Biomolecules – JEE Main ChemistryPractice Questions & PYQs

DNA molecules are constructed from a specific set of nitrogenous bases, namely adenine, thymine, cytosine, and guanine, which serve as the fundamental building blocks for genetic information. Since quinoline acts as a separate aromatic compound unrelated to the composition of these nucleotides, it remains entirely absent from the structure of DNA.

Photosynthesis utilizes the Calvin cycle to convert inorganic carbon dioxide into energy-rich glucose, a process that demands a significant input of chemical energy. Because the synthesis of one molecule of glucose requires the fixation of six molecules of carbon dioxide, the energy budget must accommodate these multiple steps by utilizing specific amounts of ATP and NADPH produced during the light reactions. The stoichiometry for this dark reaction is represented as:

$6\mathrm{CO}_2+12\mathrm{NADPH}+18\mathrm{ATP}\rightarrow\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6+12\mathrm{NADP}+18\mathrm{ADP}+6\mathrm{H}_2\mathrm{O}$

This balanced equation confirms that the cell must consume exactly 18 molecules of ATP to successfully complete the synthesis of one glucose molecule.

Optical activity in amino acids depends on the presence of a chiral center, which requires the central $\alpha$-carbon to be bonded to four distinct chemical groups. In most amino acids, the side chain provides this fourth distinct group, ensuring the molecule is asymmetric and thus optically active. However, glycine is unique because its side chain is simply a hydrogen atom, making its $\alpha$-carbon bonded to two identical hydrogen atoms, which renders it achiral. Consequently, every common amino acid exhibits optical activity except for glycine.

Molisch's test serves as a universal diagnostic indicator specifically designed to detect the presence of carbohydrates, which are chemically classified as sugars. During the procedure, a few drops of an alcoholic alpha-naphthol solution are added to 2 ml of the sample, followed by the careful addition of 1 ml of concentrated $\mathrm{H}_2\mathrm{SO}_4$ along the test tube walls. The formation of a distinct violet ring at the interface of the two liquid layers provides positive verification for the presence of sugars, thereby excluding classes like nitro compounds, amines, or primary alcohols.

The fundamental distinction between DNA and RNA arises from the specific pentose sugar utilized in their molecular structure. While RNA incorporates D(-)-ribose, DNA contains 2-deoxy-D(-)-ribose, where the "deoxy" prefix explicitly indicates the loss of an oxygen atom. This structural variation is localized at the 2nd carbon atom, which features a hydroxyl $-\text{OH}$ group in RNA but lacks it in DNA.

The Biuret test functions by detecting the specific $-\text{C}(=\text{O})-\text{NH}-$ peptide linkage found in molecules, which produces a violet color upon reaction with an alkaline solution containing $\text{CaSO}_4$. Since proteins, polypeptides, and urea all possess this fundamental amide backbone, they test positive for the presence of these bonds. Carbohydrates, however, are characterized by their glycosidic linkages rather than peptide bonds, which renders them incapable of interacting with the reagent to produce the expected color change.

Carbohydrates are chemically defined as polyhydroxy aldehydes or ketones, meaning their molecular structure is characterized by the presence of multiple hydroxyl groups alongside a carbonyl group. Considering glucose as the standard model for a typical carbohydrate, it acts as an aldose which features an aldehyde group, denoted as $-\mathrm{CHO}$, and several hydroxyl groups, written as $-\mathrm{OH}$. These two specific functional groups are the distinguishing chemical features present in this sugar.

Cyclic sugars such as glucose contain a specialized chiral center known as the anomeric carbon, which arises at the C-1 position during the formation of the ring structure. Stereoisomers that differ in their configuration exclusively at this C-1 atom are defined as anomers. Because $\alpha$-D-(+)-glucose and $\beta$-D-(+)-glucose possess identical molecular structures apart from the orientation of the hydroxyl group attached to their C-1 position, they are classified as anomers.

The secondary structure of a protein describes the specific spatial arrangement or folding pattern adopted by the long polypeptide backbone. This level of organization is characterized by two distinct, regular conformations: the right-handed $\alpha$-helix and the extended $\beta$-conformation, which resembles pleated drapery. Since the $\alpha$-helical arrangement is a direct result of how the backbone folds to stabilize the protein, it serves as a defining feature of the secondary structure.

