NEET Biomolecules Previous Year Questions with Solutions

Tertiary structure represents the overall folding of the polypeptide chains (further folding of the secondary structure), giving rise to fibrous or globular shapes. The spatial arrangement of two or more polypeptide subunits with respect to each other is defined as QUATERNARY structure, not tertiary.

• • Statement 1 is incorrect.

Biological activity does not strictly require quaternary structure. Numerous single-chain globular proteins (such as myoglobin, lysozyme, and ribonuclease) possess only primary, secondary, and tertiary structures and exhibit full native biological activity.

• • Statement 2 is incorrect.

In the β-pleated sheet structure, individual polypeptide strands (extended in a zig-zag arrangement) line up adjacent to one another (either parallel or antiparallel). The backbone >C=O group of one chain forms hydrogen bonds with the >N-H group of an adjacent chain. Because these bonds occur between distinct polypeptide chains (or distinct segments), they are INTERMOLECULAR hydrogen bonds.

• • Statement 3 is scientifically correct and matches textbook definitions.

In an α-helix, a polypeptide chain twists into a coil by forming intramolecular hydrogen bonds between the -NH of each amino acid residue and the >C=O of the fourth residue ahead. The natural helical conformation of proteins composed of L-amino acids is exclusively a RIGHT-HANDED screw, not left-handed.

• • Statement 4 incorrectly claims a left-handed helix.

Examine the side chains of the four amino acids in the options: 1. Leucine: (CH₃)₂CH-CH₂-CH(NH₂)COOH (Contains C, H, N, O). 2. Threonine: CH₃-CH(OH)-CH(NH₂)COOH (Contains C, H, N, O). 3. Serine: HO-CH₂-CH(NH₂)COOH (Contains C, H, N, O). 4. Methionine: CH₃-S-CH₂-CH₂-CH(NH₂)COOH (Contains C, H, N, O, and S).

• • Methionine is the unique sulfur-containing amino acid among the choices.

When an organic compound containing sulfur (and nitrogen) is fused with sodium metal, covalent sulfur is converted into ionic sodium sulfide (or sodium thiocyanate if N and S are both present and Na is not in excess): 2Na + S → Na₂S Na + C + N + S → NaSCN

• • Organic sulfur is mineralized into sulfide / thiocyanate ions.

Sulfide ions react with sodium nitroprusside [Fe(CN)₅NO]²⁻ in alkaline medium to form the brilliant purple-violet complex ion [Fe(CN)₅NOS]⁴⁻ (often noted alongside red-blood thiocyanate coloration in dual N+S analysis). Because leucine, threonine, and serine lack sulfur entirely, they cannot give this test.

• • Only methionine yields the sulfur diagnostic test.
• • Corresponds to Option (3).

Vitamin A is a fat-soluble vitamin essential for night vision (rhodopsin synthesis). Rich dietary sources include carrots (beta-carotene precursor), fish liver oil, milk, and butter.

• • Match: A -> IV.

Vitamin B₁₂ is a water-soluble coordination complex containing cobalt. It cannot be synthesized by higher plants and is obtained in human diets exclusively through animal foods such as meat, liver, fish, and eggs.

• • Match: B -> I.

Vitamin E is an antioxidant fat-soluble vitamin primarily sourced from vegetable seed oils, including sunflower oil, wheat germ oil, and cotton seed oil.

• • Match: C -> II.

Vitamin K is responsible for blood clotting (prothrombin synthesis) and is richly abundant in green leafy vegetables like spinach, cabbage, and kale.

• • Match: D -> III.
• • Full Match: A-IV, B-I, C-II, D-III (Option 2).

Starch is a homopolymer of α-glucose molecules and serves as the principal energy storage polysaccharide in plant tissues. In animals, excess energy is stored as glycogen.

• • Forms helical secondary structures capable of trapping iodine molecules (giving a blue-black colour).

Antibodies (immunoglobulins) are defensive glycoproteins produced by B lymphocytes and plasma cells in response to antigens. Their primary function is to fight infectious pathogens.

• • Listed under "Some Proteins and their Functions" in NCERT Table 9.5.

Concanavalin A is an agglutinating carbohydrate-binding protein extracted from the jack bean (Canavalia ensiformis). In plant biochemistry and NCERT classification, it is categorized under Lectins (Secondary Metabolites).

• • Listed under Table 9.3: Some Secondary Metabolites (Category: Lectins).

GLUT-4 (Glucose Transporter 4) is an insulin-regulated transmembrane transport protein found primarily in adipose tissues and striated muscle. It facilitates the cellular uptake and transport of glucose.

