Lipids (Biomolecules): structure, classification and functions

Lipids (Biomolecules): structure, classification and functions | Biochemistry

Diksha Bhatla

Lipids (Biomolecules): structure, classification and functions | Biochemistry

07 Jul 2022

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Introduction

Lipids (Greek: lipos–fat) are of great importance to the body as the chief concentrated storage form of energy, besides their role in cellular structure and various other biochemical functions. As such, lipids are a heterogeneous group of compounds and, therefore, it is difficult to define them precisely. Lipids may be regarded as organic substances relatively insoluble in water, soluble in organic solvents (alcohol, ether etc.), actually or potentially related to fatty acids and utilized by the living cells.

Unlike the polysaccharides, proteins and nucleic acids, lipids are not polymers. Further, lipids are mostly small molecules.

Classification of lipids

Lipids are broadly classified into simple, complex, derived and miscellaneous lipids, which are further subdivided into different groups

1. Simple lipids: Esters of fatty acids with alcohols. These are mainly of two type

a) Fats and oils (triacylglycerols) : These are esters of fatty acids with glycerol.

The difference between fat and oil is only physical. Thus, oil is a liquid while fat is a solid at room temperature.

(b) Waxes : Esters of fatty acids (usually long chain) with alcohols other than glycerol. These alcohols may be aliphatic or alicyclic. Cetyl alcohol is most commonly found in waxes. Waxes are used in the preparation of candles, lubricants, cosmotics, ointments, polishes etc.

2. Complex (or compound) lipids: These are esters of fatty acids with alcohols containing additional groups such as phosphate, nitrogenous base, carbohydrate, protein etc. They are further divided as follows

(a) Phospholipids : They contain phosphoric acid and frequently a nitrogenous base. This is in addition to alcohol and fatty acids.

3. Derived lipids: These are the derivatives obtained on the hydrolysis of group 1 and group 2 lipids which possess the characteristics of lipids. These include glycerol and other alcohols, fatty acids, mono- and diacylglycerols, lipid (fat) soluble vitamins, steroid hormones, hydro- carbons and ketone bodies.

4. Miscellaneous lipids: These include a large number of compounds possessing the characteristics of lipids e.g., carotenoids, squalene, hydrocarbons such as pentacosane (in bees wax), terpenes etc.

NEUTRAL LIPIDS: The lipids which are uncharged are referred to as neutral lipids. These are mono-, di-, and triacylglycerols, cholesterol and cholesteryl esters.

Functions of lipids

Lipids perform several important functions

1. They are the concentrated fuel reserve of the body (triacylglycerols).

2. Lipids are the constituents of membrane structure and regulate the membrane permeability (phospholipids and cholesterol).

3. They serve as a source of fat soluble vitamins (A, D, E and K).

4. Lipids are important as cellular metabolic regulators (steroid hormones and prostaglandins).

5. Lipids protect the internal organs, serve as insulating materials and give shape and smooth appearance to the body.

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Lipids (Biomolecules): structure, classification and functions | Biochemistry

Lipids (Biomolecules): structure, classification and functions | Biochemistry

Questions

Introduction

Classification of lipids

Functions of lipids

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