Biomolecules – JEE Main ChemistryPractice Questions & PYQs

When forming a tripeptide sequence from three distinct amino acids, the specific order of arrangement is critical because each unique sequence represents a different chemical structure. Since each amino acid must be utilized exactly once, we are essentially calculating the total number of possible permutations for three unique items. By selecting any of the 3 options for the first position, 2 for the second, and 1 for the last, the total number of arrangements is determined by the factorial operation expressed as $3! = 3 \times 2 \times 1 = 6$.

Fehling's reagent reacts specifically with reducing sugars that possess a free hemiacetal or hemiketal group capable of reducing cupric ions ($Cu^{2+}$) to cuprous oxide ($Cu_2O$). Sucrose is a non-reducing sugar because its glycosidic bond involves the anomeric carbons of both glucose and fructose, which effectively locks the structure and prevents the formation of an open-chain form containing a free hemiacetal group. Consequently, while glucose, lactose, and maltose contain this necessary structural feature to generate the characteristic reddish-brown precipitate, sucrose remains unreactive.

The high solubility of glucose in water is driven by its polyhydroxy nature, specifically the presence of multiple hydroxyl ($-OH$) groups that facilitate extensive hydrogen bonding with the solvent, rather than the aldehyde functional group as implied in the incorrect statement. Glucose is accurately classified as an aldohexose because it contains six carbon atoms and a terminal aldehyde group. In an aqueous environment, glucose undergoes mutarotation, existing in a dynamic equilibrium between an open-chain structure and various cyclic isomeric forms. Finally, sucrose is a disaccharide derived from the condensation of two monosaccharide units, one of which is glucose and the other fructose.

Biological polymers are complex macromolecules assembled from specific monomeric building blocks through repeating chemical linkages. Starch functions as an energy storage polysaccharide in plants and is comprised of $\alpha$-glucose units, while cellulose provides structural support to plant cell walls through the polymerization of $\beta$-glucose. Nucleic acids are long chains made up of nucleotides, which serve as the fundamental components of genetic material. Finally, proteins are polypeptides formed by the condensation of $\alpha$-amino acids, establishing the clear correspondence where Starch pairs with $\alpha$-glucose, Cellulose with $\beta$-glucose, Nucleic acid with nucleotide, and Protein with $\alpha$-amino acid.

The ninhydrin test is a chemical procedure used to identify the presence of amino acids and proteins by reacting with free amino groups. Egg albumin is a protein, acting as a natural polymer of amino acid subunits, which ensures that it will yield a positive result during this test. In contrast, substances such as cellulose, starch, and polyvinyl chloride do not contain these structural units and therefore do not undergo the specific reaction required for a positive ninhydrin test.

The structure of DNA relies on specific base pairing rules where adenine connects to thymine through two hydrogen bonds, often represented as $A=T$, while cytosine connects to guanine through three hydrogen bonds, represented as $C \equiv G$. When determining the complementary sequence for the template ATGCTTCA, we substitute every adenine with thymine, every thymine with adenine, every guanine with cytosine, and every cytosine with guanine. Applying this pairing logic to each position, the template yields the complementary sequence TACGAAGT.

Tyrosine is an aromatic amino acid defined by a central alpha-carbon attached to an amino group, a carboxyl group, and a p-hydroxybenzyl side chain. By examining its molecular formula, $C_9H_{11}NO_3$, we can account for the two carbons inherent in the amino acid backbone along with the seven carbons that comprise the benzyl ring and its associated methylene group. Summing these structural components confirms that the total number of carbon atoms present in a single molecule of tyrosine is 9.

Carbohydrate isomerism arises from structural variations within cyclic sugars, which can be categorized based on the specific carbon positions involved or the nature of functional groups present. $\alpha$-Glucose and $\alpha$-Galactose differ solely at the C-4 chiral center, identifying them as epimers. In the case of $\alpha$-Glucose and $\beta$-Glucose, the structural difference is restricted to the C-1 anomeric carbon, making them classic examples of anomers. Since $\alpha$-Glucose is an aldohexose and $\alpha$-Fructose is a ketohexose, they contain different functional groups and are thus classified as functional isomers. Lastly, $\alpha$-Glucose and $\alpha$-Ribose are distinguished by their differing carbon chain lengths, which places them in a homologous relationship.

Vitamins are categorized based on their solubility, which dictates how the body manages their retention. Fat-soluble vitamins such as $A$, $D$, $E$, and $K$ are stored within the liver and adipose tissue. Because vitamin $B_{12}$ is a notable exception that is also preserved within these same sites despite being water-soluble, there are five vitamins from the provided list that the body can store.