• • Listed in NCERT Table 9.5: "GLUT-4: Enables glucose transport into cells."

Matching all components yields: A-II, B-I, C-IV, D-III, which exactly corresponds to Option (2).

• • A -> II (Energy storage)
• • B -> I (Fights infection)
• • C -> IV (Lectin)
• • D -> III (Glucose transport)

Living tissues are predominantly composed of water (70–90% of total cell mass) along with proteins, lipids, nucleic acids, and carbohydrates. Because water ($H_2O$) has a molecular weight of 18 with 16 parts Oxygen and 2 parts Hydrogen, Oxygen contributes the overwhelming majority of body mass.

• • Calcium (~1.5%) and Phosphorus (~1.0%) follow Nitrogen.

Arranging in descending order (highest percentage to lowest percentage):

• • (a) Oxygen: 65%
• • (b) Carbon: 18.5%
• • (c) Hydrogen: ~9.5 - 10%
• • (d) Nitrogen: ~3.3%

Sequence: (a), (b), (c), (d). This corresponds directly to Option (1).

• • Matches official NTA final answer key Option A / (1).

Examine the biochemical roles and clinical deficiency symptoms from NCERT Table 14.1:

• • A matches IV (A-IV).
• • Vitamin D (Calciferol): Regulates calcium and phosphate absorption. Deficiency leads to bone softening and malformation in children known as Rickets (III).
• • B matches III (B-III).

Examine water-soluble B-complex vitamins:

• • C matches I (C-I).
• • Vitamin B₆ (Pyridoxine): Involved in amino acid metabolism and neurotransmitter synthesis. Severe deficiency manifests clinically as Convulsions (II).
• • D matches II (D-II).

Combine the individual pairings:

• • Matches Option (2) exactly.

Carbohydrates are classified as aldoses or ketoses based on the functional carbonyl group:

• • Maltose is a disaccharide composed of two α-D-glucose units.
• • Sucrose is a non-reducing disaccharide composed of glucose and fructose.
• • Therefore, among monosaccharides, only D-Fructose is a keto sugar.

Examine optical rotation properties of D-monosaccharides:

• • Because of its strong negative rotation, D-fructose is universally known as levulose.
• • This property completely excludes D-glucose and matches D-fructose.

Cyclization of the C2 keto group with the C5 hydroxyl group produces a five-membered hemiketal furanose ring:

• • Honey is nature’s richest source of free fructose.
• • Thus Sugar X is unequivocally D-Fructose, matching Option (2).

Enzymes are divided into 6 functional classes (Classes 1 through 6) based on the type of reaction they catalyze:

• • Class 1: Oxidoreductases/dehydrogenases catalyze oxidation-reduction between two substrates $S$ and $S'$ ($S_{\text{reduced}} + S'_{\text{oxidized}} \rightarrow S_{\text{oxidized}} + S'_{\text{reduced}}$).
• • Class 2: Transferases catalyze the transfer of a chemical group, $G$ (other than hydrogen), between a pair of substrates $S$ and $S'$ ($S - G + S' \rightarrow S + S' - G$).
• • Class 3: Hydrolases catalyze the hydrolytic cleavage of ester, ether, peptide, glycosidic, C-C, C-halide, or P-N bonds.
• • Class 4: Lyases catalyze removal of groups from substrates by mechanisms other than hydrolysis, forming double bonds.
• • Class 5: Isomerases catalyze interconversion of optical, geometric, or positional isomers.
• • Class 6: Ligases catalyze the linking together of two compounds (e.g. C-O, C-S, C-N, P-O bonds) coupled with ATP hydrolysis.

The given reaction: $S - G + S' \rightarrow S + S' - G$, where $G$ is a chemical group other than hydrogen (such as methyl, phosphate, or amino group). Group $G$ is transferred directly from donor substrate $S$ to acceptor substrate $S'$. This is the canonical defining reaction of Transferases.

The enzyme class is Transferase, corresponding to Option (3).

Enzymes can be simple proteins or conjugated holoenzymes consisting of a protein part (apoenzyme) and a non-protein part (co-factor):

• • Co-factors are of three types: prosthetic groups, co-enzymes, and metal ions.
• • Prosthetic groups are organic compounds characterized by being tightly or permanently bound to the apoenzyme.

NCERT explicitly cites Catalase and Peroxidase as the prime examples:

• • Catalase catalyzes the rapid decomposition of hydrogen peroxide into water and molecular oxygen: $2 H_2O_2 \xrightarrow{\text{Catalase}} 2 H_2O + O_2$.
• • Haem (an iron-protoporphyrin complex) is the prosthetic group tightly integrated into the catalytic active site of catalase.
• • The central iron ($Fe^{3+}/Fe^{4+}$) undergoes reversible redox changes during the catalytic cycle.

