Lipids

Lipids are esters of long chain fatty acids. These are a diverse group of naturally-occurring organic compounds, such as fat, waxes, sterols, glycerides and phospholipids (Figure 1).

Learning Objective: Identify different kinds of lipids, their characteristics, and major functions in the body.

These compounds are similar to each other as they are soluble in non-polar organic solvents (e.g. ether, chloroform, acetone & benzene) and insoluble in water.

This is what you will learn: What are lipids and their composition To identify different kinds of lipids Various applications of lipis How soap works

Characteristics of Lipids

• Lipids are broadly defined as hydrophobic or amphiphilic molecules. Due to their amphiphilic nature, they form structures, such as, vesicles or liposomes.
• Unlike polysaccharides and proteins, lipids are not polymers—they lack a repea­ting monomeric unit.
• Lipids are made of the elements carbon, hydrogen and oxygen.
• They may be saturated or unsaturated.

The main biological functions of lipids is energy storage. They also act as signaling molecules and form structural components of the cell membrane.

Types of Lipids

Lipids are mainly categorized into simple lipids, compound lipids, and steroids.

Simple Lipids

They contain carbon, hydrogen and oxygen. Triglycerides (fats and oils) and waxes are simple lipids.

Waxes

Waxes are lipids formed by joining two monomers, i.e.,  fatty acid bonded through an ester linkage to an alcohol.

The hydrocarbon chain in the alcohol monomer of waxes varies from a short linear chain to complex carbon ring structures.

Waxes provide protective barriers to prevent water loss and protect cells.

Compound Lipids

They contain other elements such as sulphur, phosphorus or nitrogen, in addition to the carbon, hydrogen, and oxygen as simple lipids.

Phospholipids, sphingolipids, and glycolipids are examples of compound lipids.

Steroids

These are fat-soluble derivatives of cyclopentanoperhydro-phenanthrene and play important roles in metabolic activities of the organism.

Steroid rings usually contain one or a few small functional groups including hydroxyls, carbonyls, or carboxyls. Cholesterol and other steroids containing a hydroxyl group are called sterols.

The most common animal steroid is cholesterol. Cholesterol gives rise to three basic types of steroid hormones:

• Androgens (C₁₉ compounds)
• Estrogens (C₁₈ compounds)
• Progesterones and corticosterones (C₂₁ compounds)

Other Examples of Lipids

Most of the lipids found in food is in the form of triacylglycerols, cholesterol, and phospholipids.

Eicosanoids, such as prostaglandins (Figure 23), leukotrienes, and thromboxanes are biologically important fatty acids.

Prostaglandins

These are a group of naturally occurring, chemically-related fatty acids derived from arachidonic acid (Figure 2).

They participate in a wide range of body functions, such as, contraction and relaxation of smooth muscle, constriction and dilation of blood vessels, control of blood pressure, and modulation of inflammation.

Cardiolipins

These (Figure 3) are lipids that are important constituents of inner mitochondrial membrane and bacterial wall. It is formed from phosphatidyl glyc­erol.

Cardiolipins play important role in apoptosis and cholesterol translocation. It also has an anticoagulant properties.

Lecithin

The lecithin’s contain glycerol and fatty acids, phosphoric acid and choline (nitrogenous base). They are soluble in ordinary fat solvents except acetone. 

Acetylcholine formed from choline has an important role in the transmission of nerv­ous impulses across synapses. 

How does soap work?

Nonpolar molecules repel water molecules and are known hydrophobic and molecules forming ionic or a hydrogen bond with the water molecule are said to be hydrophilic.

Soap

Soaps are metallic salts of fatty acids. They are formed by adding alkali to fatty acids.

Soaps of unsaturated fatty acids are softer and more water soluble than those of satu­rated fatty acids. 

Soap can mix with both water and with oil.  The soap molecule has two ends, hydrophilic (polar head), that binds with water and the hydrophobic (non-polar hydrocarbon tail) that binds with grease and oil.

When greasy dirt or oil is mixed with soapy water, the soap molecules arrange themselves into tiny clusters called micelles (Figure 5).

The water-loving (hydrophilic) part of the soap molecules sticks to the water and points outwards, forming the outer surface of the micelle.

The oil-loving (hydrophobic) parts stick to the oil and trap oil in the center where it can’t come into contact with the water.

With oil in the center, the micelle is soluble in water. As the soapy water is rinsed away the greasy dirt goes along with it.

Soap finally tears up the grease by pulling it into the water using its hydrophilic ends.

Things to Remember

• Lipids are a class of macromolecules that are nonpolar and hydrophobic. Major types of lipids are fats and oils, waxes, phospholipids, and steroids.
• Fatty acids may be unsaturated or saturated, depending on the presence or absence of double bonds in the hydrocarbon chain.
• Cholesterol is a type of steroid and is an important constituent of the plasma membrane.
• Prostaglandins are a group of naturally occurring, chemically-related fatty acids derived from arachidonic acid.
• Cardiolipins are lipids that are important constituents of inner mitochondrial membrane and bacterial wall.
• Phospholipids make up the matrix of membranes.
• Soaps are metallic salts of fatty acids. They are formed by adding alkali to fatty acids.

External References

https://www.britannica.com/science/lipid

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/lipids

https://www.nature.com/subjects/lipids