What are Lipids?

The heterogeneous classes of organic compounds that are not water-soluble but are dissolved in organic solvents that are non-polar in nature are termed lipids. They are a long chain of fatty acids and esters of alcohols. Lipids are generally seen in several plants, microorganisms, and animals. They are utilized as insulation, components of the cell membrane, hormones, and molecules for the storage of energy.

What are Lipids made of?

Lipids majorly contain hydrocarbons in their most reduced structure. This makes lipids a significant component for the storage of energy, because, during metabolic reactions of hydrocarbon, they get oxidized to liberate large quantities of energy. Certain lipids contain small quantities of sulfur, nitrogen, and phosphorus. Triglyceride is a kind of lipid, which is seen in cells of fat. Triglyceride is an ester that is produced by the condensation reaction of fatty acid molecules (three) with glycerol.

Structure of Lipids

The elements hydrogen, carbon, and oxygen make up lipids. They also have small amount of water when compared to other molecules like carbohydrates. Lipids are not polymers because they contain do not include any repeating monomeric unit. The lipid formation is through two molecules namely glycerol and fatty acid. A molecule of glycerol contains three atoms of carbon along with hydroxyl group linked to it but the rest of the positions are accommodated by the atoms of hydrogen. Fatty acids have carboxylic acid functional group present at one terminal, and a chain of hydrocarbon which is given by “R”. Fatty acids are unsaturated or saturated. A saturated fatty acid contains hydrogen bonds made with an atom of hydrogen and no C=C  bond is found whereas an unsaturated fatty acid contains  bond. When an unsaturated fatty acid has one C=C bond, then it is known as mono-unsaturated fatty acid whereas when an unsaturated fatty acid contains more than one C=C bond, it is known as polyunsaturated fatty acid. Omega-6 fatty acids and omega-3 fatty acids are polyunsaturated fatty acids. Both these fatty acids are dietary fats.

Triglyceride structure

The lipid that has one glycerol molecule linked to fatty acid molecules (three) is triglycerides. The nature of bonds found among the molecules is covalent and bonds are esters linkages. For triglycerides that are naturally found, the chains of fatty acid contain 16, 18, or 20 atoms of carbons. The formation of triglycerides occurs at the time of condensation reaction. The distribution of charges is even enclosing the molecules so the molecule of water does not lead to the formation of any hydrogen bonds that result in their insoluble nature in water.

The figure shows the chemical reaction for the formation triglyceride from three molecules of fatty acids and glycerol

What are the several kinds of Lipids?

The classification of lipids is dependent on the products of hydrolysis and based on the resemblances in their molecular structures.

Depending on the products given out by hydrolysis, the two categories of lipids are given as:

  • Nonsaponifiable lipids.
  • Saponifiable lipids.

Nonsaponifiable Lipids

A lipid that could not be broken down into smaller molecules through the hydrolysis reaction is known as a nonsaponifiable lipid. Certain examples of nonsaponifiable lipids are fat-soluble vitamins such as vitamin A, vitamin D, vitamin E, and vitamin K, prostaglandins, and cholesterol.

Saponifiable Lipids

A lipid, which has one group of ester or more than one group of ester making it going through hydrolysis in the presence of an enzyme, a base, or an acid named as saponifiable lipid. Certain of the examples include sphingolipids, waxes, phospholipids, triglycerides, etc.

Classes of saponifiable Lipids

The three main classes of lipids are given as,

  • Simple lipids.
  • Compound lipids or complex lipids.
  • Derived lipids.

Simple lipids

Esters of fatty acid with several alcohols are simple lipids. Simple lipids comprise fats and waxes.

  • Fats and Oils: Both oils and fats on hydrolysis gives out glycerol and fatty acids as products. Fats and oils are triacylglycerols as they are ester containing three molecules of fatty acids bonded to glycerol. The difference among them is their dissimilarity in their physical state at normal temperature. At normal temperature, if the lipid is solid, then it is named as fat and at normal temperature or same temperature, if the lipid is liquid, then it is named as oil.
  • Waxes: An ester that includes a fatty acid and monohydroxyl long-chain of alcohol is wax. The numbers of atoms of carbons in length seen in the acids and alcohols of waxes are in order of 12-34 atoms of carbon. Natural waxes are combination of esters and could also have hydrocarbons.

Complex lipids or Compound lipids

Esters of fatty acids having groups other than fatty acid and alcohol are complex lipid or compound lipid. Complex lipids are lipoproteins, phospholipids, and glycolipids.

  • Phospholipids: Phospholipids on hydrolysis gives phosphoric acid, fatty acids, sphingosine, glycerol and an alcohol comprising of nitrogen as their products. They are major entities of plasma membrane. They could be sphingophospholipid or glycerophospholipid depending on the presence of alcohol group. The alcohol found in glycerophospholipid is glycerol. The alcohol found in sphingophospholipid is sphingosine.
The figure shows the structure of phospholipid
  • Glycolipids: Glycolipids on hydrolysis gives a molecule of carbohydrate, glycerol or sphingosine, and fatty acids as their products. They could be sphingoglycolipid or glyceroglycolipid depending on the presence of alcohol group. The alcohol found in glyceroglycolipid is glycerol. The alcohol found in sphingoglycolipid is sphingosine.
  • Other complex lipids: Lipoprotein and sulfolipids are few of the examples of other complex lipids. Lipoprotein is made up of fat molecule as well as protein molecule. The major role of lipoprotein is to transfer the water-fearing molecules of lipid in water. A lipoprotein has triglyceride and cholesterol center, enclosed by outer shell which is phospholipid, whereas the water-loving regions are arranged outside in the direction of enclosing water and lipophilic regions are arranged inside with respect to lipid center. Based on their density, lipoproteins are classified into five types and they are,
  • Chylomicrons.
  • Very low density lipoprotein (VLDL).
  • Intermediate-density lipoprotein (IDL).
  • Low density lipoprotein (LDL).
  • High Density lipoprotein (HDL).

