Introduction to Biomolecules – Definition, Types, Structure and Functions

What are Biomolecules?

Biomolecules are the molecules present in living organisms that serve as a building block and carry out various biochemical functions in the body of living organisms, such as cell division, digestion, growth, and development. Biomolecules include macromolecules like carbohydrates, proteins, lipids, nucleic acids, and micromolecules like metabolites and other natural products. About \(96\% \) of the human body’s mass is made up of these biomolecules that consist of elements such as carbon, hydrogen, oxygen, and nitrogen, along with some other elements. These biomolecules such as carbohydrates, proteins, lipids, nucleic acids, and vitamins have complex chemical structures and are classified accordingly.

Types of Biomolecules

Based on the functions and structures of biomolecules, they are classified as the following major types:

1. Carbohydrates
2. Proteins
3. Nucleic Acids
4. Lipids

Let’s discuss their subtypes, structures, and functions in detail:

Carbohydrates

Carbohydrates are a vast group of naturally occurring organic compounds composed of Carbon, Hydrogen, and Oxygen in the ratio \(1:2:1,\) having the general chemical formula \({{\text{C}}_{\text{x}}}{\left({{{\text{H}}_2}{\text{O}}} \right)_{\text{y}}}.\) As observed from its formula, they are commonly known as hydrates of carbon, hence named as carbohydrates. Cane sugar, glucose, starch, etc., are some common examples of carbohydrates. Chemically, carbohydrates are optically active polyhydroxy aldehydes or ketones. Due to their sweet taste, they are also called sugars.

Carbohydrates are classified as monosaccharides, disaccharides, oligosaccharides, and polysaccharides based on their structures and units. Let us discuss them one by one in detail:

1. Monosaccharides – It is a class of carbohydrates having only one sugar molecule. They cannot be further hydrolysed into simpler sugar molecules. Glucose, fructose, and galactose are some examples of monosaccharides.
2. Disaccharides – It is a class of carbohydrates that contain two monosaccharide units that are fused by a glycosidic linkage—for example, sucrose, lactose, and maltose.
3. Oligosaccharides – A class of carbohydrates that consists of a small number of monosaccharide units, such as three to ten units joined by an \({\text{o}}\)-glycosidic linkage, are known as oligosaccharides, like raffinose.
4. Polysaccharides – As the name suggests, polysaccharides are known as the polymer of saccharides or complex carbohydrates. They form long polymeric chains of monosaccharide units joined together by glycosidic bonds—for example- starch, cellulose, glycogen, and chitin.
Starch is a polysaccharide that is stored in plants as a form of energy. It consists of two types of polymeric chains as amylose and amylopectin. Amylose forms a linear structure with \(\alpha \,1 – 4\) glycosidic linkage, while amylopectin consists of a branched structure with \(\alpha \,1 – 4\) and \(\alpha \,1 – 6\) glycosidic linkages.

Functions of Carbohydrates

1. Carbohydrates help in the metabolism of complex substances and are the primary source of energy of our body in the form of glucose.
2. Cellulose, which is a disaccharide, is a primary constituent of a plant cell. Thus, it is a structural carbohydrate.
3. Carbohydrates are one of the primary sources of calories in our bodies.
4. Starch is stored in plants as a source of energy that contains thousands of glucose units.
5. Glycogen is another complex carbohydrate stored in animal cells and, under stress and prolonged muscular activity, breaks into simple glucose molecules.
6. The exoskeleton of arthropods or insects is made up of a complex carbohydrate known as chitin.

Proteins

A class of nitrogenous organic compounds that consists of macromolecules composed of one or more long chains of amino acids are known as proteins. They are an essential part of all living organisms due to their structural components of body tissues such as muscles, hairs, etc.

Amino Acids

Amino acids contain \( – {\text{N}}{{\text{H}}_2}\) and \( – {\text{COOH}}\) functional groups. They are the monomers of protein that are joined through the polypeptide bond. Depending upon the relative number of amino and carboxyl functional groups, amino acids are classified as acidic, basic, or neutral. They are also classified as:

a. Non-essential Amino acids – The amino acids, which can be synthesised in the body itself, are known as non-essential amino acids.
b. Essential Amino acids – The amino acids, which cannot be synthesised in the body and should be obtained externally through diet, are known as essential amino acids.

Structure and Functions of Proteins

As we know that proteins are the polymers of α-amino acids and are connected to each other by peptide bonds. Chemically, this peptide bond or linkage is an amide formed between \( – {\text{N}}{{\text{H}}_2}\) and \( – {\text{COOH}}\) group, which results in the elimination of a \({{\text{H}}_2}{\text{O}}\) molecule and formation of a peptide bond, i.e., \({\text{CO-NH-}}\).

Based on the number of peptide linkages, amino acids are classified as:

1. Dipeptides – When two amino acids react, the product of the chemical reaction is called a dipeptide because it is made up of two amino acids. For example, Glycine’s carboxyl group combines with the amino group of alanine to get a dipeptide ‘glycylalanine’.
2. Tripeptide – If a third amino acid is attached to a dipeptide through another peptide bond, then the product formed is called a tripeptide. A tripeptide is composed of three amino acids linked by two peptide linkages.
3. Polypeptide – Similarly, when more than ten amino acids are linked to two peptide linkages, then the products are called polypeptides. A polypeptide with more than a hundred amino acid residues, having a molecular mass greater than \(10,000{\text{u}},\) is known as a protein.

