NCERT Solutions

Monosaccharides are the simplest carbohydrates that cannot be hydrolysed into simpler polyhydroxy aldehydes or ketones. They typically contain 3 to 7 carbon atoms.

Examples: Glucose (\(C_6H_{12}O_6\)), Fructose (\(C_6H_{12}O_6\)), Ribose (\(C_5H_{10}O_5\)).

Carbohydrates containing a free aldehyde (\(-CHO\)) or ketone (\(>C=O\)) group (or hemiacetal group) capable of reducing Tollens’ reagent (to metallic silver) and Fehling’s solution (to red precipitate of \(Cu_2O\)) are called reducing sugars.

Examples: All monosaccharides (Glucose, Fructose), Maltose, Lactose.

1. Structural Material: Cellulose, a polysaccharide, provides structure to the cell walls of plants.
2. Energy Reserve: Starch, a polysaccharide, serves as the major food storage reserve in plants.

• Monosaccharides: Ribose, 2-deoxyribose, Galactose, Fructose.
• Disaccharides: Maltose, Lactose.

A glycosidic linkage is an ether linkage (\(C-O-C\)) formed by the condensation of two monosaccharide units with the loss of a water molecule. It joins two monosaccharide units to form di-, oligo-, or polysaccharides.

• Glycogen: Also known as “animal starch”, it is the storage polysaccharide in animals (stored in liver and muscles). It consists of highly branched chains of \(\alpha\)-D-glucose.
• Difference: While both are polymers of \(\alpha\)-D-glucose, glycogen is more highly branched than amylopectin (a component of starch). Starch is found in plants, whereas glycogen is found in animals/fungi.

• (i) Sucrose: On hydrolysis, it gives an equimolar mixture of D-(+)-Glucose and D-(-)-Fructose.
  \(C_{12}H_{22}O_{11} + H_2O \to C_6H_{12}O_6 (Glucose) + C_6H_{12}O_6 (Fructose)\)
• (ii) Lactose: On hydrolysis, it gives an equimolar mixture of D-(+)-Galactose and D-(+)-Glucose.

• Starch: A polymer of \(\alpha\)-D-glucose. It consists of Amylose (linear, C1-C4 \(\alpha\)-linkage) and Amylopectin (branched, C1-C6 \(\alpha\)-linkage).
• Cellulose: A linear polymer of \(\beta\)-D-glucose. The glucose units are joined by \(\beta\)-glycosidic linkages between C1 of one unit and C4 of the next. It does not contain branches.

Glucose is reduced to n-Hexane. This proves the straight chain carbon skeleton.

\(Br_2\) water is a mild oxidizing agent. It oxidizes the aldehyde group (-CHO) to carboxylic acid (-COOH), forming Gluconic Acid.

\(HNO_3\) is a strong oxidizing agent. It oxidizes both the aldehyde and the primary alcohol group to carboxylic acids, forming Saccharic Acid (Glucaric acid).

1. Glucose does not give the 2,4-DNP test or Schiff’s test, and does not form the hydrogensulphite addition product, indicating the absence of a free -CHO group.
2. The pentaacetate of glucose does not react with hydroxylamine (\(NH_2OH\)), indicating the absence of a free -CHO group.
3. Glucose exists in two crystalline forms, \(\alpha\) and \(\beta\), which have different melting points and optical rotations (Mutarotation). This isomerism is not explained by the open chain structure.

• Essential Amino Acids: Cannot be synthesized by the human body and must be obtained from the diet. Examples: Valine, Leucine.
• Non-essential Amino Acids: Can be synthesized by the body. Examples: Glycine, Alanine.

• (i) Peptide Linkage: An amide linkage (\(-CO-NH-\)) formed between the carboxyl group of one amino acid and the amino group of another, with the elimination of a water molecule.
• (ii) Primary Structure: The specific sequence in which various amino acids are linked to each other in a polypeptide chain. Any change in this sequence creates a different protein.
• (iii) Denaturation: The process where a protein loses its biological activity due to physical changes (like heating) or chemical changes (like pH change). During denaturation, hydrogen bonds are disturbed, globules unfold, and helixes uncoil. The primary structure remains intact. Example: Coagulation of egg white.

The two main types are:

• \(\alpha\)-Helix: The polypeptide chain coils into a right-handed screw-like structure, stabilized by intramolecular Hydrogen bonds between the -NH group of one amino acid and the >C=O group of the 4th amino acid turn.
• \(\beta\)-Pleated Sheet: Polypeptide chains are stretched out and laid side-by-side, held together by intermolecular Hydrogen bonds, resembling a pleated sheet.

