Biomolecules – JEE Main ChemistryPractice Questions & PYQs

Enzymes operate with a high degree of specificity, meaning they are tailored to catalyze particular reactions with specific substrates rather than acting on a wide, non-specific range of molecules. This structural and functional selectivity is fundamental to their behavior as biological catalysts, which work by lowering the activation energy of reactions and generally consist of protein chains. Furthermore, these catalysts are sensitive to their environment, functioning optimally only within specific temperature and pH ranges, which invalidates the claim that they are non-specific agents.

The structure of the DNA double helix relies on specific hydrogen bonding interactions between opposing nitrogenous bases to maintain stability and genetic integrity. In this molecular arrangement, the pyrimidine base thymine is chemically structured to form hydrogen bonds exclusively with the purine base adenine. Because these two molecules function as complementary partners, thymine pairs directly with adenine to ensure accurate base pairing along the DNA strands.

Vitamin K functions as a critical co-factor in the biochemical cascade that enables blood coagulation. A deficiency in this nutrient prevents the body from synthesizing essential clotting factors, directly hindering the process that typically stops hemorrhage. Consequently, without adequate Vitamin K to facilitate this rapid solidification, the blood takes much longer to clot, which may manifest as prolonged bleeding internally or externally.

Barfoed's test relies on the distinct reaction rates of sugars within a mildly acidic environment to differentiate between molecular structures. Because monosaccharides are effective enough as reducing agents to form a red cuprous oxide precipitate much more rapidly than disaccharides under these specific conditions, this test is uniquely suited to distinguish one from the other.

Proteins are classified based on their structural conformation and their interaction with solvents, where globular proteins are distinguished by their ability to dissolve readily in water. Albumin is a classic example of this category, as it functions as a highly water-soluble protein that plays a vital role in maintaining osmotic pressure and transporting substances through the blood. In contrast, proteins such as fibrin, myosin, and collagen serve primarily as fibrous structural or contractile elements and are naturally insoluble in aqueous environments.

A non-reducing sugar is defined by the absence of a free anomeric carbon capable of participating in oxidation-reduction reactions, distinguishing it from monosaccharides like glucose, fructose, and galactose which contain active carbonyl groups. Sucrose, which corresponds to sugar A, acts as a non-reducing sugar because its glycosidic linkage involves the anomeric carbons of both its constituent parts, preventing the molecule from opening into a form that can act as a reducing agent. This bond is cleaved during hydrolysis to produce a mixture of one D-glucose molecule and one D-fructose molecule, both of which are reducing sugars, as illustrated by the following reaction:
$C_{12}H_{22}O_{11} + H_2O \rightarrow C_6H_{12}O_6 + C_6H_{12}O_6$

Proteins adopt distinct folding patterns dictated by specific molecular forces at each structural level. While the primary sequence is defined by the covalent peptide bonds linking individual amino acids, the secondary structure-which includes arrangements like alpha-helices and beta-sheets-is stabilized by regular hydrogen bonding between the carbonyl oxygen ($-\text{C}=\text{O}$) and the amide hydrogen ($-\text{NH}-$) groups of the peptide chain. In contrast, glycosidic linkages are characteristic of carbohydrate structures, specifically joining monosaccharides together, rather than determining the folding of protein chains.

Peptides are generated via a condensation reaction where the carboxyl group of one amino acid bonds to the amino group of another, eliminating a water molecule in the process. For any peptide chain containing $n$ amino acid units, the total number of peptide bonds is determined by the formula $n - 1$. Because the molecule consists of 4 distinct amino acids-glycine, leucine, aspartic acid, and histidine-the reaction produces $4 - 1$ or 3 peptide linkages.

Tyrosine is classified as a non-essential amino acid uniquely characterized by the presence of a phenolic group on its side chain. Its fundamental structure is built upon the standard alpha-amino acid backbone, $NH_2-CH(R)-COOH$, where the R-group is specifically a $CH_2-C_6H_4-OH$ substituent. This specific arrangement of an aromatic ring bonded to a hydroxyl group is the defining feature that differentiates it from other amino acids and identifies the correct chemical representation.

Uracil functions within RNA by bonding with a ribose sugar to create the ribonucleoside uridine, which subsequently transforms into uridine $5'$-mono-phosphate upon the addition of a phosphate group. Chemical evidence confirms that among the available isomers, the lactam structure is the most stable and prevalent configuration found in these biological systems.

The $\alpha$-helix is a fundamental secondary structure of proteins that relies on consistent, recurring intramolecular interactions to maintain its stable coiled shape. This specific conformation is secured through hydrogen bonding between the hydrogen atom of an $-\mathrm{NH}$ group and the oxygen atom of a carbonyl $-\mathrm{C}=\mathrm{O}$ group located several residues along the polypeptide backbone.

