Correct Option: (c)

Explanation: Maltose is actually a **reducing sugar**, not a non-reducing sugar. It consists of two $\alpha$-D-glucopyranose units linked by a $\text{C}_1\text{-C}_4$ glycosidic bond. The hemiacetal group (at $\text{C}_1$) of the second glucose unit remains free in solution and can easily open up to yield a free, reactive aldehyde group. Consequently, maltose reduces Fehling's solution and Tollens' reagent, giving a positive reducing sugar test.

• Reducing Sugars: Carbohydrates that contain a free, reactive carbonyl group (aldehyde or ketone) in the form of a hemiacetal or hemiketal. Because this group is free, they are capable of reducing Fehling's solution (to red $\text{Cu}_2\text{O}$) and Tollens' reagent (to a metallic silver mirror).
  Example: Glucose, Maltose, Lactose.
• Non-Reducing Sugars: Carbohydrates in which the carbonyl groups of all constituent monosaccharide units are locked in a glycosidic linkage and are not free in solution. They do not react with Fehling's or Tollens' reagents.
  Example: Sucrose, Starch, Cellulose.

Structural Evidence:

Sucrose is a disaccharide formed by the condensation of $\alpha$-D-glucose and $\beta$-D-fructose. The glycosidic linkage is formed between the hemiacetal carbon ($\text{C}_1$) of the glucose unit and the hemiketal carbon ($\text{C}_2$) of the fructose unit:

Because the potential carbonyl carbons of **both** monosaccharides ($\text{C}_1$ of glucose and $\text{C}_2$ of fructose) are involved in forming the glycosidic linkage, neither unit has a free hemiacetal or hemiketal group. In aqueous solution, the rings cannot open to generate free aldehyde or ketone groups. As a result, sucrose cannot reduce Fehling's solution or Tollens' reagent, making it a non-reducing sugar.

Anomers: Diastereomers of cyclic monosaccharides that differ in configuration **only** at the hemiacetal/hemiketal carbon atom ($\text{C}_1$ in aldoses, $\text{C}_2$ in ketoses). This carbon atom is called the anomeric carbon.

Comparison in Glucose:

• In **$\alpha$-D-glucopyranose**, the hydroxyl group ($-\text{OH}$) attached to the anomeric carbon ($\text{C}_1$) lies on the right side in the Fischer projection (or projects downwards in the Haworth projection).
• In **$\beta$-D-glucopyranose**, the hydroxyl group ($-\text{OH}$) attached to the anomeric carbon ($\text{C}_1$) lies on the left side in the Fischer projection (or projects upwards in the Haworth projection).

Although D-glucose contains a potential aldehyde group, its open-chain structure fails to account for several experimental observations:

1. No reaction with Schiff's reagent or Sodium Bisulfite: Despite containing an aldehyde group, glucose does not form a bisulfite addition product with $\text{NaHSO}_3$, nor does it restore the pink color of Schiff's reagent.
2. Absence of reaction in Glucose Pentaacetate: When glucose is reacted with acetic anhydride, it forms glucose pentaacetate. Interestingly, this pentaacetate does **not** react with hydroxylamine ($\text{NH}_2\text{OH}$) to form an oxime, indicating the absence of a free $-\text{CHO}$ group in the acetylated form.
3. Existence in two crystalline forms ($\alpha$ and $\beta$): Glucose crystallizes in two distinct forms with different physical properties (such as melting points and specific rotations). $\alpha$-D-glucose is obtained by crystallization from a concentrated solution at $303\text{ K}$, while $\beta$-D-glucose is obtained by crystallization from a hot saturated solution at $371\text{ K}$. The open-chain model cannot explain these two isomeric forms.

Chemical Mechanism:

In aqueous solution, free glucose exists in an equilibrium between its cyclic hemiacetal forms and its open-chain aldehyde form ($\approx 0.02\%$). Hydroxylamine reacts with the open-chain aldehyde form to yield an oxime, which continuously shifts the hemiacetal equilibrium to produce more open chain:

However, when glucose is treated with acetic anhydride, all five hydroxyl groups—including the highly reactive **hemiacetal hydroxyl group at $\text{C}_1$**—are acetylated to form glucose pentaacetate.

Since the $\text{C}_1$ oxygen is locked as an ester (acetate) linkage, the ring **cannot undergo ring-opening** in solution to generate a free aldehyde group. Without a free $-\text{CHO}$ group, glucose pentaacetate is completely inert to nucleophilic attack by hydroxylamine and fails to form an oxime.