Intramolecular Hydrogen Bonding between the oxygen of the carbonyl group (>C=O) of one amino acid residue and the hydrogen of the amide group (-NH) of the fourth amino acid residue along the chain stabilizes the \(\alpha\)-helix.

Amino acids contain both an acidic carboxyl group (\(-COOH\)) and a basic amino group (\(-NH_2\)). In aqueous solution, the carboxyl group loses a proton and the amino group accepts a proton, forming a dipolar ion called a Zwitter ion (\(H_3N^+-CH(R)-COO^-\)).

In acidic medium, it accepts a proton (acts as base/cation). In basic medium, it loses a proton (acts as acid/anion). This dual behavior explains its amphoteric nature.

Enzymes are biological catalysts. They are globular proteins that catalyze specific biochemical reactions in living organisms. They are highly specific and efficient, working at optimal temperature (37°C) and pH.

During denaturation, the secondary and tertiary structures of the protein are destroyed due to the disruption of hydrogen bonds and other weak forces. The protein globules unfold and helixes uncoil. However, the primary structure (sequence of amino acids) remains intact. The protein loses its biological activity.

Vitamins are classified based on solubility:

• Fat-soluble vitamins: Soluble in fat/oils but insoluble in water. Stored in liver/adipose tissue. Examples: A, D, E, K.
• Water-soluble vitamins: Soluble in water. Must be supplied regularly (except B12) as they are excreted in urine. Examples: B-complex, C.

Vitamin K (Phylloquinone) is responsible for the coagulation of blood.

• Vitamin A: Essential for vision (formation of retinal pigment). Deficiency causes Night Blindness and Xerophthalmia. Sources: Carrots, Milk, Butter, Fish liver oil.
• Vitamin C: Essential for healthy gums, immune system, and collagen formation. Deficiency causes Scurvy. Sources: Citrus fruits (Amla, Orange, Lemon), Green leafy vegetables.

Nucleic acids are long-chain polymers of nucleotides (Polynucleotides) found in the nucleus of cells. They are responsible for the transmission of hereditary characteristics and protein synthesis.

Functions:

1. Replication: DNA replicates itself to pass genetic information from one generation to the next.
2. Protein Synthesis: RNA directs the synthesis of specific proteins in the cell.

• Nucleoside: Consists of a Pentose Sugar + Nitrogenous Base (attached at C1′). Example: Adenosine.
• Nucleotide: Consists of a Nucleoside + Phosphate Group (attached at C5′). Example: Adenylic acid.

The two strands are complementary because the hydrogen bonding occurs between specific pairs of bases: Adenine (A) always pairs with Thymine (T) via 2 H-bonds, and Guanine (G) always pairs with Cytosine (C) via 3 H-bonds. Thus, the sequence of bases on one strand automatically determines the sequence on the other strand.

There are three main types of RNA:

1. Messenger RNA (mRNA): Carries genetic code from DNA to ribosomes.
2. Ribosomal RNA (rRNA): Structural component of ribosomes.
3. Transfer RNA (tRNA): Carries amino acids to the ribosome for protein synthesis.

Glucose and sucrose contain many polar -OH groups. These groups form strong hydrogen bonds with water molecules, making them soluble. Cyclohexane and benzene are non-polar hydrocarbons that cannot form hydrogen bonds with water, making them insoluble.

Lactose is a disaccharide composed of \(\beta\)-D-galactose and \(\beta\)-D-glucose. On hydrolysis, it yields equal moles of D-Galactose and D-Glucose.

The pentaacetate of glucose does not react with hydroxylamine. This indicates that the aldehyde group is not free but is involved in the formation of a cyclic hemiacetal structure (specifically, the C1-OH is part of the ring and acetylated, preventing ring opening to the free aldehyde form).

Amino acids exist as Zwitter ions (dipolar ions) at physiological pH. This imparts them with a strong salt-like crystalline character. The strong electrostatic forces of attraction between the positive (\(NH_3^+\)) and negative (\(COO^-\)) poles lead to higher melting points and water solubility compared to simple halo acids which do not exist as zwitter ions.

When an egg is boiled, the proteins inside (albumin) undergo denaturation and coagulation. The water present in the egg gets adsorbed or hydrogen-bonded into the coagulated protein mass, forming a solid/semi-solid gel. Thus, no free water is left.