Several adhesins, toxins, enzymes, antigens, transporters come under lipoprotein.

Derived lipids

Derived lipids are the hydrolysis product of simple lipids as well as complex lipids. Examples of derived lipids are glycerol, fatty acid, derivatives of steroids, and sphingosine.

Fatty acids: Carboxylic acids with aliphatic tails that are in long chains which are saturated or unsaturated are called as fatty acids. A fatty acid is said to be saturated, when no C=C  bond is seen in the compound. A fatty acid is said to be unsaturated, when the compound comprises of one or more than C=C  one bond. The saturated fatty acids exhibit greater melting points when compared to unsaturated ones.

The figure shows the example of saturated fatty and unsaturated fatty acid

Fatty acids are broken down by beta-oxidation to acetyl-CoA by the acetylation reaction of CoA.

  • Cholesterol: Sterols are derived lipids that contain steroids alcohol which occurs in certain bacteria, animals, fungi, and plants. Cholesterol is the most common type of sterol and can be inserted in the membrane of cells. Cholesterol is a precursor to hormones and fat-soluble vitamins like vitamin D. The generation of cholesterol is through four bonded hydrocarbon rings producing the bulk structure of steroid. A hydrocarbon tail is found at the one terminal of cholesterol, whereas the alcohol group is attached at the other terminal. The hydroxyl group is bonded to additional groups of hydroxyl or with phospholipids containing carbonyl oxygen. The liver produces cholesterol in the body for the production of bile acids. One of the primary bile acids is cholic acid which is formed by the liver.
The figure shows the structure of cholesterol

Properties of Lipids

The high amount of energy is given out by lipids and is accountable for the several functions inside the human body. Certain of the significant properties of lipids are discussed below:

  • At room temperature, the physical states of lipids are either non-crystalline solids or liquids.
  • The nature of pure oils and fats are odorless, colorless, and tasteless.
  • They are not water soluble whereas dissolved in organic solvents.
  • High proportions of fatty acids, which are saturated, are seen in solid triglycerols, which are fats.
  • High proportions of fatty acids that are unsaturated are seen in liquid triglycerols, which are oils.

Role of Lipids

  • In human body, a sufficient amount of source of energy is supplied by fats and adipose tissue stores them.
  • The significant structural compounds of biological membrane are sterols and phospholipid.
  • The dynamic feature of the plasma membrane is because of the phosphoplipids.
  • The significant substances of the membrane of cell and mitochondria are lipoproteins. Lipoproteins are formed by combination of fats with proteins. An important role is played by lipoproteins in transferring lipids to tissues from liver.
  • Cholesterol plays the role of precursor to hormones and fat-soluble vitamins like vitamin D.
  • Lipids activate enzymes such as glucose-6-phosphatase, stearyl CoA desaturase, beta-hydroxybutyric dehydrogenase etc.

Context and Applications

This topic is needed for Bachelors in Chemistry, Biochemistry, and Masters in Biochemistry.

Practice Problems

Question 1: Which of the following is an example of fatty acid that is monounsaturated?

  1. Oleic acid
  2. Arachidonic acid
  3. Palmitic acid
  4. Linolenic acid

Answer: Option (1) is correct.

Explanation: The structure of oleic acid is given as,

The figure shows the structure of oleic acid

Oleic acid contains one C=C bond; therefore oleic acid is a monounsaturated fatty acid.

Question 2: The one which stores the energy in terms of per unit weight of any molecule in the body?

  1. Protein
  2. Nucleic acids
  3. Lipids
  4. Carbohydrates

Answer: Option (3) is correct.

Explanation: Lipids have the tendency to store the maximum energy in terms of per unit mass of any molecule. When a fatty acid undergoes complete oxidation, the energy obtained is 9kcal/g when compared to proteins and carbohydrates which give 4kcal/g.

Question 3: The one which does not belong to the predominant kind of membrane lipid?

  1. Free fatty acid
  2. Glycolipid
  3. Cholesterol
  4. Phospholipid

Answer: Option (1) is correct.

Explanation: The important sections of membranes are glycolipids, cholesterol, and phospholipids. In the membranes, free fatty acids are not present.

Question 4: Which of the following fits the best for fatty acid?

  1. Hydrophilic
  2. Inorganic
  3. Hydrophobic
  4. Amphipathic

Answer: Option (4) is correct.

Explanation: As fatty acids contain non-polar regions and polar regions, they are amphipathic.

Question 5: When there is an increase in double bonds, the melting point of triacylglyceride could

  1. Increase
  2. Decrease
  3. Either increase or decrease
  4. No change

Answer: Option (2) is correct.

Explanation: When there is an increase in double bonds, the melting point of triacylglyceride could decrease.

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