Lipids are classified as those organic compounds which do not interact with water. In simple terms, most substances made of lipids will not be miscible with water. The study of the function and structure of lipids constitutes a sizable part of the study of organic compounds because they make up such substances as oils, fats, wax, membranes, and energy-storing molecules in plants and animals.

Lipid types

Lipids are categorised into two classes: Non-saponifiable lipids and Saponifiable lipids.

Non saponifiable lipids

A non-saponifiable lipid does not break down into smaller molecules through hydrolysis. E.g., prostaglandins, cholesterol, etc.

Saponifiable lipids

A saponifiable lipid consists of one or more ester groups. Ester group enables hydrolysis in acid or base, including waxes, triglycerides, phospholipids, sphingolipids, etc.These categories are divided into non-polar and polar lipids.

Nonpolar lipids(triglycerides) are utilised as fuel and to store energy.

Polar lipids are utilised in membranes. Polar lipids are sphingolipids and glycerophospholipids.

Structure of lipids

Lipids, though not as large as macromolecules (polysaccharides, proteins, etc.), are made up of smaller molecules arranged in particular combinations. Lipids are classified based on these building block molecules. These classifications also differentiate between the functions and structure of lipids. They are as follows: 

1. Fatty acids: Part of carboxylic acids of a hydrocarbon chain have the terminal group COOH. The remaining fragment is called an acyl group if the hydroxyl ion (OH) is removed from the carboxylic acid. This acid is usually without a hydrogen ion in water, forming a negatively charged COO or carboxylate group. For this reason, fatty acids are amphipathic; that is, their hydrocarbon chain is hydrophobic, and their negatively charged COO end is hydrophilic. 

1. Saturated fatty acids: These contain single-bonded carbon atoms that have formed bonds with as many hydrogen atoms as possible.
2. Unsaturated fatty acids: These contain double-bonded carbon atoms. The number of such double-bonded pairs of carbon atoms determines whether they are monounsaturated or polyunsaturated.

A kind of particular polyunsaturated fatty acid is commonly called trans fat. This kind of fat is synthesised in the stomach of cud-chewing animals and can be created by the hydrogenation of some oils and/or fats. These kinds of fats are detrimental to metabolic health.

Functions of lipids in the body

There are many vital lipids in the body. They are as follows:

• Forming membranes: The wide variety of higher-order organisms (most plants and animals) we see around us are eukaryotic. This means that their cells and their cell organelles are separated from their surrounding environments by membranes. These are called biological membranes and consist of the plasma membrane and other intracellular membranes. The main building blocks of these membranes are glycerophospholipids. These are:

○ Amphipathic molecules

○ Have a head group joined to the core by a phosphate ester link

○ Have fatty acid-derived tails that link to the core with ester links

○ The core is glycerol.

Lipid metabolism

Triglycerides, sterols, and biological membrane lipids (primarily phospholipids) are the main lipids in the diets of most animals, including humans. These are metabolised in the following ways

• Biosynthesis:

○ Extra carbohydrates in animals are converted into triglycerides. This is done by synthesising fatty acids with the help of acetyl-CoA, followed by the esterification of the fatty acids. This process is called lipogenesis. The acids can then be converted into triglycerides.

○ Unsaturated fatty acids are synthesised by introducing the double bond by a chemical reaction called a desaturation reaction.

○ Triglycerides are synthesised in the endoplasmic reticulum.

• Degradation:

○ Fatty acids are broken down in the cell by a process called beta-oxidation. Most fatty acids are oxidised by a process almost identical to the reversal of the synthesis of fatty acids.

○ Unsaturated fatty acids require additional steps which involve enzymes for degradation.

Importance of lipids 

Lipids play a wide range of roles in the functioning of organisms. Some of which are as follows:

• The fat in the body is stored in the form of lipids. These include triglycerides, cholesterol, and phospholipids.
• Lipids need to be included in the dirt to facilitate the absorption of fat-soluble vitamins like A, D, E, and K.
• Linoleic acid is a major component of most plant-based oils and fats. These are mostly omega-6 and omega-3 fatty acids. Studies have shown them to be beneficial for the development of infants, alleviating cancer-related problems, treating cardiovascular and psychological ailments.
• Biological membranes are thought to be essential to the development of life itself. They separate the cells from their environments, cell organelles that perform specialised tasks and make metabolism and other functions possible. Consequently, the biological importance of lipids is also very high since they make up these biological membranes.

Conclusion

Since lipids play such an essential role in the body, it is crucial to understand the structure and function of lipids. This understanding helps in solving several metabolism-related problems. Moreover, lipids form the building blocks of the three large food groups, namely carbohydrates, proteins, and fats. Since the biological importance of lipids is so high, studying them is a valuable pursuit.