Lipids are classified as those organic compounds which do not interact with water. Put 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.

Interaction with water

Most biological substances can interact with water. However the majority of lipids are hydrophobic, that is, they do not interact with water. Some lipids show a small tendency of being hydrophilic. But this is usually a small part of the molecule, the rest of the molecule is usually hydrophobic. Therefore, such lipid molecules are called amphipathic. Such lipids are usually those which make cellular membranes because of the way they behave in water. When amphipathic lipids come in contact with whatever their hydrophilic parts stay in contact with water and the hydrophobic parts stay away from water.

Most lipids are soluble in non-polar solvents. These non-polar solvents are usually hydrocarbons that dissolve other organic hydrophobic substances.

•  Lipids categories  into two  classes:  Non Saponifiable lipid   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  is enabling  for hydrolysis in the presence of  acid, base , including waxes, triglycerides, phospholipids and sphingolipids etc
• These categories are divided into non-polar and polar lipids
• Nonpolar lipids(triglycerides) are utilized as fuel and to store energy
• Polar lipids are utilized 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 that are arranged in particular combinations. Lipids are classified on the basis of these building block molecules. These classifications also differentiate between the  functions and structure of lipids. They are as follows: 

• Fatty acids: Part of carboxylic acids made up of a hydrocarbon chain have the terminal group COOH. If the hydroxyl ion (OH) is removed from the carboxylic acid, the remaining fragment is called an acyl group. In water this acid is usually without a hydrogen ion, 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 they can
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 special polyunsaturated fatty acid is commonly called trans fat. This kind of fat is synthesized in the stomach of cud-chewing animals and can be created by the hydrogenation of some oils and/or fats. These kinds of fats have been found to be detrimental to metabolic health.

Functions of lipids in the body

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

•   Forming membranes: The vast 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.

• Storage of energy

Another important part of the lipids functions in the body is storage of energy. Let us see how this is done. Energy is stored mostly in the form of triglycerides because they are the most efficient for this function in animals as well as in plants. There are a specialized group of cells called adipocytes that break down triglycerides. This releases energy. This constant process is controlled by the enzyme lipase.

Triglycerides release energy at the rate of about 38kJ/g(9kcal/g). When compared to the 17kJ/g (4kcal/g) of carbohydrates this is quite a bit higher. That is why triglycerides are preferable for storing energy. The adipose tissue of animals is where the triglycerides are stored. In some animals, they even act as insulation against the cold.

•     Signaling

Cell signaling is the communication that happens among cells and between cells and their environment. Lipids are a part of this process. Several lipids have been discovered to be cellular messengers and have roles in regulating cellular activity, cellular growth etc. 

•     Other functions

Fat soluble vitamins such as A, D, E and K are lipids and are also essential nutrients needed by organisms. They are stored in the liver and fatty tissue and are used in several functions in the body. Acyl-carnitine transports fatty acids to the mitochondria. Polyprenols and some of their derivatives transport oligosaccharides into and out of the membrane.

Metabolism

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

•     Biosynthesis

○      Surplus carbohydrates in animals are converted into triglycerides. This is done by synthesizing fatty acids with the help of acetyl-CoA which is 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 synthesized by introducing the double bond by a chemical reaction called a desaturation reaction

○      Triglycerides are synthesized in the endoplasmic reticulum.

•     Degradation

○      Fatty acids are broken down in the cell by a process called beta-oxidation. Most fatty acids are oxidized by a process that is 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 specialized 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 very important 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 useful pursuit.