Biomolecules – JEE Main ChemistryPractice Questions & PYQs

The backbone of a polypeptide is formed by joining amino acid residues sequentially, where each bond serves as the linkage between a pair of amino acids. In any chain containing $n$ amino acids, there are necessarily $n-1$ peptide bonds connecting them together. Therefore, a tetrapeptide with 4 amino acids and 3 peptide bonds results in the following difference:
$4 - 3 = 1$

The fermentation of glucose by the enzyme zymase breaks down the hexose sugar into ethanol and carbon dioxide, represented by the chemical equation $\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6 \rightarrow 2\mathrm{C}_2 \mathrm{H}_5 \mathrm{OH} + 2\mathrm{CO}_2$. Consequently, compound $A$ is ethanol, which contains a methyl group adjacent to the hydroxyl function, making it susceptible to the iodoform reaction upon treatment with sodium hypoiodite ($\mathrm{NaOI}$). This process oxidizes the ethanol and cleaves it to form iodoform ($\mathrm{CHI}_3$) and the sodium salt of formic acid, specifically sodium formate ($\mathrm{HCOONa}$), which is identified as compound $B$. Because the formate ion contains only one carbon atom, the number of carbon atoms present in product $B$ is 1.

Amylose constitutes a component of starch formed by a long linear chain of $\alpha\text{-D}$ glucose units connected through 1,4-glycosidic linkages. While the reason correctly describes the molecule's structural composition as a polymer exceeding 200 glucose units, the assertion that amylose is insoluble in water is factually incorrect because amylose is actually water-soluble. Consequently, the first statement is false, while the second statement accurately reflects the biochemical structure of this polysaccharide.

Aldopentoses undergo acid-catalyzed dehydration to eliminate three molecules of water, resulting in the formation of the aromatic aldehyde furfural. This specific furfural molecule subsequently undergoes a condensation reaction with resorcinol to yield a colored complex, serving as a diagnostic chemical test for the presence of pentose sugars. Unlike hexoses, which primarily yield 5-(hydroxymethyl)furfural under similar conditions, the direct production of furfural is characteristic of five-carbon monosaccharides.

The nitrogenous base specifically found in RNA is uracil, which possesses two oxygen atoms within its ring structure. A nucleotide is formed when this base attaches to a ribose sugar, which is subsequently phosphorylated to form a phosphate ester. Throughout these condensation reactions, the chemical assembly of uridine monophosphate, represented by the formula $C_9H_{13}N_2O_9P$, incorporates exactly nine oxygen atoms after accounting for the water molecules eliminated during the formation of the glycosidic and phosphodiester linkages.

Vitamins are categorized by their solubility, where Vitamin C and the entire B-complex group are water-soluble, whereas Vitamins A, D, E, and K are fat-soluble. Unlike other water-soluble vitamins that are typically flushed out of the system quickly, Vitamin $B_{12}$ is stored significantly in the liver, which prevents it from being excreted as easily.

Proteins function based on a hierarchy of structural levels, where denaturation involves the disruption of hydrogen bonds that hold higher-order shapes together. Because the primary structure is held together by strong covalent peptide bonds, it remains unaffected when the destabilizing forces of denaturation unravel the secondary and tertiary structures. This means the specific sequence of amino acids persists even after the protein loses its complex three-dimensional configuration.

The reaction of glucose with $HI$ results in the reduction of the entire carbon chain to form $n$-hexane, confirming the presence of six straight carbon atoms. Treating glucose with $Br_2$ water leads to the mild oxidation of the aldehyde group to a carboxylic acid, forming gluconic acid, whereas oxidation with $HNO_3$ utilizes stronger conditions to oxidize both the aldehyde and the terminal primary alcohol group into dicarboxylic saccharic acid. Furthermore, the reaction of glucose with acetic anhydride achieves the acetylation of all five available hydroxyl groups, resulting in the formation of glucose pentacetate.

The body relies on Vitamin $K$ for effective blood coagulation, so it is the deficiency of this nutrient that directly leads to an increase in blood clotting time. While the presence of Vitamin $K$ is necessary to facilitate clotting, its absence effectively hinders the process, causing the delay mentioned. Other nutrients serve entirely different functions: Vitamin $C$ is essential for treating scurvy, Vitamin $B$ complex vitamins are required for cell metabolism and the synthesis of red blood cells, and Vitamin $E$ acts as a fat-soluble antioxidant that protects cell membranes and prevents the fragility of red blood cells.

