Biomolecules – JEE Main ChemistryPractice Questions & PYQs

Understanding these complex carbohydrates requires identifying the specific glycosidic linkages and biological origins unique to each molecule. Amylose functions as a linear plant starch defined by $\alpha-C_1-C_4$ glycosidic linkages, while cellulose provides essential structural stability in plants through the use of $\beta-C_1-C_4$ glycosidic bonds. Glycogen serves as a primary energy reserve in animals and is characterized by an $\alpha-C_1-C_4$ linkage configuration. In contrast, amylopectin acts as a branched starch found in plants, distinguishable by the presence of both $\alpha-C_1-C_4$ and $\alpha-C_1-C_6$ glycosidic linkages.

Sucrose is a disaccharide formed by the condensation of $\alpha-D\text{-glucose}$ and $\beta-D\text{-fructofuranose}$, where the fructose unit is the levorotatory $D-(-)\text{-fructose}$ isomer rather than the $D-(+)$ isomer indicated in the first statement. The hydrolysis of sucrose yields an equimolar mixture of $D-(+)\text{-glucose}$ and $D-(-)\text{-fructose}$, and because the levorotatory nature of fructose ($-92.4^\circ$) exceeds the dextrorotatory contribution of glucose ($+52.5^\circ$), the resulting mixture becomes levorotatory, a process known as the inversion of sucrose that defines the product as invert sugar.

Hydrogen bonding in a DNA double helix follows Watson-Crick base pairing rules, where Guanine (G) pairs with Cytosine (C) via three hydrogen bonds, and Adenine (A) pairs with Thymine (T) via two hydrogen bonds. Analysis of the provided thirteen-base sequence yields four Gs, three Cs, four As, and two Ts. This configuration results in seven total G-C pairs and six total A-T pairs across the entire helix. The total number of hydrogen bonds is calculated as:
$(7 \times 3) + (6 \times 2) = 33$

Proteins are composed of $\alpha$-amino acids, so the claim that they hydrolyze to produce $\beta$-amino acids is scientifically inaccurate. Furthermore, although acid-induced denaturation does involve the unfolding and uncoiling of protein structures, the specific assertion that this process transforms water-soluble fibrous proteins into water-insoluble forms is false, as fibrous proteins are typically already insoluble in water. Because both propositions rely on erroneous premises regarding protein composition and structural behavior, both statements are invalid.

The determination of L-isomerism in a ketohexose relies on the configuration of the chiral center furthest from the carbonyl group, specifically at the $C_5$ position of the Fischer projection. While D-fructose features the hydroxyl group on the right side at $C_5$, L-fructose is the mirror image enantiomer, requiring the hydroxyl group at the $C_5$ position to be oriented on the left side.

The reaction of an $\alpha$-amino acid with nitrous acid involves the diazotization of the primary amino group followed by hydrolysis, which converts the amino acid into an $\alpha$-hydroxy acid. Since alanine ($\mathrm{CH}_3\mathrm{CH}(\mathrm{NH}_2)\mathrm{COOH}$) produces lactic acid ($\mathrm{CH}_3\mathrm{CH}(\mathrm{OH})\mathrm{COOH}$) under these conditions, component 'y' is identified as alanine. Because glycine is known to readily form a 2,5-diketopiperazine cyclic anhydride upon heating, component 'z' is glycine, confirming that the dipeptide 'x' is alanine-glycine.

The number of unique sequences for a tetrapeptide containing glycine, alanine, valine, and leucine in equimolar proportions is determined by calculating the permutations of these four distinct amino acids. Because there are four distinct positions in the peptide chain to be filled by four unique components, the total count is found using the factorial of the number of amino acids, expressed as $4!$. By calculating $4 \times 3 \times 2 \times 1$, we determine there are exactly 24 possible tetrapeptide arrangements.

Peptide bond formation typically occurs through a nucleophilic substitution reaction, where the lone pair on an amine group attacks the electrophilic carbonyl carbon of an activated carboxylic acid. When synthesizing the dipeptide $Gly-Ala$, the process requires a glycine moiety in which the carboxylic acid group has been converted into an acid chloride, providing a highly reactive site for the incoming nucleophile. The amino group of the alanine molecule acts as the nucleophile, displacing the chloride ion from the glycyl chloride to establish the amide linkage. This specific chemical pathway effectively joins the two amino acids in the sequence $Gly-Ala$ while concurrently releasing $HCl$ as the byproduct of the condensation.

The classification of nitrogenous bases relies on identifying specific molecular arrangements and substituent patterns present in the heterocyclic rings. By examining these structural features, we can map Adenine to structure III and Cytosine to structure IV. Following this analytical pattern, Thymine is correctly identified with structure II, while Uracil corresponds to structure I, confirming their positions within the provided series.

Vitamins are essential organic compounds that the body requires in small quantities for various metabolic functions, and they are defined by their specific chemical structures. Among the options provided, ascorbic acid is the scientific name for Vitamin C, which is a vital nutrient for humans that must be obtained through our diet. While the other acids listed, such as aspartic acid and adipic acid, serve important roles in protein synthesis or industrial manufacturing, they do not function as vitamins within the human body.