Reviewing the other candidate enzymes:

• • RuBisCo: Requires $Mg^{2+}$ as an essential divalent activator ion.
• • Carbonic anhydrase: Contains a zinc ($Zn^{2+}$) metal ion at its active site, not haem.
• • Succinate dehydrogenase: Uses FAD (flavin adenine dinucleotide) and iron-sulfur clusters ($Fe-S$), but not haem as its prosthetic group.

Catalase is the enzyme containing haem as its prosthetic group, matching Option (4).

Understanding the hierarchical composition of nucleic acid building blocks and amino acids:

• • A. Adenosine: Formed when the purine base adenine is covalently linked to a pentose sugar (ribose or deoxyribose) through an N-glycosidic linkage. It is a Nucleoside. Hence, A matches III.
• • B. Adenylic acid (Adenosine monophosphate / AMP): Formed when a phosphate group is esterified to the 5'-OH of the pentose sugar in adenosine. It is a Nucleotide. Hence, B matches II.
• • C. Adenine: A substituted purine (6-aminopurine) consisting of a heterocyclic carbon-nitrogen double ring. It is a free Nitrogen base. Hence, C matches I.
• • D. Alanine: An organic molecule containing an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a methyl group (-CH₃) attached to the $\alpha$-carbon. It is an Amino acid. Hence, D matches IV.

Pairing the components:

• • A -> III (Adenosine = Nucleoside)
• • B -> II (Adenylic acid = Nucleotide)
• • C -> I (Adenine = Nitrogen base)
• • D -> IV (Alanine = Amino acid)

The unambiguous combination is A-III, B-II, C-I, D-IV, corresponding to Option (3).

Many enzymes are conjugated proteins composed of two distinct biochemical entities:

• • Apoenzyme: The macromolecular protein portion of the enzyme that provides the 3D tertiary structure, substrate-binding pocket, and catalytic active site framework.
• • Co-factor: The non-protein constituent bound to the enzyme to make it catalytically active.

Distinguishing non-protein co-factors:

• • Prosthetic group: An organic co-factor tightly and permanently bound to the apoenzyme (e.g. haem in catalase).
• • Coenzyme: An organic co-factor only transiently associated with the apoenzyme during catalysis (often derived from vitamins, e.g. NAD, NADP from niacin).
• • Metal ions: Inorganic co-factors coordinating with active site residues (e.g. $Zn^{2+}$ in carbonic anhydrase).

The protein portion of an enzyme is called the Apoenzyme.

Although glucose contains an aldehyde carbonyl group in its Fischer projection, it fails to exhibit several characteristic reactions of aliphatic aldehydes: 1. It does not react with Schiff’s reagent to restore the pink/magenta color. 2. It does not form a sodium bisulphite addition product with NaHSO3. 3. The pentaacetate of glucose does not react with hydroxylamine (NH2OH), proving the absence of a free -CHO group.

The -OH at C-5 adds intramolecularly to the C-1 carbonyl carbon, locking glucose into a stable six-membered cyclic hemiacetal (glucopyranose). The concentration of free open-chain aldehyde is less than 0.02%. Bulky bisulphite ions (HSO3-) and Schiff’s reagent cannot drive ring opening because their addition products are thermodynamically less stable than the closed hemiacetal ring.

In contrast: - Tollen’s reagent is alkaline and drives tautomerization (Lobry de Bruyn-Alberda van Ekenstein rearrangement) and oxidation to gluconic acid. - HCN and NH2OH are powerful, unhindered nucleophiles that irreversibly trap the minute open-chain form, pulling the equilibrium forward to completion.

Some lipids have phosphorus and a phosphorylated organic compound in them. These are phospholipids. They are found in cell membranes.

Lecithin (phosphatidylcholine) consists of glycerol, two fatty acid chains, a phosphate group, and choline. It is an amphipathic lipid and a major structural constituent of biological membranes.

Cofactors are non-protein constituents bound to an enzyme to make it catalytically active. They are divided into three kinds: prosthetic groups, coenzymes, and metal ions.

A number of enzymes require metal ions for their activity which form coordination bonds with side chains at the active site and simultaneously form one or more coordination bonds with the substrate.

NCERT explicitly states: "For example, zinc is a cofactor for the proteolytic enzyme carboxypeptidase."

