Revision: Biomolecules Chemistry Science (English Medium) Class 12 CBSE

Biomolecules are organic compounds present in living organisms as essential constituents of different cells, such as carbohydrates, proteins, fats, and amino acids.

Carbohydrates that are crystalline solids, sweet in taste and soluble in water are called sugars.

Carbohydrates are optically active polyhydroxy aldehydes or polyhydroxy ketones or compounds that can be hydrolysed to polyhydroxy aldehydes or polyhydroxy ketones.

Carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or compounds which produce such units on hydrolysis, such as cellulose, glycogen, starch, etc.

The sugars that reduce the Tollen's reagent and Fehling's solution are called reducing sugars.

Carbohydrates that are amorphous solids, tasteless and insoluble in water are catled non-sugars.

A monosaccharide that contains one ketonic carbonyl group is called a ketose.

A ketose with six carbon atoms is called a ketohexose.

An aldose monosaccharide that has six carbon atoms (e.g., Glucose) is called an aldohexose.

Monosaccharides that contains one aldehydic group is called aldose.

Fructose is another commonly known monosaccharide having the same molecular formula as glucose. It is levorotatory and a ketohexose. It is present abundantly in fruits, and hence it is also called fruit sugar.

Proteins are complex polyamides formed from amino acids. They are essential for the proper growth and maintenance of the body. They have many peptide (-CO–NH )bonds.

The bond that connects α-amino acids to each other is called a peptide bond.

Enzymes are biological catalysts that speed up chemical reactions in living cells without being consumed in the process.

Chemically, proteins are polyamides, which are high molecular weight polymers of the monomer units called \[\alpha\]-amino acids.

Chemically proteins are polyamides which are high molecular weight polymers of the monomer units, i.e., α-amino acids. OR It can also be defined as proteins are the biopolymers of a large number of α-amino acids and they are naturally occurring polymeric nitrogenous organic compounds containing 16% nitrogen and peptide linkages (-CO-NH-)

Bifunctional organic compounds containing a carboxylic and an amino group either at the same carbon atom or at nearby carbon atoms are called amino acids.

α-Amino acids are carboxylic acids having an amino (–NH₂) group bonded to the α-carbon, that is, the carbon next to the carboxyl (–COOH) group.

Amino acids that cannot be synthesised in the human body and must be obtained through diet are known as essential amino acids.

An ∝-amino acid molecule contains both acidic carboxyl (-COOH) group as well as basic amino (-NH₂) group. Proton transfer from acidic group to basic group of amino acid forms a salt, which is a dipolar ion called zwitter ion.

Amino acids which contain more number of amino groups than carboxyl groups are called basic amino acids.

Proteins may have one or more polypeptide chains. Each polypeptide in a protein has amino acids linked with each other in a specific sequence and it is this sequence of amino acids that is said to be the primary structure of that protein. Any change in this primary structure, i.e., the sequence of amino acids, creates a different protein.

Amino acids which are synthesised by the body itself are called non-essential amino acids.

Amino acids which contain more number of carboxyl groups than amino groups are called acidic amino acids.

Amino acids which contain equal number of amino groups and carboxyl groups are called neutral amino acids.

Chemically, peptide linkage is an amide formed between the –COOH group and –NH₂ group. The reaction between two molecules of similar or different amino acids proceeds through the combination of the amino group of one molecule with the carboxyl group of the other. This results in the elimination of a water molecule and the formation of a peptide bond –CO–NH–. The product of the reaction is called a dipeptide because it is made up of two amino acids.

For example, when the carboxyl group of glycine combines with the amino group of alanine, we get a dipeptide, glycylalanine.

Denaturation is the process in which the secondary and tertiary structure of a protein is disrupted due to heat, a change in pH, or chemicals, while the primary structure remains unchanged. In denaturation, peptide bonds are not broken; only the weak bonds (like hydrogen bonds) are disturbed.

When a protein, in its native form, is exposed to changes, such as temperature or pH, the hydrogen bonds are disrupted. Due to this, globules unfold, the helix uncoils, and the protein loses its biological activity. It is called denaturation of proteins, e.g., coagulation of egg white on boiling, curdling of milk, etc.

A colloidal solution of protein which works as a biological catalyst is known as an enzyme.

Vitamins are organic compounds essential for the average growth of life for animals, some bacteria and microorganisms.

Nucleic acids are large biological macromolecules that store and transmit genetic information in living organisms.

The unit formed by joining the anomeric carbon of the furanose (sugar) with a nitrogen of a base is called nucleoside.

