Biomolecules – JEE Main ChemistryPractice Questions & PYQs

Maltose is a disaccharide sugar, meaning its structure is built from two monosaccharide units connected by a glycosidic bond. This carbohydrate is formed by the condensation of two molecules of $\alpha$-D-glucose joined specifically by an $\alpha(1 \to 4)$ glycosidic linkage. Consequently, the breakdown of a maltose molecule consistently yields two identical units of $\alpha$-D-glucose.

Acetylation is an esterification process where acetic anhydride, $(CH_3CO)_2O$, reacts with available hydroxyl ($-OH$) groups to form acetate esters, and the number of equivalents required is directly determined by the number of free hydroxyl groups present in the reactant molecule. Glucose typically possesses five such hydroxyl groups in its structure. When glucose reacts with alcohol in the presence of dry HCl to form an acetal, the hydroxyl group at the anomeric position is converted into an ether linkage ($-OR$), which protects that site from acetylation, leaving only 4 free hydroxyl groups available and identifying x as 4. Conversely, the reduction of glucose using $Ni/H_2$ converts the aldehyde group into a primary alcohol group ($-CH_2OH$), which effectively adds an extra hydroxyl site to the molecule and increases the total reactive count to 6, identifying y as 6. Finally, when glucose reacts directly with acetic anhydride, all 5 original hydroxyl groups are free and available for esterification, which sets the value of z to 5.

Vitamins serve as critical cofactors in metabolic processes, and their absence results in specific deficiency diseases characterized by distinct clinical symptoms. Riboflavin is essential for maintaining healthy skin and mucous membranes, so its lack typically leads to cheilosis, whereas thiamine plays a pivotal role in energy metabolism, with its deficiency manifesting as the condition known as beriberi. Pyridoxine is vital for neurotransmitter synthesis, meaning a shortage often triggers neurological issues like convulsions, and ascorbic acid is fundamental for the production of collagen, meaning its absence results in the well-documented disease scurvy.

Gluconic acid is produced by the mild oxidation of the terminal aldehyde group of glucose to a carboxylic acid group, typically utilizing mild oxidizing agents like bromine water while keeping the primary alcohol group intact. This specific reaction constitutes a partial oxidation of glucose, whereas strong oxidation with nitric acid ($HNO_3$) oxidizes both the aldehyde and the terminal primary alcohol group to form saccharic acid, which is a dicarboxylic acid. Because the anomeric carbon is oxidized to a carboxylic acid group, gluconic acid cannot form the cyclic hemiacetal structure characteristic of glucose, although it does readily form a stable cyclic lactone.

Essential amino acids are defined as those which the human body cannot produce internally and must therefore be obtained directly from dietary intake. While Valine, Leucine, and Lysine fall into this category because they are indispensable nutrients that our bodies cannot synthesize, Tyrosine is classified as a non-essential amino acid because our metabolic pathways allow us to generate it from other compounds.

Maltose is a disaccharide composed of two glucose units connected by an $\alpha(1 \to 4)$ glycosidic bond, which creates an acetal functional group at the linkage site. The second glucose unit retains a free anomeric carbon that exists in equilibrium with its open-chain aldehyde form, identifying it as a hemiacetal functional group. As a result, the complete structure contains exactly one acetal group and one hemiacetal group.

Although glucose is classified as an aldohexose, it exists primarily in a stable cyclic hemiacetal form in equilibrium with its open-chain structure. This structural arrangement effectively masks the free aldehyde group, meaning the aldehyde functionality is not sufficiently available to react with Schiff reagent to restore its color. Consequently, glucose gives a negative test with Schiff reagent, making the claim that it produces a positive result chemically incorrect.

Sucrose is a disaccharide formed by joining an alpha-D-glucopyranose unit and a beta-D-fructofuranose unit through a glycosidic bond between their respective anomeric carbons. In the glucopyranose component, the carbons at positions 1, 2, 3, 4, and 5 are all chiral because each is bonded to four distinct groups. The fructofuranose ring contributes four additional chiral centers at positions 2, 3, 4, and 5, while the remaining carbon atoms do not exhibit chirality due to their bonding patterns. Summing these contributions from the two monosaccharide rings confirms that the sucrose molecule contains a total of 9 chiral carbons.

Structure of Tri peptide Asp - Glu - Lys

The reaction involving $CHCl_3$ and alcoholic $KOH$ is a diagnostic test known as the carbylamine test, which is specifically used to detect the presence of primary amine functional groups. Since both adenine and lysine contain these primary amine groups, they are able to react with this reagent mixture to form isocyanides. Other compounds like thymine and proline lack the necessary free primary amine structure required to undergo this specific chemical transformation.