The determination of whether a sugar belongs to the D or L series depends on the orientation of the hydroxyl group attached to the chiral carbon atom furthest from the aldehyde carbonyl, which for glucose is the $C_5$ position. As the enantiomer of D-glucose, the structure of L-glucose is the direct mirror image, meaning that the hydroxyl group at $C_5$ is placed on the left side, while the configurations at the $C_2$, $C_3$, and $C_4$ centers are all inverted relative to their positions in D-glucose.

The hardening of an egg upon heating is a classic example of protein denaturation, a process where thermal energy disrupts the higher-order three-dimensional structure of proteins. During this transformation, the secondary and tertiary structures are broken down as the protein chains unfold, but the sequence of amino acids-known as the primary structure-remains completely intact. This modification renders the protein biologically inactive while causing it to coagulate into a solid state, without actually cleaving the peptide bonds that define its primary structure.

Glucose exhibits specific chemical behavior depending on the strength of the reagents involved; for instance, $\mathrm{Br}_2$ water acts as a mild oxidizing agent that converts the aldehyde functional group into a carboxylic acid to form gluconic acid, whereas glucose does not undergo any transformation when heated with $\mathrm{NaHCO}_3$. Stronger reagents induce more drastic changes, as $\mathrm{HNO}_3$ oxidizes the sugar to produce saccharic acid, while heating with $\mathrm{HI}$ acts as a powerful reducing agent to fully deoxygenate the carbon chain, resulting in n-hexane.

Tooth enamel is primarily composed of hydroxyapatite, which undergoes a chemical substitution reaction when exposed to fluoride ions. During this process, the hydroxide group within the crystal lattice is replaced by fluoride ions, resulting in the formation of a more acid-resistant and stable mineral structure known as fluoroapatite. This chemical transformation is represented by the specific formula $\left[ 3\mathrm{Ca}_3(\mathrm{PO}_4)_2 \cdot \mathrm{CaF}_2 \right]$.

Nucleic acids are synthesized by covalently joining individual nucleotides, which creates the stable backbone required for DNA and RNA. This joining process specifically involves a phosphate group linking the 3' carbon of one sugar to the 5' carbon of the next, resulting in a structural arrangement called a phosphodiester linkage. This precise connection is what allows for the linear arrangement of genetic information, setting it apart from other bonding types such as peptide or glycosidic linkages.

Proteins function as natural polymers that are constructed by linking amino acid units together through specific peptide bonds. When subjected to hydrolysis, the water molecules essentially break these peptide linkages, effectively reversing the polymerization process. This cleavage restores the original monomeric building blocks, which are structurally identified as $\alpha$-amino acids.

A heteroatom is defined as any atom other than carbon or hydrogen present within a molecular structure. Evaluating the given list, Thiophene ($C_4H_4S$) contains a sulfur atom within its aromatic ring, and the amino acid Cysteine ($HSCH_2CH(NH_2)COOH$) contains a sulfur atom in its thiol side chain. Since Furan, Pyridine, Pyrrole, and Tyrosine do not contain sulfur, only two compounds satisfy the specified condition.

DNA is composed of four distinct nitrogenous bases-adenine, guanine, cytosine, and thymine-each of which possesses a highly specific and invariant molecular structure essential for genetic stability. Any diagram that misrepresents the precise atomic connectivity or functional group arrangement of these bases fails to reflect their true biochemical identity. Consequently, the third option stands out as incorrect because its structural depiction contradicts the established chemical requirements of these four fundamental components of the DNA molecule.

Protein denaturation occurs when external stressors like heat disrupt the weak chemical bonds-such as hydrogen bonds and hydrophobic interactions-responsible for maintaining a protein's specific three-dimensional folding. As an egg boils, this thermal energy causes the quaternary, tertiary, and secondary structures to unfold or uncoil, losing their functional shape. Because the primary structure relies on strong covalent peptide bonds to maintain the specific sequence of amino acids, it remains unaffected by this process and stays intact.

$\mathrm{Br}_2$ water acts as a mild oxidizing agent that selectively oxidizes the aldehyde group of an aldose into a carboxylic acid, resulting in the formation of a monocarboxylic acid known as gluconic acid. This invalidates the claim of producing a dicarboxylic acid, whereas the other observations are chemically accurate: the molecule does not produce a positive Schiff's test because its aldehyde group exists predominantly in a cyclic hemiacetal form, it contains 4 asymmetric carbon atoms, and it remains in equilibrium with 2 other cyclic anomeric structures in aqueous solution.

Genetic information is anchored in DNA, which serves as the permanent repository for inheritance, contradicting the notion that RNA performs this role. During cellular division, DNA replicates itself to ensure that identical strands are passed down to daughter cells, preserving genetic consistency. While DNA harbors the specific instructions for building proteins, it does not carry out the synthesis directly, meaning three of the provided statements accurately characterize these biological processes.