Based on the structure, proteins are classified as under:

1. Fibrous proteins – The fibres formed when the polypeptide chains run parallel and are linked together by hydrogen and disulphide bonds are known as fibrous proteins. They are not soluble in water. For example, keratin (present in hair, wool, silk) and myosin fibres (present in muscles), and many more.
2. Globular proteins – When the chains of polypeptides coil around to form a spherical shape, then it is known as globular protein. These are usually water-soluble. For example, Insulin and albumins are common examples of globular proteins. Based on the structure and shape of proteins, globular proteins can be studied as under:
I. Primary proteins: Proteins that have one or more amino acids linked with each other in a specific sequence forming a polypeptide chain that lead to the primary structure of that protein.
II. Secondary proteins: A long polypeptide chain can exist in two different types of structures:
a. \(\alpha \)-helix – It is one of the most common ways in which a polypeptide chain forms hydrogen bonds by twisting into a right-handed screw (helical form) with the –NH group of each amino acid residue bonded to the C O of an adjacent turn of the helix by hydrogen bonding.
b. The \(\beta \)-pleated sheet is another structure of proteins where all peptide chains are stretched out to almost maximum extension and then laid side by side, resembling the pleated folds of drapery held together by intermolecular hydrogen bonds.
3. Tertiary proteins: The further folding of the secondary structure leads to the tertiary structure of proteins representing the overall folding of the polypeptide chains. The main forces of attraction which stabilise the secondary and tertiary structures of proteins are hydrogen bonds, disulphide linkages, Van der Waals, and electrostatic forces.
4. Quaternary proteins: Proteins that are composed of two or more polypeptide chains as sub-units that undergo spatial arrangement with respect to each other are known as quaternary structures.

Denaturation of Protein

When a protein in its original form is subjected to some physical or chemical changes like changes in temperature, pH, hydrogen bonds, etc., its structure gets disturbed. As a result, globules unfold, and the helix gets uncoiled, and protein loses its biological activity. This phenomenon is called the denaturation of proteins. For example, the coagulation of egg white on boiling, curdling of milk results from the denaturation of proteins.

Lipids

Basically, lipids are esters of alcohol (glycerol) and fatty acids. Structurally based on glyceride units, they are divided as monoglycerides, diglycerides, triglycerides. They include fats and oils. Fatty acids are of two types:

1. Saturated Fatty Acids – They are fatty acids that do not have any double bonds. For example, animal fat, palm oil, coconut oil, etc.
2. Unsaturated Fatty Acids – They are fatty acids that have an unsaturated double bond. For example, vegetable oils, sunflower oil, walnuts, fish oil, etc.

Structure and Functions of Nucleic Acids

The genetic material is made up of nucleic acids that collectively refer to DNA (DeoxyriboNucleic Acid) and RNA (Ribonucleic Acid).

They are made from nucleotides and are composed of five chemicals bases:

1. Guanine (G)
2. Thymine(T)
3. Uracil (U)
4. Adenine(A)
5. Cytosine (C)

1. These bases combine with the help of hydrogen bonds along with starch and phosphate. In DNA, base pairs are formed as \({\text{A=T}}\) with two hydrogen bonds and \({\text{G}} \equiv {\text{C}}.\)
2. In RNA, instead of Thymine, Uracil is present. So base pairs formed are \({\text{A=U}}\) and \({\text{G}} \equiv {\text{C}}.\)
3. The genetic information of the cell is stored as DNA.
4. RNA is a polymer of ribose, which is a pentose sugar.

Summary

In short, biomolecules are organic compounds that are building blocks of the living body. They are composed of carbohydrates, proteins, lipids, and nucleic acids. Carbohydrates are mostly optically active polyhydroxy aldehydes or ketone molecules that are broadly classified into three groups such as monosaccharides, disaccharides and polysaccharides that are held together by glycosidic linkages. Glucose is the most important source of energy for mammals. Proteins are the polymers of different \(\alpha \)-amino acids, which are linked together by peptide bonds.

Essential amino acids cannot be synthesised by our body, while non-essential amino acids are synthesised by our body. The secondary or tertiary structure of proteins get ruptured on change of \({\text{pH}}\) or temperature and are unable to perform their functions. This is termed as denaturation of proteins. The entire genetic constituent of our body is made up of nucleic acids, i.e., DNA and RNA, that are responsible for genetic coding and various traits in the body of living beings. Thus, these biomolecules are though non-living molecules, but they support the entire living system to survive.

FAQs on Introduction to Biomolecules

Q.1. What are biomolecules and their importance?
Ans: Biomolecules are organic molecules that are composed of carbon, hydrogen, oxygen, nitrogen, etc., present in all living organisms. They are fundamental building blocks of living beings as they support the biological processes which are essential for sustaining life.

Q.2. What are biomolecules and their types?
Ans: Biomolecules are the molecules present in living organisms that serve as a building block and carry out various biochemical functions in the body of living organisms, such as cell division, digestion, growth, and development. Different types of biomolecules include carbohydrates, proteins, lipids, and nucleic acids.

Q.3. What are the characteristics of biomolecules?
Ans: The various characteristics of biomolecules are that most of them are organic compounds. They have specific shapes, dimensions and the functional group determines their chemical properties. Biomolecules are macromolecules that are constructed from small building block molecules.

Q.4. What are the 5 biomolecules?
Ans: Based on the functions and structures of biomolecules, they are classified into the following five major types, including carbohydrates, proteins, nucleic acids, lipids, and vitamins.