Lipids are organic compounds that contain hydrocarbons which are the foundation for the structure and function of living cells. Lipids are nonpolar so they are soluble in nonpolar environments thus not being water soluble because water is polar.
The Lipids are a heterogeneous group of compounds, including fats, oils, steroids, waxes, and other compounds, which are related more by their physical than by their chemical properties. Lipids are a class of compounds distinguished by their insolubility in water and solubility in nonpolar solvents. Lipids are important in biological systems because they form the cell membrane, a mechanical barrier that divides a cell from the external environment. Lipids also provide energy for life and several essential vitamins are lipids. Lipids can be divided into two major classes, nonsaponifiable lipids, and saponifiable lipids. A nonsaponifiable lipid cannot be broken up into smaller molecules by hydrolysis, which includes triglycerides, waxes, phospholipids, and sphingolipids.
A saponifiable lipid contains one or more ester groups allowing it to undergo hydrolysis in the presence of an acid, base, or enzymes. Nonsaponifiable lipids include steroids, prostaglandins, and terpenes. Within these two major classes of lipids, there are several specific types of lipids important to life, including fatty acids, triglycerides, glycerophospholipids, sphingolipids, and steroids.
Each of these categories can be further broken down. Nonpolar lipids, such as triglycerides, are used for energy storage and fuel. Polar lipids, which can form a barrier with an external water environment, are used in membranes. Polar lipids include glycerophospholipids and sphingolipids. Fatty acids are important components of all of these lipids.
Classification of Lipids
Simple lipids: Esters of fatty acids with various alcohols.
- Fats: Esters of fatty acids with glycerol. Oils are fats in the liquid state.
- Waxes: Esters of fatty acids with higher molecular weight monohydric alcohols
Complex Lipids: Esters of fatty acids containing groups in addition to an alcohol and a fatty acid.
- Phospholipids: Lipids containing, in addition to fatty acids and an alcohol, a phosphoric acid residue. They frequently have nitrogen-containing bases and other substituents, eg, in glycerophospholipids the alcohol is glycerol and in sphingophospholipids the alcohol is sphingosine.
- Glycolipids (glycosphingolipids): Lipids containing a fatty acid, sphingosine, and carbohydrate.
- Other complex lipids: Lipids such as sulfolipids and amino lipids. Lipoproteins may also be placed in this category
Precursor and derived lipids
These include fatty acids, glycerol, steroids, other alcohols, fatty aldehydes, and ketone bodies, hydrocarbons, lipid soluble vitamins, and hormones. Because they are uncharged, acylglycerols (glycerides), cholesterol, and cholesteryl esters are termed neutral lipids.
Fatty acids are carboxylic acids (or organic acid), often with long aliphatic tails (long chains), either saturated or unsaturated.
Saturated fatty acids
When a fatty acid is saturated it is an indication that there are no carbon-carbon double bonds and if the fatty acid is saturated it is an indication that it has at least one carbon-carbon double bond. As the following data indicate, the saturated acids have higher melting points than unsaturated acids of the corresponding size.
Unsaturated fatty acid
If a fatty acid has more than one double bond then this is an indication that it is an unsaturated fatty acid. “Most naturally occurring fatty acids contain an even number of carbon atoms and are unbranched.” Saturated fatty acids have higher melting points due to their ability to pack their molecules together thus leading to a straight rod-like shape. Unsaturated fatty acids, on the other hand, have a cis-double bond(s) that create a kink in their structure which doesn’t allow them to group their molecules in straight rod-like shape.
Role of Fats
Waxes are “esters” (an organic compound made by replacing the hydrogen with an acid by an alkyl or another organic group) formed from long-chain carboxylic acids and long-alcohols. Waxes are seen all over in nature. The leaves and fruits of many plants have waxy coatings, which may protect them from dehydration and small predators. The feathers of birds and the fur of some animals have similar coatings which serve as a water repellent. Carnauba wax is valued for its toughness and water resistance(great for car wax).
Membranes are chiefly made of phospholipids which are Phosphoacylglycerols. Triacylglycerols and phosphoacylglycerols are similar however the terminal OH group of the phosphoacylglycerol is esterified with phosphoric acid instead of a fatty acid which leads to the formation of phosphatidic acid. The name phospholipid comes from fact that phosphoacylglycerols are lipids that contain a phosphate group.
The chemical messengers in our bodies are known as Hormones which are organic compounds synthesized in glands and delivered by the bloodstream to certain tissues in order to stimulate or inhibit the desired process. Steroids are a type of hormone which is usually recognized by their tetracyclic skeleton, consisting of three fused six-membered and one five-membered ring, as shown in the diagram below. The four rings are designated A, B, C & D as noted in blue, and the numbers in red represent the carbons.
Cholesterol is an important lipid found in the cell membrane. It is a sterol, which means that cholesterol is a combination of a steroid and an alcohol. It is an important component of cell membranes and is also the basis for the synthesis of other steroids, including the sex hormones estradiol and testosterone, as well as other steroids such as cortisone and vitamin D. In the cell membrane, the steroid ring structure of cholesterol provides a rigid hydrophobic structure that helps boost the rigidity of the cell membrane. Without cholesterol, the cell membrane would be too fluid. In the human body, cholesterol is synthesized in the liver.
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