Cytosine is a heterocyclic pyrimidine base built upon a six-membered ring containing two nitrogen atoms at positions 1 and 3. Its molecular structure, represented by the formula $C_4H_5N_3O$, is uniquely identified by the presence of a carbonyl group at the 2 position and an amino group attached to the carbon at the 4 position. This specific substitution pattern on the aromatic ring is what defines the chemical identity of cytosine within the context of nucleic acids.

Maltose is a disaccharide built from two $\alpha$-D-glucose units connected by a glycosidic linkage that joins the anomeric $C_1$ of the first unit to the $C_4$ carbon of the second. In this $\alpha$-anomer, the geometry requires the oxygen bridge to descend from the first ring to the second, preserving the specific downward orientation of the hydroxyl group on the anomeric carbon of the second unit. This arrangement of the two glucopyranose rings, linked exclusively between these specific carbon positions, accurately depicts the molecular structure.

Sucrose is a non-reducing disaccharide because the glycosidic linkage involves the anomeric carbons of both monosaccharide units, preventing the presence of any free aldehyde or ketone groups capable of undergoing oxidation. While the assertion accurately reflects these chemical properties, the reason provided contains a factual error regarding the configuration of the participating isomers. The glycosidic bond in sucrose actually forms between the $C_1$ of $\alpha$-D-glucose and the $C_2$ of $\beta$-D-fructose, rendering the provided reason incorrect.

Biochemical transformations rely on highly specific enzymes to catalyze distinct reactions, where the hydrolysis of sucrose into glucose and fructose is effectively mediated by the enzyme $A$, known as invertase. Following this initial breakdown, the fermentation of glucose into ethanol and carbon dioxide, expressed as $\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{Zymase}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$, is carried out by the enzyme $B$, which is zymase. This enzyme-specific activity identifies the catalysts for the two steps as invertase and zymase respectively.

Understanding the classification of chemical compounds in pharmacology requires identifying the specific physiological effect each substance produces. Valium is widely recognized as a tranquilizer, frequently prescribed to alleviate symptoms of anxiety. Morphine is identified as an analgesic, commonly administered to provide relief from severe pain. Norethindrone serves as an antifertility drug, acting as a synthetic hormone that regulates ovulation and reproductive cycles. Vitamin ${B}_{12}$ is a crucial nutrient for physiological health, and its deficiency leads to a specific blood disorder known as pernicious anaemia. This mapping process correctly connects each drug to its primary medicinal application.

Lactose is a disaccharide formed by a $\beta$-1,4-glycosidic linkage between the $\beta$-D-galactopyranose unit and the D-glucopyranose unit, which makes the claim of an $\alpha$-glycosidic linkage incorrect. This $\beta$-configuration is a fundamental structural feature that distinguishes lactose from other disaccharides like maltose, which contains an $\alpha$-linkage. Consequently, the assertion that lactose possesses an $\alpha$-glycosidic linkage is scientifically inaccurate.

Seliwanoff's test is a standard diagnostic tool used to differentiate between aldose and ketose sugars, as the latter reacts with the reagent to rapidly form a specific color. When the glycosidic bond in sucrose undergoes cleavage, the reaction $\text{Sucrose} \xrightarrow{\text{Hydrolysis}} \text{Glucose} + \text{Fructose}$ produces two monosaccharides. Because fructose is a ketose sugar, its interaction with the Seliwanoff reagent results in the formation of a red colour.

A chiral carbon atom is defined as an $sp^3$ hybridized carbon atom bonded to four distinct chemical groups. In the amino acid Isoleucine, the $\alpha$-carbon is chiral, and the $\beta$-carbon in the side chain ($CH(CH_3)CH_2CH_3$) is also chiral because it is bonded to a hydrogen atom, a methyl group, an ethyl group, and the $\alpha$-carbon, yielding 2 chiral centers. Arginine contributes 1 chiral center located at its $\alpha$-carbon, as the carbons within its side chain are not bonded to four distinct groups. Proline also contributes 1 chiral center at its $\alpha$-carbon, given that the carbons in its pyrrolidine ring are achiral. Summing these individual contributions results in a total of $2 + 1 + 1 = 4$ chiral carbons for the peptide Ile-Arg-Pro.

A chiral center occurs when a carbon atom is covalently bonded to four different chemical substituents, creating a point of asymmetry within an organic molecule. Examining the structure of threonine, $CH_3-CH(OH)-CH(NH_2)-COOH$, reveals that both the alpha-carbon atom and the beta-carbon atom are attached to four unique groups. Because both of these specific carbons satisfy this definition, there are 2 chiral centers present in the molecule.