Barfoed's test is a specific chemical assay designed to distinguish between reducing monosaccharides and disaccharides by reducing copper(II) acetate to copper(I) oxide in a weakly acidic environment. In contrast, the Biuret test identifies peptide bonds, the Xanthoproteic test detects aromatic amino acids, and the Ninhydrin test reacts with free alpha-amino groups to produce a characteristic purple color. Since the chemical principle of Barfoed's test is limited to the reduction of sugars and does not interact with the structural functional groups of amino acids, it cannot be used for their identification.

Maltose is a disaccharide composed entirely of two D-glucose units joined by a glycosidic bond. Treatment with dilute hydrochloric acid facilitates the hydrolysis of this linkage, causing the molecule to break down into its constituent monosaccharides. As both units are identical, the process results specifically in the formation of D-glucose according to the reaction $C_{12}H_{22}O_{11} + H_2O \xrightarrow{\text{dil. HCl}} 2C_6H_{12}O_6$.

Ribonucleic acid (RNA) is fundamentally characterized by its single-stranded molecular structure, which distinguishes it from the double-stranded helical configuration often associated with DNA. Because of this single-stranded nature, the claim that RNA possesses a double-stranded $\alpha$-helix structure is factually incorrect. In reality, RNA is found within the cell nucleus and serves as the essential blueprint for protein synthesis, typically without undergoing the self-replication processes characteristic of DNA.

Acidity is inversely proportional to $pK_a$, meaning more acidic molecules correspond to lower values, whereas basic side chains correspond to higher $pK_a$ values. Aspartic acid, which contains an acidic side-chain carboxylic group, is the strongest acid in the group, followed by glycine, which has a neutral side chain. Among the basic amino acids, lysine has a lower $pK_a$ than the strongly basic arginine, resulting in the overall increasing order of $Asp < Gly < Lys < Arg$.

Amino acid sequences in a polypeptide chain are conventionally read from the N-terminus, which bears the free amino group ($-\text{NH}_2$), to the C-terminus, which bears the free carboxyl group ($-\text{COOH}$). By examining the distinctive side chains attached to the $\alpha$-carbon atoms of the provided structure, the N-terminal residue is identified as Valine, the central residue as Serine, and the C-terminal residue as Threonine. This linear progression of side chains determines the specific sequence as Val-Ser-Thr.

Glycogen functions as the primary energy storage polysaccharide in animal cells, yeast, and fungi, characterized by its reliance solely on $\alpha$-linkages between glucose units. While amylose consists of simple, linear chains, glycogen possesses a highly branched architecture that is structurally similar to amylopectin. Because this extensive branching is a fundamental property of the molecule, it is incorrect to classify it as a straight-chain polymer like amylose.

The ester test serves as a method to detect carboxylic acid groups, making Aspartic acid the correct match because it contains additional carboxylic acid functional groups. The carbylamine test is specific to primary amines, identifying Lysine due to the presence of an $NH_2$ group on its side chain. Finally, the phthalein dye test is a characteristic procedure for identifying phenolic compounds, which links directly to Tyrosine given that it features a phenolic group in its structure.

The ionization state of histidine depends on whether the solution pH is higher or lower than the pKa of each of its ionizable functional groups. Because a pH of 2 is slightly higher than the pKa of the carboxyl group (approximately 1.8) but significantly lower than the pKa values of both the imidazole ring and the alpha-amino group, the carboxyl group loses its proton to exist as $\text{-COO}^-$. Consequently, the imidazole and amino groups remain protonated as $\text{-NH}^+$ and $\text{-NH}_3^+$, respectively, resulting in a molecule with a net positive charge in this acidic environment.

Sucrose is a non-reducing disaccharide composed of a glucose and a fructose molecule, held together by a glycosidic bond that leaves no free anomeric carbon. The hydrolysis of this sugar, expressed as $\text{Sucrose} \xrightarrow{\text{hydrolysis}} \text{glucose} + \text{fructose}$, leads to a mixture that is called invert sugar because the optical rotation changes sign during the process. While the other characteristics described are accurate, the glycosidic linkage specifically joins the $C_1$ carbon of $\alpha$-glucose to the $C_2$ carbon of $\beta$-fructose, which proves the assertion regarding a $C_1-C_1$ linkage is false.

Seliwanoff's test differentiates between keto-sugars and aldo-sugars by utilizing the dehydration of fructose to form a compound that reacts with resorcinol to produce a colored complex, which leads to a positive result for fructose while glucose remains negative. In contrast, tests such as Benedict's, Fehling's, and Barfoed's are based on the reducing ability of the carbonyl group, a feature possessed by both fructose and glucose, which causes all three tests to yield a positive result for both sugars.