DNA is a double-stranded nucleic acid that stores and transmits hereditary information and can replicate itself.

A nucleotide is the basic structural unit of nucleic acids, composed of a nitrogenous base, a pentose sugar, and a phosphate group.

A nitrogenous base is an organic molecule (purine or pyrimidine) that carries genetic information in nucleic acids.

RNA is a single-stranded nucleic acid that helps in protein synthesis and information transfer.

A nucleoside consists of a nitrogenous base linked to a pentose sugar without a phosphate group.

The technique of identifying an individual by analyzing the unique DNA sequence present in each person, similar to fingerprints, is called DNA fingerprinting.

Biological catalysts which increase the rate of biochemical reactions are called enzymes.

Organic compounds required in small amounts in the diet to perform specific biological functions are called vitamins.

The unit formed when a nucleoside is linked to phosphoric acid is called nucleotide.

Long chains of nucleotides joined by phosphodiester linkage are called nucleic acids.

Chemical messengers secreted by endocrine glands and transported through blood are called hormones.

The sequence of amino acids in a polypeptide chain is called primary structure of protein.

The linkage formed between two monosaccharide units through an oxygen atom is called glycosidic linkage.

Carbohydrates which yield a large number of monosaccharide units on hydrolysis are called polysaccharides.

Carbohydrates which yield two to ten monosaccharide units on hydrolysis are called oligosaccharides.

Carbohydrates which cannot be hydrolysed to simpler units are called monosaccharides.

Optically active polyhydroxy aldehydes or ketones or the compounds which yield such units on hydrolysis are called carbohydrates.

The cyclic hemiacetal forms of a sugar differing in configuration at the anomeric carbon are called anomers.

• Common Composition - All living organisms are made of the same elements, like carbon, hydrogen, and oxygen, detected through elemental analysis.
• Same Elements, Different Abundance - Both living and non-living matter contain the same elements, but carbon and hydrogen are more abundant in living organisms.
• Major Elements in Human Body - Oxygen (65%), Carbon (18.5%), Nitrogen (3.3%), Hydrogen (0.5%).
• Earth's Crust vs Human Body - Silicon (27.7%) and Oxygen (46.6%) dominate Earth's crust, while Carbon dominates living matter despite being only 0.03% in the crust.
• Significance - Living organisms selectively concentrate certain elements, making their composition different from non-living matter.

• Carbohydrates are organic biomolecules made of C, H and O, usually fitting the general formula Cx(H₂O)y and existing as aldoses or ketoses.
• They are classified into monosaccharides, disaccharides and polysaccharides; monosaccharides cannot be hydrolysed further, disaccharides are formed by two monosaccharides via glycosidic bonds, and polysaccharides are long polymers.
• Some sugars like digitoxose (C₆H₁₂O₄) and rhamnose (C₆H₁₂O₅) do not obey the typical Cx(H₂O)y formula.
• All monosaccharides are reducing sugars because they possess a free aldehyde or ketone group.
• Cellulose is a linear polymer of β‑D‑glucose, unlike starch and glycogen, which are polymers of α‑glucose and show branching.
• Biologically, carbohydrates supply energy for metabolism; glucose is the main substrate for ATP synthesis, and lactose provides energy to infants.
• Polysaccharides such as starch and glycogen act as storage products and also contribute to structural components of cell membranes and cell walls.

• Glucose is a monosaccharide, an aldohexose, and a reducing sugar, commonly found in fruits and also known as dextrose.
• It can be prepared by hydrolysis of sucrose (using dilute acid) or hydrolysis of starch under heat and pressure.
• Glucose confirms a straight-chain structure of six carbon atoms when reduced to n-hexane.
• Presence of functional groups is shown by reactions: –CHO (aldehyde), five –OH groups, and formation of derivatives like oxime and cyanohydrin.
• Oxidation reactions indicate the formation of gluconic acid (mild oxidation) and saccharic acid (strong oxidation), confirming functional groups in glucose.

• Glucose is an aldohexose with molecular formula \[C_{6}H_{12}O_{6},\mathrm{M.P.146^{\circ}C.}\]
• 'D' in D-(+)-Glucose = configuration; (+) = dextrorotatory nature; 'D'/'L' have no relation to optical activity.
• Glucose has five —OH groups (confirmed by glucose pentaacetate) and one aldehydic carbonyl group (confirmed by oxime & cyanohydrin formation).
• Glucose is soluble in water, sparingly soluble in alcohol, and insoluble in ether.
• The additional chiral centre in glucose ring structures is formed due to ring closure.