When the inhibitor closely resembles the substrate in its molecular structure and inhibits the activity of the enzyme by competing for the active binding site, it is known as competitive inhibition.

Inhibition of succinic dehydrogenase by malonate is the classic demonstration: malonate closely resembles succinate in chemical structure, binding to the active site and preventing succinate oxidation to fumarate.

Competitive inhibition can be overcome by increasing the concentration of the substrate (succinate).

- Collagen: Intercellular ground substance. - Trypsin: Enzyme. - Insulin: Hormone. - Antibody: Fights infectious agents. - Receptor: Sensory reception (smell, taste, hormone, etc.). - GLUT-4: Enables glucose transport into cells.

A -> IV B -> I C -> II D -> III

Matches Option (3) uniquely.

- In proteins, amino acids are linked by peptide bonds formed by condensation between carboxyl and amino groups. - In polysaccharides, monosaccharides are linked by glycosidic bonds. - In lipids (triglycerides), fatty acids are esterified to glycerol by ester bonds. - In nucleic acids, nucleotides are linked along the backbone by 3'-5' phosphodiester bonds.

- Lipase hydrolyzes ester bonds (A -> II). - Nuclease hydrolyzes phosphodiester bonds (B -> IV). - Protease hydrolyzes peptide bonds (C -> I). - Amylase hydrolyzes glycosidic bonds (D -> III).

A-II, B-IV, C-I, D-III corresponds to Option (1).

The catalytic cycle of an enzyme action can be described in the following steps: 1. First, the substrate binds to the active site of the enzyme, fitting into the active site (Step E). 2. The binding of the substrate induces the enzyme to alter its shape, fitting more tightly around the substrate, forming the enzyme-substrate complex (Step A).

3. The active site of the enzyme, now in close proximity of the substrate breaks the chemical bonds of the substrate and the new enzyme-product complex is formed (Step D). 4. The enzyme releases the products of the reaction (Step C). 5. The free enzyme is ready to bind to another molecule of the substrate and run through the catalytic cycle once again (Step B).

The sequence is E -> A -> D -> C -> B, corresponding to Option (3).

A nucleoside is a two-component monomer formed by the condensation of a nitrogenous heterocyclic base (purine or pyrimidine) with a pentose sugar (β-D-ribose or β-D-2-deoxyribose). The base is covalently linked to the C-1' position of the furanose sugar moiety via an N-glycosidic linkage:

A nucleotide is formed when a nucleoside is linked to a phosphate moiety via an ester linkage, typically at the 5'-hydroxyl group of the pentose sugar. Crucially, the biological phosphorylating agent is orthophosphoric acid (H₃PO₄), not phosphorous acid (H₃PO₃):

Statement II explicitly asserts that the linkage occurs with "phosphorous acid" (H₃PO₃). Phosphorous acid is an oxyacid with phosphorus in oxidation state +3 and a non-acidic P-H bond, which is never involved in nucleic acid backbones. Therefore, Statement II is factually and chemically false.

Amylose in starch forms a helical secondary structure with internal hydrophobic cavities:

Cellulose is a linear, unbranched homopolymer of $\beta$-D-glucose linked via $\beta$-1,4-glycosidic bonds. Extensive inter- and intramolecular hydrogen bonding holds adjacent chains parallel in planar sheets, preventing helical coil formation:

Because cellulose does not possess the prerequisite helical secondary structure to trap polyiodide ions, it produces a negative result (no blue colouration) upon addition of iodine solution.

When an inhibitor closely resembles the natural substrate in its molecular structure and competes for the active catalytic site of the enzyme, it is a competitive inhibitor:

NCERT states under Enzymes: "Inhibition of succinic dehydrogenase by malonate which closely resembles the substrate succinate in structure. Such competitive inhibitors are often used in the control of bacterial pathogens."

Because the inhibitor binds reversibly to the catalytic active site without altering the enzyme permanently, competitive inhibition can be overcome by increasing substrate concentration ($K_m$ increases, $V_{\max}$ remains unchanged).

Polypeptides are synthesized in the $N \to C$ direction from the ribosome. NCERT defines the positional convention of a primary protein structure:

NCERT states verbatim: "A protein is imagined as a line, the left end represented by the first amino acid and the right end represented by the last amino acid. The first amino acid is also called as N-terminal amino acid. The last amino acid is called the C-terminal amino acid." Because Statement I claims the left end is C-terminal and right end is N-terminal, it is definitively FALSE.

