Lipids

Lipids are organic compounds made up of carbon (C), hydrogen (H), and oxygen (O), with a much lower oxygen content than carbohydrates. They are hydrophobic (insoluble in water) but soluble in organic solvents such as ether, chloroform, and benzene. Lipids are essential biomolecules that function as energy storage, structural components of cell membranes, and signalling molecules in biological systems.

Introduction

• Fats and their derivatives are collectively known as lipids.
• Compounds of C, H, O, but the ratio of Hydrogen and Oxygen is not 2:1. The amount of oxygen is considerably less.
• Lipids are insoluble in water and soluble in organic solvents like Acetone, Chloroform, Benzene, Hot alcohol, ether, etc.
• Lipids occur in protoplasm as minute globules. 
• Lipids do not form a polymer.
• Lipids provide more than twice the energy as carbohydrates.
• In animals, fats are present in the subcutaneous layer and work as a food reservoir and shock absorber.
• Lipids require less storage space than carbohydrates because lipid molecules are hydrophobic and tend to condense.
• Animals store the maximum amount of food in the form of lipids.
• Lipids are micro molecules.
• Some lipids also contain phosphorus, such as lecithin.

1.0Classification of Lipids

Lipids can be classified into several main categories based on their structure and function.

1. Simple Lipids

• These are esters of long-chain fatty acids and alcohol. In the majority of simple lipids, the alcohol is a Trihydroxy sugar alcohol, i.e. glycerol.
• Three molecules of fatty acids are linked with one molecule of glycerol. The linkage is called "ester bond". This type of lipid is called as Triglycerides. Three molecules of water are released during the formation of triglycerides (dehydration synthesis)
• Glycerol is also known as trihydroxypropane

Simple lipids contain two types of Fatty Acids.

Saturated Fatty Acids 

• Only single bonds are present between C-C atoms.
• Also known as non-essential fatty acids, because they can be synthesised in the animal body, so it is not essential to take them with food.
• Lipids with higher amounts of saturated fatty acids are primarily present in solid form at normal temperatures. e.g. Butter, Ghee
• Metabolically, they are very reactive. So, mainly don't participate in metabolism. So it tends to be stored in an animal's body, leading to obesity. Some saturated fatty acids are converted into cholesterol in the liver. This cholesterol deposits on the inner walls of blood vessels, causing high blood pressure & different types of cardiovascular diseases.
• Most animal fat contains a high amount of saturated fatty acids.
• Examples                    No. of C
• Palmitic acid      =         16
• Stearic acid        =        18
• Arachidic acid    =        20

Unsaturated Fatty Acids

• Double bonds are also present with single bonds between C-C atoms.
• Also known as essential fatty acids because they can't be synthesised in the animal body, so it is essential to take them with food.
• Lipids with higher amounts of unsaturated fatty acids are primarily present in liquid form at normal temperatures. e.g. Coconut oil, Mustard oil.
• Metabolically, they are highly reactive, so mostly participate in metabolic reactions. So it doesn't tend to accumulate in the animal's body. So no obesity, no cholesterol formation, no cardiovascular disease, no high B.P. & no harmful effect
• Most plant fat contains high amount of unsaturated fatty acids.
• Examples   - Oleic acid, Linoleic acid, Linolenic acid, and Arachidonic acid.

2. Compound Lipids

These lipids contain additional chemical groups besides fatty acids and alcohol. Key examples include:

Phospholipids, phosphatides or phospholipins 

• 2 Molecules of fatty acid + Glycerol + H₃PO₄ + Nitrogenous compound.
• Phospholipids have both hydrophilic polar end (H₃PO₄ and nitrogenous compound) and hydrophobic non-polar end (fatty acids). Such molecules are called amphipathic. 
• Due to this property, phospholipids form a bimolecular layer in the cell membrane.
• The lipid component of the membrane mainly consists of phosphoglycerides. 
• Phospholipids are the most abundant type of lipids in protoplasm.

Some biologically important phospholipids are as follows:

• Lecithin or Phosphatidyl choline -: Nitrogenous compound in Lecithin is Choline. Lecithin occurs in the cell membrane.
• Cephalin – Similar to lecithin, but the nitrogenous compound is Ethanolamine. Cephalin occurs in Nervous tissue, egg yolk and blood platelets.
• Sphingolipids, or sphingomyelins, are similar to lecithin but, in place of glycero, contain an amino alcohol, sphingosine. Sphingolipids occur in the myelin sheath of nerves.

Glycolipids

• 1 fatty acid + sphingosine + galactose
• e.g. Cerebrosides, which occur in the white matter of the brain.
• Gangliosides - These occur in nerve ganglia and the spleen.

3. Derived Lipids

Lipid is derived from simple or conjugated lipids. Derived lipids are complex in structure. They are insoluble in water and soluble in organic solvents.

• Steroids: Steroids exhibit a tetracyclic structure called "Cyclopentano perhydrophenanthrene nucleus", based on the functional group. Steroids are of two types –
• Sterols:– Alcoholic steroids, e.g. Cholesterol– Cholesterol abundantly occurs in the brain, nervous tissue, the Adrenal gland and skin. Cholesterol is a parent steroid. Several other biologically important steroids are derived from cholesterol. (7-dehydrocholesterol) which occurs in the skin is a provitamin. On exposure to ultraviolet radiation, it transforms into Cholecalciferol, i.e. vitamin D
•  Sterones :– Ketonic steroids, for example, sex hormones, Adrenocorticoids.

Chromolipid 

• It is also called a terpene. 
• The most complex lipid in protoplasm, Chromolipids, are composed of repeated isoprene units.

2.0Lipids Functions

Lipids perform a wide range of essential functions in living organisms:

• Energy Storage: Lipids, particularly triglycerides, store more than twice the amount of energy per gram compared to carbohydrates. This makes them an efficient long-term energy reserve.
• Structural Components: Phospholipids are the fundamental building blocks of cellular membranes, forming the barrier that encloses cells and their organelles.
• Insulation and Protection: A layer of subcutaneous fat provides thermal insulation, helping to maintain body temperature. It also acts as a protective cushion around vital organs.
• Hormones and Signaling: Steroid hormones, such as estrogen and testosterone, are derived from cholesterol and act as chemical messengers.
• Vitamins: Some vitamins, like vitamins A, D, E, and K, are fat-soluble, meaning they require lipids for proper absorption and transport in the body.

3.0Lipid Examples

• Fats: Butter, ghee, lard
• Oils: Coconut oil, olive oil, sunflower oil
• Phospholipids: Lecithin (found in egg yolk)
• Steroids: Cholesterol, testosterone, estrogen
• Waxes: Cutin on plant leaves, beeswax

4.0Lipid Deficiency and Disorders

• Lipid Deficiency: Can cause dry skin, hormonal imbalance, poor vitamin absorption, and fatigue.
• Lipid Metabolic Disorders:
• Atherosclerosis: Cholesterol deposition in arteries.
• Obesity: Excess lipid accumulation.
• Fatty Liver Disease: Excess fat buildup in liver cells.
• Hyperlipidemia: High levels of lipids in the blood.