Biomolecules Class 12: Examples, Definition, Notes

Biomolecules are the building blocks of life. Simply, the molecules that keep every living thing running, from the smallest bacteria to us, human beings, are biomolecules.

These are organic compounds that are naturally produced by living organisms and are essential for all biological processes. Examples of biomolecules are glucose, hemoglobin, cholesterol, fats and oils, DNA, and RNA.

Types of Biomolecules

Living systems contain four major types of biomolecules: carbohydrates- the primary energy source, and structural components, proteins- structural materials, and biological catalysts, nucleic acids- information storage and transfer, DNA and RNA, and lipids- energy storage and membrane components. There are smaller molecules, like vitamins and minerals, that play crucial supportive roles.

Carbohydrates are the most easily available, with more than a quantity of organic compounds on Earth, primarily produced by plants through photosynthesis. You encounter them daily as sugar, starch, and cellulose.

What are Carbohydrates?

In Chemical definition, Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that produce such units on hydrolysis.

Carbohydrates general formula Cx(H₂O)y, making it appear like “hydrates of carbon.”

Classification of Carbohydrates

Carbohydrates are classified based on their hydrolysis behavior. Classification of carbohydrates is into three groups: Monosaccharides, oligosaccharides, and polysaccharides.

1. Monosaccharides

These are the simplest carbohydrates that cannot be broken down further.

Classification by carbon atoms:

• Triose: 3 carbons (e.g., glyceraldehyde)
• Tetrose: 4 carbons
• Pentose: 5 carbons (e.g., ribose, deoxyribose)
• Hexose: 6 carbons (e.g., glucose, fructose)

Classification by functional group:

• Aldose: Contains aldehyde group (-CHO)
• Ketose: Contains the ketone group (>C=O)

Examples of monosaccharide carbohydrates are: Glucose, fructose, galactose, and ribose.

Glucose

• Molecular formula: C₆H₁₂O₆
• Type: Aldohexose
• Occurrence: Fruits, honey, blood

Preparation of Glucose

From Sucrose:

$C_{12}{H}_{22}{O}_{11}+H_{2}O\to C_6{H}_{12}{O}_6+C_6{H}_{12}{O}_6$

From Starch:

$\left(C_6{H}_{10}{O}_5\right)n+n{H}_{2}O\to n{C}_6{H}_{12}{O}_6$

Structure of Glucose:

Molecular formula confirmed as C₆H₁₂O₆. Forms n-hexane on heating with HI (proves 6-carbon straight chain). Reacts with hydroxylamine and hydrogen cyanide to produce gluconic acid with mild oxidizing agents, and forms pentaacetate (confirms 5 -OH groups)

Cyclic structure:

Glucose exists mainly in cyclic form, where the -OH at C-5 adds to the -CHO group, forming a six-membered ring called pyranose.

Anomers: Two forms (α and β) differing in configuration at C-1 (anomeric carbon).

Also Read: Difference between Glucose and Fructose

2. Oligosaccharides

Oligosaccharides yield 2-10 monosaccharide units on hydrolysis.

Disaccharides (most common):

• Sucrose: Glucose + Fructose
• Maltose: Glucose + Glucose
• Lactose: Galactose + Glucose

3. Polysaccharides

Large molecules yield many monosaccharide units on hydrolysis.

Examples of polysaccharides are Starch, cellulose, and glycogen.

Importance of Carbohydrates

Carbohydrates are important for everyone, plants, and animals.

• Primary energy source (4 kcal/g)
• Structural components (cellulose in plants)
• Energy storage (starch in plants, glycogen in animals)

Industrial applications of carbohydrates are in Textiles (cotton - cellulose), the Paper industry, the Food industry, the Pharmaceuticals industry, etc.

Most abundant biomolecules in living systems that are essential for life are proteins. They’re like the workhorses of our cells, performing vital functions.

What are Proteins?

Proteins are complex biomolecules made up of long chains of amino acids connected by peptide bonds. 

Amino Acids

Amino acids contains amino group (-NH2) and carboxyl group (-COOH)

Structure of Amino Acids

All amino acids have a common basic structure:

    R

    |

H₂N-CH-COOH

Where R is the side chain that makes each amino acid unique.

Classification of Amino Acids by Position

Based on the position of the amino group relative to the carboxyl group:

• α-amino acids: Amino group attached to the carbon next to the carboxyl group
• β, γ, δ amino acids: Less common in proteins

Only α-amino acids are found in proteins.