Adult human hemoglobin ($HbA$) is an allosteric tetrameric metalloprotein:

Enzyme activity exhibits a bell-shaped curve across temperature. Sub-optimal low temperatures reduce thermal molecular kinetic collisions, locking the active site in a dormant state without breaking covalent bonds. Supra-optimal high temperatures disrupt weak hydrogen and ionic bonds stabilizing tertiary folding, causing irreversible denaturation:

When a chemical agent shares geometric and charge complementarity with the catalytic active site of an enzyme, competing with the physiological substrate:

NCERT states verbatim: - "Low temperature preserves the enzyme in a temporarily inactive state whereas high temperature destroys enzymatic activity because proteins are denatured by heat." (Statement I is TRUE) - "When the inhibitor closely resembles the substrate in its molecular structure and inhibits the activity of the enzyme, it is known as competitive inhibitor." (Statement II is TRUE)

Vitamin $B_{12}$ (cyanocobalamin) is a cobalt-containing water-soluble vitamin required for DNA synthesis, erythropoiesis (maturation of red blood cells in the bone marrow), and normal myelin sheath synthesis in nerve cells.

In the absence of Vitamin $B_{12}$ (or lack of intrinsic factor required for its absorption in the terminal ileum), erythroblasts fail to proliferate properly, resulting in megaloblastic/pernicious anaemia characterized by a severe deficiency of mature functional red blood cells (RBC deficiency) and low haemoglobin count.

- Vitamin $B_1$ (Thiamine): Causes Beri-beri. - Vitamin $B_2$ (Riboflavin): Causes Cheilosis (cracked lips and corners of mouth), digestive disorders, burning sensation of skin. - Vitamin $B_6$ (Pyridoxine): Causes Convulsions.

Biomolecules in plant and fungal cells are categorized based on their roles in organismal survival:

- Morphine, Codeine: Alkaloids (secondary metabolites). - Vinblastin, Curcumin: Anti-cancer and anti-inflammatory drugs (secondary metabolites). - Rubber, Gums: Polymeric substances (secondary metabolites). - Amino acids, Glucose: Universal building blocks and energy substrates $\implies$ Primary metabolites.

Biopolymers are assembled from specific building blocks via characteristic condensation bonds:

Matching (a)-(iv), (b)-(i), (c)-(ii), (d)-(iii) points uniquely to Option (1).

Plants, fungi, and microbes synthesize thousands of organic compounds not directly involved in primary growth, termed secondary metabolites. NCERT organizes them into distinct functional chemical classes: - Pigments: Carotenoids, Anthocyanins - Alkaloids: Morphine, Codeine - Terpenoids: Monoterpenes, Diterpenes - Essential oils: Lemon grass oil - Toxins: Abrin, Ricin - Lectins: Concanavalin A - Drugs: Vinblastin, Curcumin - Polymeric substances: Rubber, Gums, Cellulose

Pair (4) lists "Drugs - Ricin". Ricin is a carbohydrate-binding protein (toxin) produced in the seeds of the castor oil plant (*Ricinus communis*). It inactivates eukaryotic 60S ribosomal subunits, causing fatal inhibition of protein synthesis. It is classified strictly as a Toxin, not a drug. The genuine medicinal drugs listed in NCERT are Vinblastin (anticancer vinca alkaloid) and Curcumin (anti-inflammatory compound from turmeric).

- Statement (a) (FALSE): Fatty acids possessing only single carbon-carbon bonds ($-\text{C}-\text{C}-$) along their aliphatic acyl chain are termed saturated fatty acids (e.g. palmitic, stearic). Fatty acids possessing one or more double bonds ($-\text{C}=\text{C}-$) are termed unsaturated fatty acids (e.g. oleic, linoleic, arachidonic). - Statement (b) (TRUE): Lecithin (phosphatidylcholine) is a major membrane phospholipid composed of glycerol esterified to two fatty acids, a phosphate group, and a choline nitrogenous head group. - Statement (c) (TRUE): Glycerol is chemically designated as trihydroxy propane, with the IUPAC structural formula $\text{CH}_2\text{OH}-\text{CHOH}-\text{CH}_2\text{OH}$ (propane-1,2,3-triol). - Statement (d) (FALSE): Palmitic acid is a 16-carbon saturated fatty acid ($ext{CH}_3(\text{CH}_2)_{14}\text{COOH}$), possessing 16 carbon atoms including the carboxyl carbon, NOT 20. - Statement (e) (FALSE): Arachidonic acid is a polyunsaturated fatty acid with 4 double bonds that contains 20 carbon atoms including the carboxyl carbon, NOT 16.

Only statements (b) and (c) are true statements. This leads exclusively to Option (3).