Acidic hydrolysis cleaves the glycosidic linkages of carbohydrates to yield their respective monosaccharide building blocks, a process exemplified by the reaction $C_{12}H_{22}O_{11} + H_2O \rightarrow C_6H_{12}O_6 + C_6H_{12}O_6$. Cane sugar, or sucrose, hydrolyzes into an equimolar mixture of glucose and fructose, whereas milk sugar, known as lactose, breaks down into glucose and galactose. Since starch is a polysaccharide composed exclusively of glucose units, its component amylose releases only glucose upon hydrolysis, confirming that statements (B), (C), and (E) are accurate.

The reaction with 2,4-dinitrophenylhydrazine serves as a specific diagnostic test for free carbonyl groups, such as those present in aldehydes or ketones. Since the aldehyde function in D-glucose pentaacetate is masked and its hydroxyl groups are acetylated, the molecule does not possess a free carbonyl group available to undergo this reaction, rendering the first statement incorrect. In contrast, the industrial hydrolysis of starch is an established chemical process used to isolate simple sugars. When starch is heated in the presence of $\text{H}_2\text{SO}_4$ at 100°C and a pressure of 2-3 atmospheres, it undergoes cleavage to yield glucose, confirming that the second statement is entirely accurate.

Although fructose is a ketone and lacks a terminal aldehyde group, it functions as a reducing sugar because its $\alpha$-hydroxy ketone moiety tautomerizes into an aldehyde in the presence of a base, allowing it to reduce Tollen's reagent. In this same alkaline medium, fructose undergoes an isomerization reaction that results in a rearrangement of its structure to yield both glucose and mannose.

Determining the D-configuration for a carbohydrate depends on the spatial orientation of its functional groups within a Fischer projection. A molecule belongs to the D-series if, when the aldehyde group ($-CHO$) is positioned at the top of the chain, the hydroxyl group ($-OH$) attached to the lowest chiral center is oriented toward the right. Comparing the provided structures against this definition reveals that exactly three of them align with the configuration of D-glyceraldehyde.

Protein structure is organized hierarchically, starting with the linear sequence of amino acids known as the primary structure. When specific regions of a polypeptide chain fold into regular, repeating patterns stabilized by hydrogen bonds between backbone atoms, these local geometries are classified as the secondary structure. The $\alpha$-Helix and $\beta$-pleated sheet are the fundamental motifs of these spatial arrangements, resulting from the polypeptide chain twisting or lining up to form stable, recurring shapes.

Glycosidic linkages define the structural architecture of carbohydrates by specifying the exact carbon atoms and anomeric configurations involved in connecting monosaccharide units. Sucrose connects glucose and fructose via an $\alpha\,1-\beta\,2$ linkage, while maltose forms through a standard $\alpha\,1-4$ bond between two glucose units. Lactose distinguishes itself with a $\beta\,1-4$ bond between galactose and glucose, and amylopectin utilizes both linear $\alpha\,1-4$ and branching $\alpha\,1-6$ linkages to support its complex, bushy structure.

Deoxyribose, the sugar backbone component found in DNA, is classified as a pentose sugar because of its five-carbon chain. It naturally adopts a furanose ring structure rather than a pyranose form and consistently exhibits the D configuration in its molecular geometry. Furthermore, this sugar functions as a reducing sugar when in a free state and specifically assumes the $\beta$-anomeric configuration within biological nucleotides. Accordingly, the statements identifying it as a pentose sugar, a molecule in the D configuration, and a reducing sugar are the correct descriptions.

Essential amino acids are defined as those that the human body cannot synthesize internally, requiring them to be obtained directly from dietary intake. Within the provided list, Valine, Lysine, and Threonine are classified as essential because our metabolism lacks the capacity to manufacture them. This fundamental biological requirement ensures that these specific amino acids must be consumed in the diet, distinguishing them from the other options.

Vitamins are categorized based on their solubility in either water or fat, a distinction that dictates how the body absorbs and stores these essential nutrients. Fat-soluble vitamins, which encompass A, D, E, and K, are stored in the liver and adipose tissue for later utilization. In this context, Vitamin E and Vitamin K are the only components that belong to the fat-soluble group, as the various B-complex vitamins and Vitamin C dissolve in water and are processed differently by the body.

Enzymes function as highly efficient biocatalysts that accelerate chemical reactions within living organisms without being consumed in the process. Structurally, the vast majority of these biological molecules are globular proteins, which provide the precise three-dimensional shape necessary for their catalytic function. Because of this specialized architecture, enzymes exhibit remarkable specificity, meaning they typically catalyze only one particular type of reaction rather than acting on various substrates indiscriminately. Consequently, an enzyme like oxidase, which is involved in oxidation-reduction processes, cannot facilitate the hydrolysis of maltose into glucose, as that specific task requires the enzyme maltase.