Disaccharides are carbohydrates that break down into two simple sugar units upon hydrolysis. Lactose specifically comprises one unit of $\beta$-D-Galactose and one unit of $\beta$-D-Glucose, joined through a bond between the $C_{1}$ atom of the galactose and the $C_{4}$ atom of the glucose. Because compound A produces these two particular monosaccharides upon hydrolysis, it is confirmed to be lactose.

Proteins are typically categorized by their overall molecular shape, with fibrous proteins forming extended, narrow strands that serve vital structural roles within the body. Examples like keratin, collagen, and myosin fit this description because they provide physical support and strength to tissues. Conversely, albumin belongs to the class of globular proteins, which are defined by their compact, rounded structure and their essential involvement in metabolic processes rather than structural integrity.

At a highly basic pH 12.5, the acidic functional groups within the tetrapeptide Gly-Glu-Asp-Tyr undergo deprotonation, losing their protons to form negatively charged ions. The carboxyl group at the C-terminus of tyrosine, the side-chain carboxyl groups of both glutamic acid and aspartic acid, and the phenolic hydroxyl group of tyrosine all convert to their respective anionic forms, represented as $COO^-$ and $O^-$. Since the N-terminal amine group remains neutral in this alkaline environment, these four independent ionization sites collectively contribute one negative charge each. Summing these individual contributions leads to a total negative charge of 4.

The B-complex vitamins comprise a family of water-soluble compounds, each distinguished by unique chemical terminology that highlights its biological significance. Thiamine is the established scientific identifier for Vitamin B1, a nutrient vital for converting carbohydrates into usable energy. In the same manner, Pyridoxine is the chemical name used for Vitamin B6, which plays an essential role in protein metabolism and cognitive development. Mapping these specific designations directly to the provided choices identifies the matching pair in option D.

Sucrose is a disaccharide that undergoes hydrolysis to break down into its two constituent monosaccharides. This reaction is represented by the equation
$\text{Sucrose} + \text{H}_2\text{O} \rightleftharpoons \alpha\text{-D-(+)-glucose} + \beta\text{-D-(-)-fructose}$
resulting in the formation of $\alpha$-D-(+)-glucose and $\beta$-D-(-)-fructose. The $(+)$ and $(-)$ symbols indicate the direction of plane-polarized light rotation, which confirms the specific optical properties of the products formed during the process.

Sugars are classified according to the specific functional group they possess and the total number of carbon atoms within their molecular structure. Fructose is identified as a ketohexose because its backbone contains six carbon atoms and features a characteristic ketonic functional group. This arrangement also includes five hydroxyl groups, distinguishing it from sugars that contain an aldehyde group instead.

Disaccharides are formed by joining two monosaccharide units through a glycosidic bond, where the specific orientation and position of this bond determine the molecule's chemical identity. Lactose is characterized by a $\beta-C_1-C_4$ glycosidic linkage, which connects the C-1 carbon of a galactose molecule to the C-4 carbon of a glucose molecule. Other carbohydrates like sucrose, maltose, and amylose utilize different bonding configurations or monosaccharide components, identifying lactose as the unique compound among the choices containing this specific structural connection.

The classification of vitamins into fat-soluble and water-soluble groups determines how they are retained by the human body. Because Vitamin A and Vitamin D are insoluble in water and dissolve in fats, the body is able to store them for relatively long durations within its tissues. In contrast, water-soluble vitamins such as Thiamine and Ascorbic acid cannot be stored and are instead excreted, which necessitates their frequent replenishment through a regular daily diet.

Disaccharides are formed by the condensation of two monosaccharide units connected through a glycosidic linkage, where the specific identity of each disaccharide depends entirely on the monomers involved. Sucrose, designated as A, arises from combining $\alpha$-D-glucose and $\beta$-D-fructose, which corresponds to entry (ii), while lactose, labeled B, is formed from $\beta$-D-galactose and $\beta$-D-glucose, corresponding to entry (i). Finally, maltose, listed as C, is uniquely composed of two $\alpha$-D-glucose units, which pairs it with entry (iii).

Disaccharides are formed through a dehydration synthesis reaction that establishes a covalent glycosidic bond between two monosaccharide units. In the specific case of lactose, this bond links a galactose molecule to a glucose molecule. The structure is characterized by a connection between the C-1 carbon atom of the galactose and the C-4 carbon atom of the glucose.

Starch is composed of two distinct components, amylose and amylopectin, both of which are polymers of $\alpha$-D-glucose. Contrary to the idea of a branched structure, amylose is specifically characterized by its linear chain configuration, whereas branching is the defining feature of amylopectin. Because of this linear arrangement, classifying amylose as a branched chain polymer is factually incorrect.