Types of Amino Acids

1. Essential amino acids: Amino acids that cannot be made by the human body.

• Histidine, Isoleucine, Leucine, Lysine
• Methionine, Phenylalanine, Threonine
• Tryptophan, Valine, Arginine

2. Non-essential amino acids: these can be made by the human body.

• Alanine, Asparagine, Aspartic acid, Cysteine
• Glutamic acid, Glutamine, Glycine, Proline
• Serine, Tyrosine

Classification by Nature of Amino Acids

• Acidic amino acids: More carboxyl groups than amino groups

Examples: Aspartic acid, Glutamic acid

• Basic amino acids: More amino groups than carboxyl groups

Examples: Lysine, Arginine, Histidine

• Neutral amino acids: Equal number of amino and carboxyl groups

Examples: Glycine, Alanine, Valine

Some of the Physical Properties of Amino Acids:

• Colorless, crystalline solids
• High melting points
• Water-soluble
• Behave like salts rather than simple amines or acids

Structure of Proteins

There are four structures of proteins: Primary Structure, Secondary Structure, Tertiary Structure, and Quaternary Structure.

• Primary Structure

It is the sequence of amino acids in the polypeptide chain. This determines other levels of structure and the functions of proteins.

• Secondary Structure

Secondary structure of proteins is the regular and repeating arrangements in space along the polypeptide backbone.

The two main types of secondary structure of proteins are:

α-Helix:

Right-handed spiral structure, stabilized by hydrogen bonds between C=O and N-H groups. Common in fibrous proteins

β-Pleated Sheet:

Extended polypeptide chains positioned side by side, with hydrogen bonds between adjacent chains, create a pleated appearance. Found in silk proteins

• Tertiary Structure

The overall 3D folding of the polypeptide chain. It forms fibrous and globular proteins. Forces that stabilize the structure of a protein are: Hydrogen bonds, Disulfide bridges (between cysteine residues, Van der Waals forces, Electrostatic interactions, and Hydrophobic interactions.

• Quaternary Structure

The arrangement of multiple polypeptide chains in proteins containing more than one chain is the quaternary structure. Example: Hemoglobin has four subunits (2α and 2β chains).

Types of Proteins: Based on Shape

Based on shape, there are two types of proteins: Fibrous proteins and Globular proteins.

Fibrous Proteins

When polypeptide chains move parallel and held together by hydrogen bonds they form fibrous proteins

They are insoluble in water and provide structural support.

Examples:

• Keratin: Hair, wool, nails
• Collagen: Tendons, skin
• Myosin: Muscle proteins
• Fibroin: Silk

Globular Proteins

Globular Proteins are formed when polypeptide chains turn into spherical shapes. It has Hydrophobic parts inside and hydrophilic parts outside. They are water-soluble and perform metabolic functions.

Examples:

• Insulin: Blood sugar regulation
• Hemoglobin: Oxygen transport
• Albumin: Blood protein
• Enzymes: Biological catalysts

Denaturation of Proteins

Loss of native structure and biological activity due to the disruption of secondary and tertiary structures of protein, while the primary structure remains intact, is known as denaturation of proteins.

Denaturation of proteins causes:

• Heat: Cooking, high fever
• pH changes: Acids or bases
• Heavy metals: Lead, mercury
• Organic solvents: Alcohol
• Radiation: UV light

Examples:

• Egg white coagulation during cooking
• Milk curdling due to lactic acid formation

Consequences of Denaturation of Proteins:

• Loss of biological activity
• Change in physical properties
• Usually irreversible under normal conditions

Enzymes are biological catalysts that make life possible by speeding up chemical reactions in living organisms. Without enzymes, the reactions necessary for life would occur too slowly to sustain biological processes.

What are Enzymes?

Enzymes are specialized proteins that catalyze specific biochemical reactions. They work by lowering the activation energy required for reactions to occur, making processes happen millions of times faster than they would naturally. 

• Almost all enzymes are globular proteins
• Have specific 3D structures essential for function
• Can be denatured like other proteins
• Required in very small quantities
• Enzymes can catalyze thousands of reactions per second
• They are not consumed in the reaction (can be reused)

Vitamins are organic compounds that our body needs in small amounts to function properly, but cannot produce in the required quantities. They’re like helpers that keep our biological system running smoothly.

What are Vitamins?

Organic compounds are required in small quantities in our diet to perform specific biological functions for growth, maintenance, and health.

Classification of Vitamins

Vitamins are classified based on their solubility:

1. Fat-Soluble Vitamins
2. Water-Soluble Vitamins

Fat-Soluble Vitamins (A, D, E, K)

They are soluble in fats and oils and insoluble in water, stored in the liver and fatty tissues. They can accumulate to toxic levels if consumed in excess.

Vitamins	Sources	Functions	Deficiency
Vitamin A (Retinol)	Fish liver oil, carrots, butter, milk	Vision, skin health, and immune function	Night blindness, xerophthalmia
Vitamin D (Calciferol)	Sunlight exposure, fish, egg yolk	Calcium absorption, bone health	Rickets (children), osteomalacia (adults)
Vitamin E (Tocopherol)	Vegetable oils, nuts, and seeds	Antioxidant, cell membrane protection	RBC fragility, muscular weakness
Vitamin K (Phylloquinone)	Green leafy vegetables	Blood clotting	Increased bleeding time

Water-Soluble Vitamins (B-Complex, C)

They are soluble in water and not stored in the body (except B12), required in the daily diet, and readily excreted in urine.