When the open-chain structure of $D-(+)$-glucose undergoes cyclization, the carbonyl carbon is converted into a new stereocenter, leading to the formation of two distinct cyclic structures known as $\alpha$ and $\beta$ glucose. Because these two molecules differ solely in their spatial arrangement at this specific hemiacetal carbon, which is $C_1$, they are defined as anomers. This structural relationship is unique to the configuration at the anomeric carbon, distinguishing these molecules from other types of stereoisomers.

Nitrogenous bases found in DNA are classified into two structural groups, known as purines and pyrimidines, depending on the complexity of their molecular ring systems. Adenine and guanine are categorized as purines, while the single-ring pyrimidine class consists exclusively of cytosine and thymine. Therefore, cytosine and thymine are the specific bases identified as the pyrimidines within the DNA structure.

Nucleotides, the fundamental building blocks of genetic material, are constructed around a pentose sugar that provides distinct anchor points for the nitrogenous base and the phosphate group. The nitrogenous base is covalently bonded to the anomeric carbon at the $C_1'$ position, while the phosphate group forms a stable ester linkage with the hydroxyl group specifically located at the $C_5'$ position.

Insulin operates as a vital chemical messenger that regulates blood glucose levels, a function which characterizes it as a hormone rather than an enzyme or antibiotic. This proteinaceous substance is produced and secreted into the bloodstream by the beta cells located in the Islets of Langerhans of the pancreas, allowing it to exert systemic control over metabolic processes throughout the body.

Enzymes are shape-selective biological catalysts that rely on well-defined active sites to interact precisely with specific substrates. Unlike industrial catalysts that might operate at extreme thermal conditions, such as 1000 K, these biological molecules function effectively at body temperature and are highly susceptible to being inhibited or poisoned. This inherent specificity and sensitivity distinguish them from heterogeneous catalysts and underscore why they are uniquely suited for the delicate chemical processes within living systems.

Genetic information is encoded through nitrogenous bases categorized into purines and pyrimidines. While both RNA and DNA contain the same purines, adenine and guanine, they utilize distinct pyrimidines because DNA incorporates thymine and cytosine, whereas RNA replaces thymine with uracil. This substitution makes uracil a characteristic component of RNA that is entirely absent from the DNA molecule.

Cellulose acts as a structural polysaccharide composed entirely of repeating $\beta$-D-glucose units joined by specific glycosidic linkages. By subjecting this polymer to hydrolysis, which requires heating in the presence of dilute $\mathrm{H}_2\mathrm{SO}_4$ under pressure, the glycosidic bonds are broken down completely to yield its individual monomeric components. This transformation, represented by the chemical equation $(\mathrm{C}_6\mathrm{H}_{10}\mathrm{O}_5)_n + n\mathrm{H}_2\mathrm{O} \xrightarrow{\mathrm{H}^+} n\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6$, confirms that the product of the reaction is solely D-glucose.

The stability of the DNA structure relies on specific intermolecular forces that allow two complementary polynucleotide chains to associate effectively. These two strands coil around a common axis to form a right-handed double helix, characterized by an anti-parallel orientation where the chains run in opposite directions, with exactly ten bases found in one turn of the spiral. This precise geometry is held together by hydrogen bonding between the nitrogenous bases, providing the necessary attraction to maintain the helical shape.

Nucleic acids are structurally distinguished by the specific pentose sugar and nitrogenous bases they incorporate within their chains. While DNA relies on D-2 deoxyribose as its sugar and includes thymine as a base, RNA is characterized by the presence of D-ribose sugar paired with the nitrogenous base uracil instead of thymine.

Amino acids serve as the fundamental building blocks of proteins, distinguished by a molecular structure that features two specific functional groups attached to the same central carbon atom. These essential compounds incorporate both an amino group, denoted as $-\mathrm{NH}_2$, and a carboxyl group, represented as $-\mathrm{COOH}$, which define their versatile chemical behavior.

The presence of an aldehyde group is established by the positive Schiff's test, which confirms the compound is an aldotetrose with the formula $\mathrm{C}_4\mathrm{H}_8\mathrm{O}_4$. This sugar contains two chiral carbons and three hydroxyl groups, the latter being consistent with the formation of a triacetate upon acetylation. When an aldotetrose is treated with $\mathrm{NaBH}_4$, the aldehyde group is reduced to a primary alcohol, yielding a corresponding tetritol. In the case of L-erythrose, the reduction produces erythritol, which is an achiral meso compound due to an internal plane of symmetry. Conversely, the reduction of L-threose yields threitol, which is a chiral compound. Because the problem specifies that the reduction product 'B' is chiral, the sugar 'X' must be identified as L-threose.