Biomolecules – JEE Main ChemistryPractice Questions & PYQs

Oxidation of an aldotetrose with concentrated $HNO_3$ converts both the terminal aldehyde and the primary alcohol groups into carboxylic acid groups, resulting in the formation of an aldaric acid. D-Erythrose yields meso-tartaric acid upon this oxidation, which is optically inactive due to the presence of an internal plane of symmetry. In contrast, D-threose produces an optically active tartaric acid, confirming that the D-aldotetrose resulting in an optically inactive product must be D-erythrose.

Oligosaccharides are carbohydrates that release two to ten monosaccharide units upon hydrolysis, and the identity of these units is not strictly uniform across all types. While certain disaccharides like maltose break down into two identical glucose units, others such as lactose, when subjected to hydrolysis in the presence of $\text{H}_3\text{O}^{\oplus}$, yield different monosaccharides like glucose and galactose. Consequently, the claim that hydrolysis of oligosaccharides always generates identical units is incorrect, as the nature of the products depends entirely on the specific composition and glycosidic linkages of the starting material.

Vitamins are essential organic compounds that possess specific chemical names, which serve as their formal identification in biological and nutritional contexts. Within this classification, Vitamin $B_1$ is scientifically recognized as Thiamine, while Vitamin $B_2$ is identified as Riboflavin. Following this nomenclature, Vitamin $B_6$ corresponds to Pyridoxine and Vitamin C is designated as Ascorbic acid, establishing the direct pairings of A-III, B-IV, C-I, and D-II.

Enzymes function by lowering the activation energy of chemical reactions, making the enzyme-catalyzed pathway more efficient than acid-catalyzed hydrolysis, thus rendering statement A correct. During denaturation, the non-covalent interactions responsible for secondary and tertiary protein structures are disrupted, but the covalent peptide bonds maintaining the primary structure remain stable, which verifies statement B. Conversely, statement C is incorrect because nucleotides are linked via phosphodiester bonds between $3'$ and $5'$ carbons, and statement D is incorrect because the quaternary structure specifically describes the spatial arrangement of multiple distinct polypeptide subunits, whereas the overall folding of a single polypeptide chain defines the tertiary structure.

Arginine is recognized as an essential or semi-essential amino acid in standard curricula alongside Tryptophan, establishing statement A as correct. Glycine lacks a chiral center because its alpha carbon is bonded to two identical hydrogen atoms, confirming the validity of statement D. Statements B, C, and E are incorrect because Histidine features a heterocyclic imidazole ring, Proline consists of a five-membered pyrrolidine ring, and the side chain $MeS-CH_2-CH_2-$ is characteristic of Methionine rather than Cysteine.

Glutamine possesses an amide functional group ($-\text{CONH}_2$) within its side chain, which serves as the specific site for Hoffmann bromamide degradation. Since lysine incorporates a primary amine ($-\text{NH}_2$) in its side chain, it undergoes the Hinsberg test, a standard method for identifying such amines. Tyrosine is characterized by a phenolic hydroxyl group that reacts with neutral $\mathrm{FeCl}_3$, while serine contains an alcoholic hydroxyl group ($-\text{OH}$) detectable through the ceric ammonium nitrate test.

The number of ways to arrange $n$ distinct items in a specific sequence is given by the permutation formula $n!$. Since there are 3 distinct amino acids (alanine, glycine, and valine) being arranged in a tripeptide of length 3, the total number of possible arrangements is $3! = 3 \times 2 \times 1 = 6$.

Boiling point elevation and the relative lowering of vapour pressure are colligative properties that depend directly on the amount of dissolved solute. The elevation in boiling point is found using the formula $\Delta T_b = T_b(\text{solution}) - T_b(\text{pure solvent})$, which results in 0.5 K. By applying the relationship $\Delta T_b = K_b \cdot m$, the molality is calculated as 0.5 divided by 0.3, or 5/3 mol kg$^{-1}$. With 0.04 kg of solvent present, the moles of solute are determined by multiplying the molality by the mass of the solvent in kilograms, which equals 5/3 times 0.04, or 0.2/3 moles.

The relative lowering of vapour pressure is expressed by the relationship $\frac{P^{\circ} - P_s}{P_s} = \frac{n_{\text{solute}}}{n_{\text{solvent}}}$. Substituting the given pressure values, the left side simplifies to 10/750, which reduces to 1/75. Because the molar ratio of solute to solvent is 1/75, the number of solvent moles must be 75 times the solute moles. Multiplying 75 by 0.2/3 yields 5.

The chirality of DNA and RNA is fundamentally derived from the D-configuration of their sugar components, specifically D-2-deoxyribose in DNA and D-ribose in RNA. These pentose sugars possess multiple chiral centers (stereocenters) that impart necessary asymmetry to the nucleotide building blocks, a property that is conserved throughout the entire nucleic acid polymer structure.

The tertiary structure of a protein defines its complex three-dimensional shape, which is maintained by specific interactions such as hydrogen bonds, disulfide linkages, van der Waals forces, and electrostatic attractions. Because these stabilizing interactions are highly sensitive to their chemical environment, shifting the $pH$ level causes the protein to lose its folded configuration, a process known as denaturation. Since this environmental stress disrupts the delicate non-covalent architecture, the assertion that the structure remains intact under $pH$ changes is fundamentally incorrect, as only the primary structure held by stable peptide bonds survives such conditions.

