Biomolecules – NEET UG ChemistryPractice Questions & PYQs

Storage polysaccharides in living organisms feature highly branched or unbranched polymeric arrangements of glucose units tailored for rapid energy mobilization. Glycogen structurally resembles amylopectin rather than amylose because both feature extensive branch points in addition to linear linkages, whereas amylose is entirely unbranched. Beyond its primary role as an energy reservoir in animal liver and muscle tissues, glycogen is also widely synthesized and stored by lower eukaryotic organisms such as yeasts and fungi. Therefore, the structural comparison to amylose is inaccurate while the occurrence in fungi remains entirely correct.

Although glucose possesses a free aldehyde group in its open-chain structure, steric hindrance and the formation of cyclic hemiacetal structures prevent it from undergoing certain characteristic reactions typical of simple aliphatic aldehydes. Specifically, it fails to restore the pink color in Schiff's reagent and is sterically hindered from forming a crystalline bisulphite addition compound with sodium hydrogen sulphite ($\mathrm{NaHSO}_3$). Consequently, reactions involving Schiff's reagent and $\mathrm{NaHSO}_3$ do not yield the expected products with glucose, making items B and E the correct choices.

Nucleic acids are constructed from monomeric building blocks whose nomenclature depends on the precise attachment of nitrogenous bases and phosphate groups to a pentose sugar backbone. Statement I is entirely accurate because joining a purine or pyrimidine base to the $1'$ carbon of the sugar via a N-glycosidic linkage creates a nucleoside. However, Statement II is scientifically flawed because the linkage at the $5'$ position involves phosphoric acid ($\mathrm{H}_3\mathrm{PO}_4$) to form a nucleotide, rather than phosphorous acid ($\mathrm{H}_3\mathrm{PO}_3$).

Protein denaturation involves the disruption of secondary, tertiary, and quaternary structures through physical or chemical alterations like heat or pH changes, leaving the primary peptide backbone intact. While this process destroys biological activity and uncoils the characteristic helical conformation, it describes the destruction of higher-order folding rather than the synthetic creation of the macromolecule. Therefore, stating that a protein is formed from amino acids linked by peptide bonds refers to peptide synthesis and polymerization rather than the denaturation phenomenon.

Addition polymerization proceeds through the sequential chain growth of unsaturated monomers without the elimination of any small-molecule byproducts. Teflon exemplifies this mechanism perfectly as it is synthesized directly from the free-radical polymerization of gaseous tetrafluoroethene molecules. In contrast, polymers like nylon-66, dacron, and novolac form through step-growth condensation reactions accompanied by the release of water or alcohol molecules.

Disaccharides like sucrose are composed of two monosaccharide units joined by a glycosidic linkage between the anomeric carbons of glucose and fructose. When cleaved by water in the presence of an acid or an enzyme, this bond breaks to yield its constituent monosaccharides. Specifically, the reaction produces alpha-D-glucose and beta-D-fructose in their respective cyclic forms.

Amino acids are classified based on the chemical nature of their side chains, where basic amino acids contain an extra amino group that makes them alkaline in aqueous solution. Among the common proteinogenic amino acids, lysine features a long aliphatic hydrocarbon chain ending in a primary amino group. This additional basic center allows the side chain to accept a proton at physiological pH values.

Protein denaturation involves the disruption of higher-order folding patterns due to external stressors like heat or pH changes, which break stabilizing hydrogen bonds and hydrophobic interactions. However, the covalent peptide bonds that link individual amino acids together in the polypeptide chain remain entirely unaffected by this process. Consequently, only the linear sequence of the protein, known as its primary structure, survives the denaturation treatment intact.

Non-essential amino acids are those that the human body can synthesize independently, meaning they do not strictly need to be obtained directly from dietary sources. Alanine is a prime example of such an amino acid, easily manufactured via transamination pathways from common metabolic intermediates like pyruvate. This capability ensures a steady cellular supply without relying entirely on nutritional intake.