(i) Prolonged heating with Hydrogen Iodide ($\text{HI}$):

Glucose is reduced to **n-hexane**, indicating that **all six carbon atoms are linked in a straight, unbranched chain**:

(ii) Oxidation with Bromine Water ($\text{Br}_2 / \text{H}_2\text{O}$):

Bromine water is a mild oxidizing agent that selectively oxidizes the aldehyde group to a carboxylic acid group, producing **gluconic acid**. This confirms that the carbonyl group in glucose is an **aldehyde group**:

(iii) Oxidation with Concentrated Nitric Acid ($\text{HNO}_3$):

Concentrated nitric acid is a strong oxidizing agent that oxidizes both the terminal aldehyde group ($\text{-CHO}$) and the primary alcohol group ($\text{-CH}_2\text{OH}$) at the other end to dicarboxylic acid, producing **saccharic acid** (glucaric acid). This indicates the presence of a **primary alcohol group** in glucose:

Mutarotation: The spontaneous, time-dependent change in the specific optical rotation of an optically active compound in solution until a stable equilibrium value is reached.

Phenomenon in D-Glucose:

D-glucose exists in two distinct crystalline anomers with different specific rotations:

• A freshly prepared solution of pure **$\alpha$-D-glucopyranose** has a specific rotation of **$+112^\circ$**. Over time, this value decreases.
• A freshly prepared solution of pure **$\beta$-D-glucopyranose** has a specific rotation of **$+19^\circ$**. Over time, this value increases.

When either anomer is dissolved in water, the ring spontaneously opens to the open-chain form, allowing conversion into the other anomer. Eventually, both solutions reach the same stable equilibrium value of **$+52.7^\circ$**, which represents an equilibrium mixture of approximately $36\% \alpha$-anomer, $64\% \beta$-anomer, and a tiny trace ($0.02\%$) of the open-chain form.

Zwitterion (Dipolar Ion): An intramolecular salt formed when an acidic proton from the carboxyl group ($-\text{COOH}$) is transferred to the basic amino group ($-\text{NH}_2$) within the same amino acid molecule, resulting in a species with coexisting positive and negative charges:

Salt-like Behavior Justification:

In the solid state, $\alpha$-amino acids exist entirely as dipolar zwitterions. Rather than experiencing weak molecular dispersion or dipole forces like simple amines or carboxylic acids, zwitterions interact through incredibly strong, multi-directional **electrostatic (ionic) attractions** (similar to $\text{NaCl}$ salts).

Because breaking these strong electrostatic forces requires a significant amount of thermal energy, amino acids exhibit high melting points, exist as crystalline solids, are soluble in polar solvents (like water), and are insoluble in non-polar organic solvents (like benzene).

Peptide Bond: A covalent amide linkage ($-\text{CO-NH}-$) formed between the carboxyl group ($-\text{COOH}$) of one amino acid molecule and the amino group ($-\text{NH}_2$) of another amino acid molecule, accompanied by the elimination of a molecule of water.

Structural Representation:

Here, the group $-\text{CO-NH}-$ is the peptide bond linking the two amino acids together in a dipeptide.

Isoelectric Point: The specific pH value at which an amino acid molecule carries **no net electrical charge** and exists entirely as a neutral, dipolar zwitterion. At this pH, the amino acid is stationary and does not migrate towards either electrode in an electric field.

Behavior in Different pH Environments:

• At pH < pI (Acidic Medium): The carboxylate ion ($-\text{COO}^{-}$) accepts a proton, converting the amino acid into a **cation ($\text{H}_3\text{N}^{+}$-CH(R)-COOH)**, which migrates toward the cathode.
• At pH > pI (Alkaline Medium): The ammonium ion ($-\text{NH}_3^{+}$) loses a proton, converting the amino acid into an **anion ($\text{H}_2\text{N-CH(R)-COO}^{-}$)**, which migrates toward the anode.

Denaturation of Proteins: A process in which a native protein (possessing specific biological activity) undergoes conformational changes due to physical or chemical stress (such as heating, agitation, or changes in pH), causing it to lose its unique three-dimensional shape and biological activity.

Why Primary Structure Remains Intact:

A native protein's structure is stabilized by different types of chemical bonds:

• The **primary structure** is held together by exceptionally strong, covalent **peptide bonds** ($-\text{CO-NH}-$) that link amino acids in a specific sequence.
• The **secondary and tertiary structures** are stabilized by weaker, non-covalent interactions such as **hydrogen bonds**, ionic interactions, disulfide bridges, and hydrophobic forces.

When a protein undergoes denaturation, the heat or pH change is energetic enough to disrupt the weak hydrogen bonds and non-covalent interactions, causing the helices and sheets to uncoil and globular structures to unfold. However, these conditions are **not energetic enough** to hydrolyze the strong covalent peptide bonds. Consequently, the primary sequence of amino acids remains completely unaffected.

