Biomolecules – JEE Main ChemistryPractice Questions & PYQs

Peptide chains are linear polymers where amino acids are linked covalently by peptide bonds, characterized by the $-CO-NH-$ functional group, which connect the carboxyl group of one residue to the amino group of the next. To determine the primary structure of a tetrapeptide, one performs total hydrolysis to break these linkages, effectively releasing the individual constituent amino acids. Analysis of the resulting hydrolysis products for this specific chain confirms the presence of phenylalanine (F), leucine (L), aspartic acid (D), and tyrosine (Y) arranged in the sequence FLDY.

An asymmetric carbon atom, or chiral center, is defined as a carbon atom bonded to four distinct groups, creating a unique spatial configuration. In the molecular structure of testosterone ($C_{19}H_{28}O_2$), the steroidal backbone features six such stereocenters located at specific ring junctions and substitution sites within the tetracyclic framework.

The chiral centers are identified at carbons C8, C9, C10, C13, C14, and C17. Specifically, carbon C8 is bonded to C7, C9, C14, and a hydrogen atom; carbon C9 is bonded to C8, C10, C11, and a hydrogen atom; carbon C10 is bonded to C1, C5, C9, and the methyl group C19; carbon C13 is bonded to C12, C14, C17, and the methyl group C18; carbon C14 is bonded to C8, C13, C15, and a hydrogen atom; and carbon C17 is bonded to a hydroxyl group, a hydrogen atom, C13, and C16. Other carbons in the molecule, such as C5, are not chiral because they participate in a carbon-carbon double bond and are therefore $sp^2$ hybridized, while the remaining carbons are bonded to multiple identical hydrogen atoms. Consequently, the total number of asymmetric carbon atoms in the structure is 6.

Sulfur-containing amino acids are identified by the presence of a sulfur atom within their side chain, which manifests as either a thiol group or a thioether linkage. Cysteine incorporates a thiol group in its structure, denoted as $\mathrm{HS}-\mathrm{CH}_2-\mathrm{CH}(\mathrm{NH}_2)-\mathrm{COOH}$, while methionine contains a thioether bridge represented by $\mathrm{CH}_3-\mathrm{S}-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{CH}(\mathrm{NH}_2)-\mathrm{COOH}$. The remaining molecules, specifically isoleucine, lysine, and glutamic acid, do not contain sulfur atoms in their side chains, as their structures are characterized by aliphatic hydrocarbon chains, basic amine groups, and acidic carboxyl groups, respectively.

Enzymes act as highly specific biological catalysts that facilitate biochemical transformations by lowering the activation energy required for a reaction to proceed. The hydrolysis of sucrose into glucose and fructose is mediated by invertase, whereas the conversion of glucose into ethyl alcohol and $\mathrm{CO}_2$ is catalyzed by zymase. Furthermore, diastase serves to convert starch into maltose, and pepsin catalyzes the breakdown of proteins into amino acids, confirming the matches as A-III, B-I, C-IV, and D-II.

The primary structural variation distinguishing DNA from RNA is found within their backbone pentose sugars. DNA incorporates $\beta$-D-2-deoxyribose, a sugar defined by the loss of an oxygen atom at the $C-2'$ position, while RNA relies on $\beta$-D-ribose, which retains the hydroxyl group at that site. This specific difference in the sugar moiety is fundamental to the nomenclature and chemical properties of both nucleic acids.

The molecular formula $\mathrm{C}_4\mathrm{H}_8\mathrm{O}_4$ corresponds to an aldotetrose, a carbohydrate containing an aldehyde group and three hydroxyl groups, which is consistent with the positive Tollen's test and the formation of a triacetate derivative upon acetylation. Oxidation with bromine water selectively converts the aldehyde group into a carboxylic acid, producing an optically active aldonic acid, whereas nitric acid ($\mathrm{HNO}_3$) oxidizes both the terminal aldehyde and primary alcohol groups to carboxylic acids, yielding an aldaric acid. Because the nitric acid oxidation product is optically inactive, the starting sugar must have a symmetric structure that forms a meso-dicarboxylic acid, which identifies the compound as an isomer of erythrose. Given the requirement for the L-isomer, the compound is L-erythrose.

Peptide linkages are amide bonds formed through the condensation reaction between the carboxyl group of one amino acid and the amino group of the next. For any polypeptide chain comprised of $n$ amino acid residues, there are exactly $n - 1$ peptide bonds connecting them. The name alanylglycylleucylalanylvaline identifies a pentapeptide, which consists of 5 amino acid residues. Consequently, applying the relationship $5 - 1$ confirms there are 4 peptide linkages.

A carbohydrate qualifies as a reducing sugar specifically when it possesses a free $-\mathrm{OH}$ group attached to the anomeric carbon, which allows the molecule to open its ring structure and act as a reducing agent. Because the chemical structure identified in option A retains this specific functional arrangement at the anomeric position, it exhibits the characteristic properties of a reducing sugar.

Protein architecture is maintained by different classes of chemical interactions, where higher-level configurations such as secondary, tertiary, and quaternary structures depend on fragile forces like hydrogen bonding that succumb to thermal energy. The primary structure, however, consists of amino acids linked together by robust covalent peptide bonds that remain stable during the denaturation process. Consequently, while heating effectively dismantles the complex folding of a protein, it leaves the linear sequence of the primary structure entirely unchanged.