Structure of Fructose:

Open Chain Structure	Ring Structure
	\[\alpha\]-D-(-)-Fructofuranose	\[\beta\]-D-(-)-Fructofuranose

Preparation of Fructose:

• From hydrolysis of cane sugar (sucrose): 
  \[ \underset{\text{Sucrose}}{\mathrm{C}_{12}\mathrm{H}_{22}\mathrm{O}_{11}} + \mathrm{H}_2\mathrm{O} \xrightarrow{\text{Dil } \mathrm{H}_2\mathrm{SO}_4} \underset{\text{Glucose}}{\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6} + \underset{\text{Fructose}}{\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6} \]

• From inulin: 
  \[ (\mathrm{C}_6\mathrm{H}_{10}\mathrm{O}_5)_n + n\mathrm{H}_2\mathrm{O} \xrightarrow{\text{Dil } \mathrm{H}_2\mathrm{SO}_4} \underset{\text{Fructose}}{n\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6} \]

• Sucrose consists of one unit each of α-D-glucopyranose and β-D-fructofuranose.
• It contains an α, β-1,2-glycosidic linkage.
• Maltose is composed of two α-D-glucopyranose units joined by an α-1,4-glycosidic bond.
• Lactose consists of β-D-galactopyranose and β-D-glucopyranose units.
• It has a β-1,4-glycosidic linkage.

• Starch — A polymer of α-D-glucopyranose. It has two components: amylose (α-1,4-glycosidic linkage) and amylopectin (both α-1,4 and α-1,6-glycosidic linkages).
• Cellulose — A polymer of β-glucopyranose units linked by β-1,4-glycosidic bonds.
• Glycogen — A polymer of glucose units.
• Linkage Comparison —

  Polysaccharide	Monomer	Linkage
  Amylose (Starch)	α-D-glucopyranose	α-1,4
  Amylopectin (Starch)	α-D-glucopyranose	α-1,4 and α-1,6
  Cellulose	β-glucopyranose	β-1,4
  Glycogen	Glucose	—

   
• Key Distinction — Starch and Cellulose are both glucose polymers but differ in linkage type: Starch has α-glycosidic bonds (digestible by humans), while Cellulose has β-glycosidic bonds (not digestible by humans).

• Primary source of energy for plants and animals
• Honey consists of a mixture of carbohydrates (source of quick energy)
• Starch and cellulose are stored in plants; glycogen is stored in animals/humans
• Used in the textile, paper, and alcohol industry
• Found in combination with proteins and lipids (e.g., glycoproteins, glycolipids)

• Proteins are polymers of amino acids (polypeptides) in which amino acids are linked by peptide bonds.
• There are 20 types of amino acids, so proteins are heteropolymers (not homopolymers).
• Amino acids are of two types: essential (must be obtained from diet) and non-essential (can be synthesised in the body).
• Proteins are high molecular weight biomolecules (polyamides) made of α-amino acids with a general structure R-CH(NH₂)-COOH.
• Proteins perform various functions such as enzymatic activity, transport, hormonal regulation, immunity, and sensory reception.
• Proteins are of two main types: fibrous proteins (insoluble, structural, e.g., keratin) and globular proteins (soluble, functional, e.g., enzymes, insulin).
• Collagen is the most abundant protein in animals, while RuBisCO is the most abundant enzyme in the biosphere.

α-Amino acids: Carboxylic acids where α-hydrogen is replaced by the –NH₂ group

General structure: \[ \mathrm{R} - \underset{\underset{\displaystyle \mathrm{NH}_2}{|}}{\overset{\overset{\displaystyle \alpha}{\displaystyle \mathrm{CH}}}{}} - \mathrm{COOH}\]
(where R = H or alkyl group at α-carbon)

Essential amino acids (not synthesised in the body; must be taken in food):

1. Leucine
2. Isoleucine
3. Lysine
4. Methionine
5. Phenylalanine
6. Threonine 
7. Tryptophan
8. Valine

Non-essential amino acids (synthesised in the body):

1. Alanine
2. Asparagine
3. Aspartic acid
4. Cysteine
5. Glutamic acid
6. Glutamine
7. Glycine
8. Proline
9. Serine
10. Tyrosine

Semi-essential (50% body + 50% food):

1. Arginine
2. Histidine

Glycine is the only optically inactive amino acid (no chiral centre; R = H)

Isoelectric Point (pI):

• pH at which an amino acid does not migrate in an electric field → exists as a zwitterion (+NH₃–CH(R)–COO⁻)
• At pI, the concentration of zwitterions is maximum; anionic and cationic forms are equal
• At pI, an amino acid has the least solubility in water (used in separation by isoelectric precipitation)
• For neutral amino acid: \[pI=\frac{1}{2}(pk_{a_1}+pk_{a_2})\]

