Lipids

LIPIDS

Introduction

The lipids are a heterogenous group of compounds related to fatty acids. They are insoluble in water but soluble in organic solvents such as ether, alcohol, chloroform and benzene.

Lipids include fats, oils, waxes, cholesterol and related compounds.

Functions

(i) Lipids as hydrophobic compounds are components of cellular membranes.

(ii) They also store energy, because of higher proportion of C-H bonds and very low proportion of oxygen, lipids store double the amount of energy as compared to the same amount of any carbohydrate.

(iii) Some lipids provide insulation against atmospheric heat and cold and also act as water proof material.

(iv) Waxes, in the exoskeleton of insects and cutin, are an additional protective layer on the cuticle of epidermis of some plant organs. Leaves, fruits and seeds etc. are some of the main examples.

Classification

Lipids have been classified as;

1. Acylglycerols
2. Waxes
3. Phospholipids
4. Sphingolipids
5. Glycolipids
6. Terpenoids

(i) Acylglycerols

It is of the most important groups of lipids.

Chemical Composition

Acylglycerols are composed of glycerol andfatty acids. Chemically, acylglycerols can be defined as esters of fatty acids and alcohol.

An ester is the compound produced as the result of a chemical reaction of an alcohol with an acid and a water molecule is released as shown below.

Example

The most widely spread acylglycerol is triacylglycerol, also called triglycerides or neutral lipids

Triacylglycerol is composed of one glycero l and three fatty acids molecules

Fatty Acids

• Fatty acids contain even numbers (2-30) of carbon atoms in straight chain attached with having an acidic groups COOH (carboxylic group).
• There may be two types of fatty acids i.e.,

1. Saturated fatty acids which contain no double bonds.
2. Unsaturated fatty acids, which contain upto 6 double bonds.

• In animals, fatty acids are straight chains.
• In plants, they may be branched or ringed.
• Solubility of fatty acids in organic solvents and their melting points increase with increasing number of carbon atoms in chain.

For example palmitic acid (C16)with melting point 63.1°C is much more soluble in organic solvents than butyric acid (C4)with melting point - 8°C.

Types of Fats

• Fats containing unsaturated fatty acids are usually liquid at room temperature and are said to be oils. Animal fats are solid at room temperature, where as most of the plants fats are liquids.
• Fats containing saturated fatty acids are solid at room temperature and are simply said as fats.

Fats and oils are lighter than water and have a specific gravity of about 0.8. They are not crystalline but some can be crystallized under specific conditions.

(ii) Waxes

Chemical Composition

Chemically, waxes are mixtures of long chain alkanes (with odd number of carbons ranging from C25 to C35), and alcohols, ketones and esters of long chain fatty acids.

Characteristics

1. Waxes are widespread as protective coatings on fruits and leaves.
2. Some insects also secrete wax.
3. Waxes protect plants from water loss and abrasive damage.
4. They also provide water barrier for insects, birds and animals such as sheep.

(iii) Phospholipids

They are widespread in bacteria, animal and plant cells and are frequently associated with membranes.

Chemical Composition

Phospholipids are derivatives of phosphatidic acid. Which are composed of glycerol, fatty acids and phosphoric acid.

Nitrogenous bases such as choline, ethanolamine and serine are important components of phospholipids.

Example

Phosphatidylcholine is one of the common phospholipids.

Terpenoids

Terpenoids are a very large and important group of compounds, which are made up of a simple repeating units, isoprenoid units.

This unit, by condensation, in different ways give rise to compounds such as rubber, carotenoids, steroids, terpenes etc.