose tissue cells consist of fat globules of triglycerides which may occupy as much as 90% of the cell volume.

In addition to energy storage, depot fat provides a number of other functions. Fat serves as a protective cushion and provides structural support to help prevent injury to vital organs such as the heart, liver, kidneys, and spleen. Fat insulates the body from heat loss and extreme temperature changes. At the same time, fat deposits under the skin may be metabolized to generate heat in response to lower skin temperatures.

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Lipids in the Blood:

Lipids ingested as food are digested in the small intestine where bile salts are used to emulsify them and pancreatic lipase hydrolyzes lipids into fatty acids, glycerol, soaps, or mono- and diglycerides. There is still some dispute about the lipid form that passes through the intestinal wall -- whether as fatty acids or as glycerides. In either case, triglycerides are found in the lymph system and the blood.

Quiz: According to properties studied earlier, are lipids generally soluble or insoluble in the aqueous portion of blood? Explain.  

Since lipids are not soluble in blood, they are transported as lipoproteins after reaction with water-soluble proteins in the blood. Fatty acids are generally transported in this form as well.

There is always a relatively constant supply of lipids in the blood, although of course, the concentration increases immediately following a meal. Lipids in the blood are absorbed by liver cells to provide energy for cellular functions. The liver is responsible for providing the proper concentrations of lipids in the blood. Some lipids are utilized by brain cells to synthesize brain and nerve tissue.

Excess lipids in the blood are eventually converted into adipose tissue. If lipid levels in the blood become too low, the body synthesizes lipids from other foods, such as carbohydrates, or removes lipids from storage. The body also excretes some lipids in the form of fats, soaps, or fatty acids as a normal component of feces.

Abnormally high levels of triglycerides and cholesterol are thought to be involved in hardening of the arteries. Lipids may be deposited on the walls of arteries as a partial consequence of their insolubility in the blood.

The summary of lipid functions is given in the graphic on the left.