• Enzymes are biological catalysts, mostly proteins, that increase the rate of biochemical reactions without being consumed.
• Some enzymes are ribozymes, which are RNA molecules that act like enzymes.
• Enzymes have primary, secondary, and tertiary structures, and their 3D structure determines their specificity and function.
• Each enzyme has a specific active site where the substrate binds to form an enzyme–substrate complex.
• Enzymes are highly specific and lower the activation energy of reactions.
• Enzyme activity is affected by temperature and pH; most enzymes are denatured at high temperatures, while thermophilic enzymes remain stable at 80–90°C.
• Examples of enzymes include amylase (starch → glucose), pepsin (proteins → amino acids), lactase (lactose → glucose + galactose), and maltase (maltose → glucose).

Mechanism of Enzyme Action (Lock and Key model):

1. Enzyme (E) binds to substrate (S) → ES complex (E + S → ES)
2. Product formation: ES → EP
3. Product released: EP → E + P (enzyme regenerated)

• Enzymes work best at 298 K to 313 K (25°C to 40°C) — optimum temperature
• Activity decreases with temperature increase or decrease beyond optimum range; stops at ~273 K

Vitamins = organic compounds essential in small amounts for normal growth and functioning

Not synthesised in adequate amounts by the body → must be supplied in the diet.

• Nucleic acids are biomacromolecules present in the acid-insoluble fraction and are responsible for the storage and transmission of genetic information (DNA and RNA).
• They are polynucleotides, formed by repeated units called nucleotides.
• Each nucleotide consists of three components: a nitrogenous base, a pentose sugar, and a phosphate group.
• Nitrogenous bases are of two types: purines (adenine, guanine) and pyrimidines (cytosine, thymine, uracil).
• The sugar present is either ribose (in RNA) or 2′-deoxyribose (in DNA).
• DNA is double-stranded and contains bases A, T, G, and C, while RNA is single-stranded and contains A, U, G, and C.
• DNA stores genetic information, while RNA plays a key role in protein synthesis and the expression of genetic information.

Nucleotide = Phosphate group + Sugar + Nitrogenous base

Hydrolysis pathway:

• DNA → Nucleotide → Phosphoric acid + Nucleoside → Sugar (2-deoxyribose) + Purine/Pyrimidine base
• RNA → Nucleotide → Phosphoric acid + Nucleoside → Sugar (Ribose) + Purine/Pyrimidine base

Nitrogen Bases:

• Nitrogenous bases in nucleic acids are of two types: purines and pyrimidines.
• Purine bases have a double-ring structure and include Adenine (A) and Guanine (G).
• Pyrimidine bases have a single-ring structure and include Cytosine (C), Thymine (T), and Uracil (U).
• Thymine is present in DNA, while Uracil is present in RNA instead of thymine.

• DNA fingerprinting is a technique used to identify an individual by analysing the unique DNA pattern present in every person (except identical twins).
• It is based on satellite DNA, especially VNTRs (Variable Number Tandem Repeats) - short sequences repeated in tandem, whose number varies among individuals and creates DNA polymorphism.
• Principle: the differences in VNTR repeat number produce DNA fragments of different lengths, which appear as a unique banding pattern.
• Steps: DNA isolation → PCR amplification → restriction digestion → gel electrophoresis → Southern blotting → probe hybridisation → autoradiography → comparison of band patterns.
• Applications: forensic identification, paternity/maternity testing, pedigree studies, medical research, conservation biology, and evolutionary/anthropological studies.

Hormones are chemical substances produced by ductless (endocrine) glands; released into the bloodstream to regulate organ functions

Classified into: Steroid hormones, Amine hormones, Peptide hormones

• Proteins are classified into two types based on their molecular shape: fibrous proteins and globular proteins.
• Fibrous proteins consist of parallel polypeptide chains held together by hydrogen and disulphide bonds; they are generally insoluble in water and provide structural support.
• Globular proteins are formed when polypeptide chains coil into a spherical shape; they are usually soluble in water and perform functional roles such as enzymatic and hormonal activities.
• Protein structure is organized into four hierarchical levels: primary, secondary, tertiary, and quaternary, each representing increasing complexity of folding and organization.
• The stability of higher-level protein structures (secondary, tertiary, and quaternary) is maintained by hydrogen bonds, disulphide linkages, van der Waals forces, and electrostatic interactions.