Vitamins serve as essential micronutrients, and recognizing the specific disease caused by their absence relies on understanding the unique physiological role each vitamin plays. Scurvy is primarily caused by a lack of Ascorbic Acid, whereas neurological symptoms such as convulsions are linked to a deficiency in Pyridoxine. Further, Riboflavin deficiency typically manifests as cheilosis, and Vitamin A deficiency is characterized by the eye condition xerophthalmia. Because Scurvy occurs with Ascorbic Acid deficiency, Convulsions with Pyridoxine deficiency, Cheilosis with Riboflavin deficiency, and Xerophthalmia with Vitamin A deficiency, the relationships between the listed disorders and their respective nutrients are clearly defined.

This mixture constitutes a basic buffer system consisting of the weak base ammonia and its conjugate acid ammonium chloride, so the hydroxide ion concentration is determined using the Henderson-Hasselbalch equation pOH = pK_b + \log \frac{[\mathrm{NH}_4^+]\}{[\mathrm{NH}_4\mathrm{OH}]}$. Because both species share the same final volume, the ratio of their molar concentrations is identical to the ratio of their millimoles. We calculate the millimoles for the base as$5 × 0.1 = 0.5$and for the salt as$250 × 0.1 = 25$. Dividing these yields a ratio of$\frac{25}{0.5} = 50$. Substituting this into the buffer equation gives$pOH = 4.74 + \log 50$. Applying the logarithm identity$\log 50 = \log 10 + \log 5$, we get$1 + 0.70 = 1.70$. Adding this value to the$pK_b$results \in$pOH = 4.74 + 1.70 = 6.44$. Calculating the pH requires subtracting the pOH from 14, giving$pH = 14 - 6.44 = 7.56$, which is equivalent to$756 × 10^{-2}$.

Thyroxine is a thyroid hormone synthesized from the amino acid tyrosine, which acts as the primary precursor molecule. The biosynthesis involves the iodination of the phenol side chain of tyrosine residues within the protein thyroglobulin, ultimately resulting in the formation of the hormone thyroxine, often denoted as $T_4$. Because Y is the standard one-letter abbreviation for tyrosine, the pair consisting of Y and thyroxine correctly identifies the specific amino acid precursor and its corresponding iodinated hormonal derivative.

Sucrose is a dextrorotatory disaccharide, but its hydrolysis produces an equimolar mixture of D-(+)-glucose and D-(-)-fructose, resulting in a net levorotatory solution due to the inversion of optical rotation. Statement II is false because D-(+)-glucose itself is dextrorotatory, and the net levorotation arises specifically because the magnitude of the levorotation of D-(-)-fructose ($-92.4^{\circ}$) is greater than the magnitude of the dextrorotation of D-(+)-glucose ($+52.5^{\circ}$).

Anomers are a specialized class of stereoisomers that differ in configuration only at the hemiacetal carbon atom, which serves as the anomeric center, specifically denoted as $C_1$ in glucose. Because $\alpha$-D-glucose and $\beta$-D-glucose possess identical structures at every other position and differ solely in the spatial orientation of the hydroxyl group at the $C_1$ position, they are accurately classified as anomers. Furthermore, a detailed comparison of the open-chain Fischer projections confirms that both D-glucose and D-fructose maintain identical configurations at the $C_3$, $C_4$, and $C_5$ chiral centers, making the second statement equally factual.

Essential amino acids are critical organic compounds that the body cannot synthesize independently and must obtain through dietary intake, with Threonine serving as a prime example of such a nutrient. Within the provided pentapeptide structure, the sequence Thr-Ser-Asp-Gly-Ala correctly identifies Threonine as the essential amino acid $(Y)$ component and accurately represents the order of the amino acid residues within the chain.

Glucose exists in two distinct cyclic hemiacetal forms known as the $\alpha$-anomer and $\beta$-anomer, which differ specifically in their configuration at the $C_1$ carbon atom. These two isomeric forms are prepared by crystallizing a saturated glucose solution at different temperatures, where the $\alpha$-anomer is obtained at 303 K and the $\beta$-anomer is obtained at 371 K. Because they possess unique molecular geometries, they exhibit different physical properties, such as the $\alpha$-anomer having a melting point of 419 K and the $\beta$-anomer melting at 423 K, which confirms the accuracy of the statements regarding their structural and preparation characteristics.

A carbohydrate is classified as a non-reducing sugar if both of its anomeric carbons are involved in the formation of the glycosidic linkage, which effectively prevents the ring from opening to yield a free aldehyde or ketone group capable of reducing oxidizing agents. Since the structure in option A features a glycosidic bond connecting the anomeric carbons of both monosaccharide units-analogous to the $(\alpha1 \rightarrow \beta2)$ linkage found in sucrose-the molecule is locked in its cyclic form and cannot undergo oxidation. This structural configuration is the defining criterion for non-reducing disaccharides, distinguishing them from other sugars that retain at least one potentially free anomeric center.

Glucose exhibits characteristic chemical reactivity due to its open-chain structure containing an aldehyde group and five hydroxyl groups, which yield specific derivatives depending on the reagent applied. Hydroxylamine reacts with the carbonyl group of the aldehyde to form an oxime known as glucoxime, while bromine water acts as a mild oxidizing agent that selectively oxidizes the aldehyde group to a carboxylic acid, producing gluconic acid. Excess acetic anhydride acetylates all five hydroxyl groups of the glucose molecule to yield glucose pentacetate, whereas the strong oxidizing action of concentrated $HNO_3$ converts both the terminal aldehyde group and the primary alcoholic group into carboxylic acid groups, resulting in the formation of saccharic acid.

The reactive thiol ($-SH$) side chain inherent to cysteine facilitates its spontaneous dimerization, where two such molecules link via covalent disulfide ($-S-S-$) bonds to form cystine. This unique chemical property of cysteine is well-documented, whereas other statements regarding amino acid structures fail because certain amino acids like glycine lack chirality, others possess multiple chiral centers, and aspartic acid also features a reactive carboxyl side chain.