Vitamin	Sources	Functions	Deficiency
Vitamin B₁ (Thiamine)	Yeast, cereals, green vegetables	Carbohydrate metabolism, nervous system	Beriberi
Vitamin B₂ (Riboflavin)	Milk, eggs, liver	Energy metabolism, skin health	Cheilosis, skin disorders
Vitamin B₆ (Pyridoxine)	Meat, cereals, and vegetables	Protein metabolism, neurotransmitter synthesis	Convulsions, skin problems
Vitamin B₁₂ (Cobalamin)	Meat, fish, dairy products	DNA synthesis, red blood cell formation	Pernicious anemia
Vitamin C (Ascorbic Acid)	Citrus fruits, tomatoes, and green vegetables	Collagen synthesis, antioxidant, and immune function	Scurvy

Read more about Vitamins

Nucleic acids store information and transfer molecules of life. They carry the genetic instructions that make each organism unique and enable the transfer of characteristics from parents to children.

What is Nucleic Acid?

Nucleic acid definition- Nucleic acids are biomolecules composed of long chains of nucleotides. Chromosomes that are responsible for heredity are made of protein and biomolecules called nucleic acids.

There are two main types of Nucleic Acids:

• DNA (Deoxyribonucleic Acid): Stores genetic information
• RNA (Ribonucleic Acid): Transfers genetic information and synthesizes proteins

Chemical Composition of Nucleic Acids

Each nucleotide consists of three components:

1. Pentose Sugar:

• Ribose in RNA
• 2-Deoxyribose in DNA (lacks -OH at C-2)

2. Phosphoric Acid: Provides acidic character
3. Nitrogenous Bases:

Purines (double ring):

Adenine (found in both DNA and RNA), Guanine (found in both DNA and RNA).

Pyrimidines (single ring):

Cytosine (found in both DNA and RNA), Thymine (found only in DNA), Uracil (found only in RNA)

Structure of Nucleic Acids

Nucleoside vs Nucleotide

• Nucleoside: Base + Sugar
• Nucleotide: Base + Sugar + Phosphate

DNA Structure

DNA structure according to the Watson-Crick model:

• Two antiparallel polynucleotide strands
• Complementary base pairing
• Spaces between helical strands

RNA Structure

• Typically single-stranded
• Can fold back on itself, creating hairpin loops
• Less stable than DNA due to the 2'-OH group

Types of RNA

Messenger RNA (mRNA)

• Transfers genetic information from DNA to ribosomes
• Template for protein synthesis
• Single-stranded and linear

Ribosomal RNA (rRNA)

• Large structural component of ribosomes
• Catalyzes the formation of peptide bonds
• Most common type of RNA

Transfer RNA (tRNA)

• Transfers amino acids to the protein strand
• Possesses a distinctive cloverleaf secondary shape
• Contain an anticodon that is complementary to the mRNA codon

DNA vs RNA

Check the table below to understand the key differences between DNA and RNA.

Feature	DNA	RNA
Sugar	2-Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Structure	Double-stranded	Single-stranded
Function	Genetic storage	Protein synthesis
Stability	More stable	Less stable
Location	Nucleus, mitochondria	Nucleus, ribosomes, cytoplasm

Biological Functions of Nucleic Acids

The functions of DNA are to store and transmit genetic information, and to copy itself during cell division. DNA contains instructions for making proteins, and Mutations provide variation for natural selection.

The functions of RNA are to control gene expression. mRNA, tRNA, and rRNA work together. Some RNAs have enzymatic activity (ribozymes).

Hormones are the chemical messengers that coordinate and control various body functions. Hormones are like the body’s internal communication system, which makes sure that organs work together.

What are Hormones?

The intercellular messengers that are produced by endocrine glands are called Hormones. They are transported through the blood to target organs. Hormones regulate biological processes in the body.

Chemical Nature of Hormones

Hormones belong to different chemical classes:

• Steroid Hormones- Cholesterol-derived, lipophilic, able to move through cell membranes with ease.

Examples: Testosterone, Estradiol, Progesterone, Cortisol.

• Peptide/Protein Hormones- Composed of amino acid chains, soluble in water, and are unable to pass through cell membranes directly

Examples: Insulin: Blood sugar control, Growth hormone: Growth and development, Glucagon: Elevates blood glucose.

• Amino Acid Derivatives- Altered amino acids, which are primarily water-soluble.

Examples: Epinephrine (Adrenaline): fight-or-flight response, Norepinephrine: stress response, Thyroxine: regulation of metabolism.

Important Hormones and Their Functions

• Insulin- Reduces blood glucose and stimulates cellular uptake of glucose.
• Glucagon- Increases blood glucose and stimulates release of glucose from the liver.
• Thyroxine- Controls metabolism
• Growth Hormone- Stimulates growth and development
• Testosterone: Secondary male features, reproduction
• Estradiol: Secondary female features, menstrual cycle
• Progesterone: Maintaining pregnancy, preparation of the uterus

For further reading, check: Biomolecules NCERT PDF