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Chapter: Biochemistry: Lipid Metabolism

Lipid Metabolism

Lipids are organic compounds of biological nature that includes fats, oils and waxes.

Lipid Metabolism


Lipids are organic compounds of biological nature that includes fats, oils and waxes. They are insoluble in water but soluble in nonpolar solvents such as ether, chloroform and benzene. Lipids are utilizable by living organisms.

In the normal mammal at least 10 to 20 percent of the body weight is lipid. They form important dietary constituent on account of their high calorific value and fat soluble vitamins (vitamins A, D, E and K) along with the essential fatty acids. Lipids are distributed in all organs, particularly in adipose tissues in which lipids represent more than 90 percent of the cytoplasm of a cell.

Biological functions of Lipids

Lipids are stored in a relatively water - free state in the tissues in contrast to carbohydrates which are heavily hydrated to perform a wide variety of functions.


1.           Body lipids are reservoir of potential chemical energy. Lipids can be stored in the body in almost unlimited amount in contrast to carbohydrates. Furthermore, lipids have a high calorific value (9.3 calories per gram) which is twice as great as carbohydrate. Large amount of energy is stored as lipid than as carbohydrates.

2.           Lipids which forms the major constituent of biomembranes are responsible for membrane integrity and regulation of membrane permeability.

3.           The subcutaneous lipids serve as insulating materials against atmospheric heat and cold and protect internal organs.

4.           They serve as a source of fat soluble vitamins (Vitamin A, D, E and K) and essential fatty acids. (Linoleic, Linolenic and Arachidonic acid).

5.           Lipids serve as metabolic regulators of steroid hormones and prostaglandins.

6.           Lipids present in inner mitochondrial membrane actively participate in electron transport chain.

7.           Polyunsaturated fatty acids help in lowering blood cholesterol.

8.           Squalamine, a steroid, is an potential antibiotic and antifungal agent.


Fatty acids

The fatty acids are the basic units of lipid molecules. Fatty acids are derivatives of aliphatic hydrocarbon chain that contains a carboxylic acid group. Over 200 fatty acids have been isolated from various lipids. They differ among themselves in hydrocarbon chain length, number and position of double bonds as well as in the nature of substituents such as oxy-, keto-, epoxy groups and cyclic structure. Depending on the absence, or presence of double bonds, they are classified into saturated and unsaturated fatty acids.

Saturated fatty acids, do not contain double bonds. The hydrocarbon chain may contain 12 to 18 carbon atoms. eg. palmitic and stearic acids

CH3 (CH2)14 COOH     -  Palmitic acid (C-16)

CH3 (CH2)16 COOH     -  Stearic acid (C-18)

Unsaturated fatty acids are classified into different types depending on the number of double bonds present in the hydrocarbon chain. These fatty acids are mainly found in plant lipids.



Name of Fatty Acid                No.of Double Bonds

Oleic acid                                          1

Linoleic acid                                      2

Linolenic acid                                    3

Arachidonic acid                               4


Essential fatty acids

Fatty acids required in the diet are called essential fatty acids (EFA). They are not synthesized by the body and are mainly polyunsaturated fatty acids (PUFA).


eg.   Linoleic acid


Linolenic acid


Arachidonic acid

Functions of essential fatty acids

They are required for membrane structure and function, transport of cholesterol, formation of lipoproteins and prevention of fatty liver.

Deficiency of essential fatty acids

The deficiency of essential fatty acid results in phrynoderma or toad skin.


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