Maltose is a disaccharide formed by the condensation of two $\alpha$-D-glucose units, where the $C_1$ atom of one glucose molecule joins the $C_4$ atom of another through a glycosidic linkage. Because this structure preserves a free hemiacetal group at the anomeric carbon, the molecule retains the ability to reduce oxidizing agents, confirming that it is a reducing sugar. Since Statement I accurately reflects this composition and chemical behavior, while Statement II provides an incorrect monosaccharide combination, an invalid linkage site, and an erroneous classification, only the first statement is true.

The determination of a pyranose structure in a Haworth projection relies on the convention that groups positioned to the right of the carbon chain in a Fischer projection are directed downwards, while those on the left are directed upwards. For D-sugars, the $-CH_2OH$ group is consistently placed pointing upwards at C5, and the anomeric configuration at C1 is determined by the hydroxyl group orientation, where an alpha-anomer has the hydroxyl group pointing down and a beta-anomer has it pointing up. The correct pyranose structure is identified by matching the spatial arrangement of the hydroxyl groups at C2, C3, and C4 of the cyclic hemiacetal to their corresponding positions in the Haworth representation, ensuring all stereochemical centers align correctly.

Sucrose is a non-reducing disaccharide formed by the condensation of $\alpha$-D-glucose and $\beta$-D-fructose through a glycosidic linkage that connects the C1 carbon of glucose to the C2 carbon of fructose. This specific $\alpha$-1,2-$\beta$-glycosidic bond is unique to sucrose, distinguishing it from other carbohydrates like maltose and lactose, which utilize C1-C4 linkages, or amylose, which is a polymer of $\alpha$-glucose units connected by C1-C4 glycosidic bonds.

The stability of the $\alpha$-helix structure in proteins relies fundamentally on intramolecular hydrogen bonding along the polypeptide backbone. Within this arrangement, a hydrogen bond is formed between the oxygen atom of a $C=O$ group of one amino acid residue and the hydrogen atom of an $N-H$ group located four residues further down the sequence. These repeating attractive interactions provide the necessary structural reinforcement to maintain the rigidity and precise geometry of the helical coil.

DNA is fundamentally composed of three distinct units: a nitrogenous base, a phosphate group, and a sugar molecule. When DNA undergoes complete hydrolysis, it is broken down into these basic structural components. Since the sugar backbone inherent in DNA consists of a five-carbon structure, it is scientifically categorized as a pentose sugar.

The Biuret test is a chemical assay that identifies the presence of proteins and peptides by requiring at least two peptide linkages to form a complex with copper ions. Within the provided list, Glycine is a simple amino acid and Glycylalanine is a dipeptide with only one peptide bond, so both yield a negative result. In contrast, a tripeptide contains two peptide linkages, and the Biuret molecule itself also tests positive, meaning a total of two compounds fulfill the necessary criteria.

The presence of $\beta$-glycosidic linkages is the defining structural characteristic that distinguishes cellulose from the starch family of polysaccharides, which consist exclusively of $\alpha$-linkages. When this polymer is boiled with dilute $\mathrm{H}_2\mathrm{SO}_4$ under the experimental conditions of 393 K and 2-3 atm pressure, it hydrolyzes into glucose, a fact confirmed because this product yields gluconic acid upon treatment with bromine water. These specific structural and chemical properties identify the polysaccharide as cellulose.

Enzymes act as highly specific biological catalysts that facilitate unique chemical transformations by targeting particular substrates to yield defined products. Understanding these pathways reveals that invertase catalyzes the conversion of cane sugar into glucose, while zymase is essential for changing glucose into ethanol. Furthermore, the hydrolysis of starch into maltose is mediated by diastase, and the subsequent breakdown of maltose into glucose is performed by maltase.

The sequence begins with the nucleophilic addition of hydrogen cyanide to the carbonyl group of a sugar to form a cyanohydrin, which is subsequently hydrolyzed to yield a carboxylic acid derivative, $A$. This acid is then treated with hydriodic acid ($HI$) and red phosphorus, a powerful combination that acts as a strong reducing agent capable of replacing all hydroxyl and carboxyl functional groups with hydrogen atoms. This reductive deoxygenation converts the sugar chain into a saturated hydrocarbon, $B$, maintaining the carbon skeleton established during the cyanohydrin synthesis. The formulas $A = C_7 H_{14} O_8$ and $B = C_6 H_{14}$ are derived from the stoichiometric progression of these reactions, where the initial carbon chain expansion and subsequent complete reduction account for the final molecular composition of the products.

The acid-catalyzed hydrolysis of the disaccharide sucrose results in the formation of two isomeric monosaccharides, D-glucose and D-fructose, both possessing the molecular formula $C_6H_{12}O_6$. Glucose ('A') contains a terminal aldehyde group and a primary hydroxyl group, which are oxidized by concentrated nitric acid to form the dicarboxylic acid known as saccharic acid, while fructose ('B') is a ketose sugar that exhibits laevorotation. This reaction, commonly known as the inversion of sucrose, produces these specific components, confirming that the sugar 'X' is sucrose.

Animals maintain their energy reserves primarily through the storage of glycogen, which is often termed animal starch to reflect its biological role. This classification is due to its intricate, branched molecular structure, which bears a striking resemblance to the architecture of amylopectin found in plant-based starches. By serving as the key carbohydrate reservoir in animals, glycogen functions in a manner structurally comparable to how plants accumulate starch for later utilization.