Mechanistic Explanation:

Egg white contains a soluble globular protein called **ovalbumin**. When the egg is boiled, the thermal energy disrupts the weak hydrogen bonds, hydrophobic interactions, and ionic attractions stabilizing the ovalbumin's native globular tertiary structure.

As these bonds break, the folded polypeptide chains uncoil and stretch out. These uncoiled, hydrophobic protein chains then randomly interact and aggregate with adjacent molecules, forming a dense, insoluble, cross-linked network. This irreversible physical transformation, known as coagulation, traps water molecules within the gel, converting the clear liquid egg white into an opaque, solid white mass.

Both secondary structures represent spatial arrangements of the polypeptide backbone, stabilized exclusively by **intramolecular or intermolecular hydrogen bonds** between the amide carbonyl oxygen ($-\text{C}=\text{O}$) and amide hydrogen ($-\text{N-H}$) groups:

• $\alpha$-Helix structure: The polypeptide chain folds in a right-handed screw configuration. The helix is stabilized by **intramolecular hydrogen bonds**, where the oxygen of the carbonyl group of an amino acid residue forms a hydrogen bond with the hydrogen of the $-\text{N-H}$ group of the fourth amino acid residue further down the chain.
• $\beta$-Pleated Sheet structure: Multiple polypeptide chains (or different segments of the same chain) align parallel or anti-parallel next to one another. The structure is stabilized by **intermolecular hydrogen bonds** between the adjacent chains, giving it a flat, pleated sheet-like appearance.

The four nitrogenous bases present in a DNA molecule are:

1. Adenine (A) [Purine]
2. Thymine (T) [Pyrimidine]
3. Guanine (G) [Purine]
4. Cytosine (C) [Pyrimidine]

Base Pairing Rules (Chargaff's Rule):

The bases form highly specific, complementary hydrogen-bonded pairs across the double-stranded helix:

• Adenine pairs with Thymine through **two** hydrogen bonds:
  $$ \text{A} = \text{T} $$
• Guanine pairs with Cytosine through **three** hydrogen bonds:
  $$ \text{G} \equiv \text{C} $$

Because the $\text{G}\equiv\text{C}$ pair is held together by three hydrogen bonds, it is thermally more stable than the $\text{A}=\text{T}$ pair.

Classification of Vitamins:

1. Water-Soluble Vitamins: Vitamins that dissolve in water, which include the Vitamin B-complex and Vitamin C ($\text{ascorbic acid}$).
2. Fat-Soluble Vitamins: Vitamins that are insoluble in water but soluble in lipids and organic solvents, which include Vitamins A, D, E, and K.

Dietary Requirement Reason:

• Water-soluble vitamins (with the exception of Vitamin $\text{B}_{12}$) are not stored in the body. Any excess ingested is readily filtered by the kidneys and excreted from the body in urine. Because they are constantly excreted, they must be supplied regularly in our daily diet.
• Fat-soluble vitamins (Vitamins A and D) are stored in the body's adipose (fat) tissues and the liver. Since the body maintains a reserves pool of these vitamins, daily ingestion is not required, as excessive consumption can actually lead to toxicity (hypervitaminosis).

1. (i) Scurvy:
   • Deficient Vitamin: Vitamin C (Ascorbic acid).
   • Dietary Source: Citrus fruits (amla, lemon, orange), green leafy vegetables.
2. (ii) Night Blindness:
   • Deficient Vitamin: Vitamin A (Retinol).
   • Dietary Source: Carrots, fish liver oil, milk, butter.
3. (iii) Increased Blood Clotting Time:
   • Deficient Vitamin: Vitamin K (Phylloquinone).
   • Dietary Source: Green leafy vegetables (spinach, cabbage), egg yolk.
4. (iv) Rickets:
   • Deficient Vitamin: Vitamin D (Calciferol).
   • Dietary Source: Exposure to sunlight, fish, egg yolk, milk.

Correct Option: (a)

Explanation: Monosaccharides like glucose have free hemiacetal groups, which can open up to yield a free aldehyde group, making them reducing. However, when glucose units polymerize to form starch, the hemiacetal carbon ($\text{C}_1$) of almost every glucose unit undergoes condensation to form glycosidic bonds (either $\text{C}_1\text{-C}_4$ in amylose chains or $\text{C}_1\text{-C}_6$ at amylopectin branches). Only one terminal glucose molecule in a massive polymer chain of thousands of units has a free $\text{C}_1$ hemiacetal group. Since the proportion of free hemiacetal groups in starch is negligible, it acts as a non-reducing carbohydrate. Thus, both statements are true, and the reason provides the correct explanation.