Lipid Metabolism

Lipid Metabolism


A) Properties:

==> Lipids are a group of heterogeneous compounds related to fatty acids.

==> Lipids have the common properties of being

1) Relatively insoluble in water,

2) Soluble in non-polar solvents, such as ether, chloroform, and benzene.

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

4) Utilizable by living organisms.

Weight and distribution in the body:

==> 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

==> Distributed in all organs, particularly in adipose tissues in which lipids represent more than 90 percent of the cytoplasm of a cell.

B) Digestion

(1) The digestion of fats starts in the small intestine where fats are emulsified (i.e. make emulsion) by the bile salts and hydrolyzed by the pancreatic lipases to form free fatty acids.

(2) These free fatty acids combine with glycerol (produced by the glycolysis) to form triglycerides.

(3) Then triglycerides combine with proteins to form lipoproteins to be able to enter into circulation (because blood is polar) to perform various biological functions such as oxidation, storage and formation of new lipids.

Thus the various fatty acids may exist in the free form as well as in the esterified form (Triglyceride) in blood.

Fatty acids are the immediate source for oxidation of fats in various tissues such as liver, adipose tissue, muscles, heart, kidney, brain, lungs and testes.

C) 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 and 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.

2) Responsible for membrane integrity and regulation of membrane permeability because lipids form the major constituent of biomembranes.

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 found in inner mitochondrial membrane actively participate in electron transport chain

7) Polyunsaturated fatty acids help in lowering blood cholesterol.

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

D) Phospholipids as the major component of biological membranes

phospholipid-bilayer1) Some lipids such as fatty acids, some phospholipids, have two groups: non-polar groups (hydrocarbon) and polar groups.

2) Those molecules are orientated at oil-water interfaces with the polar group in the water phase and the non-polar group in the oil phase.

3) A bilayer of such polar lipids has been regarded as a basic structure in biologic membranes.

 Lipogenesis and Lipolysis 

A) Lipogensis:

It is the process in which lipid is formed form the union of glycerol and fatty acids obtained from the diet or by conversion of glucose into fat since acetyl CoA is the starting molecule for the synthesis of fatty acids.

B) Lipolyssis:

It is the breakdown of stored fates into free glycerol and fatty acids where:

==> Glycerol enters glycolysis and form acetyl coA which enters Kerbs cycle and produce energy

==> Fatty acids is converted to acetyl coA which enters Kerbs cycle and produce energy

Kerbs cycle starts with the combination with acetyl coA and Oxaloacetic acid, therefore oxaloacetate is necessary for the complete oxidation of fat and without it, acetyl CoA is converted into ketones (ketogenesis)

 Fatty acids 

A) Properties of Fatty acids

1) The fatty acids are the basic units of lipid molecules.

2) Fatty acids are derivatives of aliphatic hydrocarbon chain that contains a carboxylic acid group.

3) They differ among themselves in hydrocarbon chain length, number and position of double bonds as well as in the nature of substituents.

4) Depending on the absence, or presence of double bonds, they are classified into saturated and unsaturated fatty acids.

5) Saturated fatty acids, do not contain double bonds.

6) The hydrocarbon chain may contain 12 to 18 carbon atoms. eg. palmi

B) Essential fatty acids


They are polyunsaturated fatty acids (FFA) that are not synthesized in the body and required in the diet e.g Linoleic acid and Linolenic acid.


1. They are required for membrane structure and function.

2. Transport of cholesterol.

3. Formation of lipoproteins.

4. Prevention of fatty liver .


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

See the Fatty acid metabolism in page 2


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