Lipids: Definition, Structure, Types, Classification

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What are Lipids?

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Lipids can be defined as the organic fatty acid compounds that are present in all living organisms. These compounds are formed when oxygen, carbon, and hydrogen are bonded together. These organic molecules are considered the building blocks of living cells as they store energy and act as the structural components of cell membranes. Sterols, triglycerides, waxes, and phospholipids are some examples of lipids.

Triglycerides is a most common type of lipids which is stored in the form of fats in the adipose tissue of the body. They are composed of long chains of hydrocarbon and a polar oxygen group. They play a major role in regulating hormones, transferring nerve impulses, and storing energy in the form of fat. A large amount of carbohydrate in our diet is converted into triglycerides which are esters formed from glycerol and three fatty acids.

Lipid Bilayer

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Properties of Lipids

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Lipids are heterogeneous organic compounds that are composed of more than one element including oxygen, hydrogen, carbon, and gases like phosphorus, nitrogen, and sulphur. The physical and chemical properties of lipids are given below:

Physical Properties of Lipids

Following are some of the important physical properties of lipids: 

• Storage: Lipids store energy in the form of fats in the adipose tissue of the body.
• Appearance: At room temperature, they are either liquid or crystalline solids.
• Solubility: Lipid molecules are soluble in non-polar solvents such as acetone, alcohol, etc. and insoluble in water.

Chemical Properties of Lipids

Lipids are energy-rich organic molecules that enable them to perform various life functions. The chemical properties of lipids are as follows: 

• Hydrolysis of Tryglerols: Triglecyrides react with sodium or potassium hydroxide to form salts of fatty acids and glycerol or soap.
• Rancidity: Due to oxidation and hydrolysis of fats and oils, packaged foods become tasteless and smell bad after a certain time, leading to rancidity.
• Halogenation: The reaction of halogen with saturated fatty acids results into the decolorization of halogen solution. 
• Saponification: The chemical process through which soaps are formed is called saponification. This involves a chemical reaction between triglycerides with alkalis such as potassium or sodium hydroxide. 

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Types of Lipids

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There are three types of lipids based on the presence of groups– Simple Lipids, Complex Lipids & Derived Lipids. 

Simple Lipids

Simple lipids are esters of fatty acids and alcohol. Fats, oils, and waxes are the types of simple lipids.

Fats and Oils

• Fats and oils are called triacylglycerol as they are composed of three fatty acids joined together by glycerol and trihydroxy alcohol.
• They are produced from fatty acid and glycerol by performing hydrolysis.
• Soyabean, coconut, peanut, butter, avocado, and walnut are some sources of fats and oils.

Waxes

• Waxes are long-chain fatty acid esters and long-chain alcohols consisting of 12-34 carbon atoms.
• They are mainly found on fruits, leaves, stems of plants, hair, and skin.
• Common examples of waxes are beeswax and ear wax (cerumen)

Complex Lipids 

Complex lipids are also called compound lipids and these are another group of lipids that combine fatty acids, alcohols, and other groups such as nitrogenous bases, proteins, carbohydrates, phosphate, etc. 

• Complex lipids are based on a group that combines different types of phospholipids and glycolipids.
• It also combines a group of lipoproteins, sulfolipids, and amino lipids. complex lipids are of three types.

Phospholipids

Depending on the group of alcohol present, it could be glycerol (glycerophospholipids) or sphingosine (sphingophospholipids). 

• They are a yield of fatty acids, glycerol, amino acid, and another alcohol group that contain nitrogen on performing hydrolysis.

Structure of Phospholipid

Glycerophospholipids 

• These complex lipids are also called glycolipids that contain fatty acids, carbohydrates, and sphingosine. 

Lipoprotein

Lipoproteins are made of protein and fat and carry cholesterol through the bloodstream. They are droplets of fat surrounded by a single layer of phospholipid molecules.

• There are two main types of cholesterol: High-density lipoprotein (HDL), or "good" cholesterol, and Low-density lipoprotein (LDL), or "bad" cholesterol.
• Another example is Chylomicron, which is one of the largest lipoproteins. 

Structure of Lipoprotein

Derived Lipids

When simple and complex lipids combine and undergo hydrolysis it gives rise to derived lipids. These lipids include fatty acid, glycerol, carotenoid, sterol, steroid, etc.

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Structure of Lipids

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The structure of lipids comprises carbon, hydrogen, and oxygen. They are not polymers as they do not have a monomeric unit.

• Glycerol and fatty acids are the two molecules that make up a lipid.
• Three atoms of carbon with a hydroxyl group (OH) attached to it complete the glycerol structure.
• Fatty acids contain one acid group and a hydrocarbon chain usually denoted by ‘R’. 
• Unsaturated and saturated are the two types of fatty acids.

Saturated Fatty Acids

saturated fatty acids are the simplest chain of fatty acids unbranched and a linear chain of CH₂ linked by carbon-carbon single bonds with a single carboxylic group at the terminal.

• They are about 12-24 carbon long and are solid at room temperature.
• Food products like beef, poultry, milk, cheese, palm oil, coconut oil, etc are examples of saturated fatty acids.

Unsaturated Fatty Acids

Unsaturated fatty acids are a long chain of one or more double bonds attached to the carbon atom.

• The term unsaturated is defined as the maximum possible number of hydrogen atoms that are bonded with other carbon molecules. 
• Carbon molecules having a single double bond are called monounsaturated and polyunsaturated are those with two double bonds.
• Olive oil, canola oil, nut oils are examples of foods with unsaturated fatty acids.

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Classification of Lipids

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On the basis of their chemical reactivity lipids are classified into two groups: nonsaponifiable lipids and saponifiable lipids.

Nonsaponifiable Lipids

Nonsaponifiable lipids cannot be saponified through alkaline hydrolysis.

• They have a complex structure.
• Cholesterol and steroids are examples of nonsaponifiable lipids.

Saponifiable Lipids

Saponifiable lipids can be saponified through alkaline hydrolysis.

• They consist of fatty acids and other compounds that can be disintegrated into simpler substances.
• Triglycerides (fats and oils) are one of the examples of saponifiable lipids.

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Functions of Lipids

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Lipids perform all the vital functions within the body. It helps in storing and preserving energy, regulating hormones, and transmitting nerve impulses. It also helps in the transport of fat-soluble nutrients. In this section, we will look into the functions of lipids in detail. 

• They are stored in the adipose tissues of the body and act as an efficient energy source.
• Lipids also act as an insulator for protecting many organs.
• They help in regulating hormones and maintain a constant temperature and the body’s internal climate.
• They are an important part of the cell membrane that helps in margining the cell's fluidity and flexibility in the membrane. 
• In the human reproductive system, lipids are required for maintaining reproductive health.

Functions of Lipids 

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Things to Remember

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• A lipid profile test is carried out to measure the cholestrol and other fat levels in the blood.
• Wax is an example of a lipid that is used in everyday products such as candles, chewing gum, nail paints, etc.
• These fatty compounds when undergo hydrolysis and oxidation cause rancidity.
• Excessive consumption of saturated fats can lead to increased cancer risk. 
• Fat helps the body in the absorption of vitamins such as vitamins A, D, E, and K.