Amino acids contain both a basic amino group and an acidic carboxyl group in the same molecule, allowing an internal proton transfer to occur. In this process, the acidic proton from the $\text{-COOH}$ group migrates to the basic $\text{-NH}_2$ group, creating a dipolar ion known as a zwitterion. Among the given choices, glycine is the simplest amino acid and readily exists in this dual-charged state where it bears both a positive and a negative charge simultaneously.

Starch consists of two distinct polysaccharide components built entirely from repeating glucose units connected exclusively through alpha-glycosidic bonds. Amylose forms a simple linear chain where monomer units link sequentially between carbon one and carbon four. In contrast, amylopectin incorporates branching points along its structure by introducing additional linkages between carbon one and carbon six alongside the standard linear connections.

Protein denaturation disrupts the secondary and tertiary structures by breaking hydrogen bonds and other non-covalent interactions, causing the peptide chains to uncoil. Because a protein's specific three-dimensional shape is essential for its function, this structural breakdown destroys its biological activity rather than enhancing it. Consequently, denatured proteins become inactive and lose their specific biochemical properties, such as enzymatic or hormonal functions.

The D or L designation of a monosaccharide depends on the stereochemical orientation of the hydroxyl group at the highest-numbered chiral center, where a right-facing group denotes the D-configuration and a left-facing group denotes the L-configuration. Erythrose and threose are diastereomeric aldotetroses characterized by whether adjacent hydroxyl groups reside on the same side or opposite sides of the carbon backbone. Evaluating the given Fischer projections based on these stereochemical assignments yields D-erythrose, D-threose, L-erythrose, and L-threose in that exact order.

Reducing sugars possess free aldehyde or ketone groups capable of reducing Tollens' or Fehling's reagents. In sucrose, the glycosidic linkage is formed between the C-1 of alpha-glucose and the C-2 of beta-fructose, which locks both anomeric carbons in the bond. Because no free carbonyl group is available to open into a reactive chain form, sucrose acts as a non-reducing sugar while maltose and lactose retain free hemiacetal groups.

Proteins are linear polymers constructed from individual amino acid monomers joined end-to-end through specialized covalent linkages. This connection forms when the carboxyl group of one amino acid reacts with the amino group of an adjacent monomer, releasing a molecule of water in a condensation reaction. The resulting amide linkage is specifically known as a peptide bond, which provides the stable backbone required to build complex polypeptide chains.

The central dogma of molecular biology establishes the unidirectional pathway by which hereditary instructions are transcribed and translated within a cellular system. Genetic information is first permanently stored and replicated in deoxyribonucleic acid, which is then transcribed into messenger ribonucleic acid transcripts. These transcripts are subsequently translated at the ribosomes into functional polypeptide chains that fold into proteins responsible for cellular structure and activity.

Nucleic acids are distinguished primarily by the specific pentose sugar incorporated into their nucleotide backbone structure. Ribonucleic acid contains a standard ribose sugar characterized by a hydroxyl group attached to the second carbon atom. In contrast, deoxyribonucleic acid features a modified sugar variant lacking one oxygen atom at that specific position, yielding 2-deoxyribose.

The fight-or-flight response is mediated by the adrenal medulla, which secretes emergency hormones during sudden stress. Adrenaline binds to specific hepatic receptors and triggers a signaling cascade that rapidly converts stored glycogen into glucose. This process, known as glycogenolysis, floods the bloodstream with readily available fuel to cope with physical demands.

The reaction of an aldose like D(+) glucose with hydroxylamine involves the nucleophilic attack of the amino group on the carbonyl carbon of the open-chain aldehyde. This condensation process selectively converts the $C=O$ group at the first carbon into an oxime linkage (\text{CH}=\text{N}-\text{OH}$), while the stereochemistry of the remaining chiral centers along the carbon chain remains completely unaffected. Consequently, the resulting glucoxime structure features this characteristic oxime functionality appended to an otherwise unchanged D-glucose carbon skeleton.

The double-helical structure of DNA is stabilized by complementary base pairing across the two polynucleotide strands. Adenine pairs exclusively with thymine through two hydrogen bonds, while guanine pairs with cytosine through three hydrogen bonds. These non-covalent hydrogen bonds provide the precise molecular recognition required for genetic replication while allowing the strands to unwind easily during transcription.