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Unit 10: Biomolecules

10.1 Carbohydrates

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or the compounds which produce such units on hydrolysis. They are primarily produced by plants. General formula: \(C_x(H_2O)_y\)

Classification

  1. Monosaccharides: Cannot be further hydrolysed. E.g., Glucose (an aldohexose), Fructose (a ketohexose), Ribose.
  2. Oligosaccharides: Yield 2 to 10 monosaccharide units on hydrolysis. Further classified as disaccharides (sucrose, maltose, lactose), trisaccharides, etc.
    • Sucrose (non-reducing sugar) yields \(\alpha\)-D-glucose and \(\beta\)-D-fructose.
    • Maltose (reducing sugar) yields two \(\alpha\)-D-glucose units.
    • Lactose (reducing sugar) yields \(\beta\)-D-galactose and \(\beta\)-D-glucose.
  3. Polysaccharides: Yield a large number of monosaccharide units on hydrolysis. They are not sweet in taste (non-sugars). E.g., Starch, Cellulose, Glycogen. All are polymers of glucose.
    • Starch: Polymer of \(\alpha\)-D-glucose. Consists of Amylose (water-soluble, linear chain of C1-C4 glycosidic linkage) and Amylopectin (water-insoluble, branched chain with C1-C6 linkages).
    • Cellulose: Polymer of \(\beta\)-D-glucose with linear \(\beta\)-1,4-glycosidic linkages.

Reducing and Non-Reducing Sugars: Carbohydrates that reduce Fehling’s solution and Tollen’s reagent are reducing sugars (all monosaccharides, maltose, lactose). Sucrose is a non-reducing sugar because its reducing groups (hemiacetal -OH) are involved in glycosidic bond formation.

D- and L- Configuration: Relates the structure to glyceraldehyde. If the -OH group on the lowest chiral carbon atom is on the right, it is D-configuration; if on the left, it is L-configuration.

10.2 Proteins

Polymers of \(\alpha\)-amino acids. They are required for growth and maintenance of the body.

Amino Acids and Peptides

Amino acids contain amino (\(-NH_2\)) and carboxyl (\(-COOH\)) functional groups. Except glycine, all naturally occurring \(\alpha\)-amino acids are optically active (L-series).

  • Zwitterion: In aqueous solution, the carboxyl group loses a proton (\(H^+\)) and the amino group gains a proton, forming a dipolar ion called a Zwitterion.
  • Peptide bond (\(-CO-NH-\) link): Formed between the carboxyl group of one amino acid and the amino group of another with the elimination of water.

Structure of Proteins

  1. Primary Structure: The specific sequence of amino acids in the polypeptide chain.
  2. Secondary Structure: The shape in which a long polypeptide chain can exist due to regular folding (e.g., \(\alpha\)-helix and \(\beta\)-pleated sheet), stabilized by hydrogen bonds between the \(-NH\) and \(-C=O\) groups of the peptide bond.
  3. Tertiary Structure: The overall 3D shape of an entire protein molecule resulting from further folding of the secondary structure. Stabilized by hydrogen bonds, disulphide linkages, van der Waals forces, and electrostatic forces.
  4. Quaternary Structure: The spatial arrangement of two or more polypeptide chains (subunits) with respect to each other (e.g., Haemoglobin).

Denaturation of Proteins

When a protein in its native form is subjected to physical change (temperature) or chemical change (pH), hydrogen bonds are disturbed. Globules unfold and helices get uncoiled, and the protein loses its biological activity. Note: During denaturation, secondary and tertiary structures are broken, but the primary structure remains intact. Example: Coagulation of egg white on boiling, curdling of milk.

10.3 Enzymes, Vitamins, and Hormones

  • Enzymes: Biological catalysts. Almost all are globular proteins. They are highly specific for particular reactions. Example: Invertase hydrolyses sucrose; Maltase hydrolyses maltose.
  • Vitamins: Organic compounds required in the diet in small amounts to perform specific biological functions for normal maintenance of optimum growth and health.
    • Water-soluble: Vitamins B and C (must be supplied regularly as they are excreted in urine).
    • Fat-soluble: Vitamins A, D, E, K (stored in liver and adipose tissues).
  • Hormones: Chemical messengers secreted directly into the bloodstream by endocrine glands. They coordinate various physiological processes.

10.4 Nucleic Acids

Polymers of nucleotides (polynucleotides). They are responsible for the transmission of inherent characters from one generation to the next (heredity) and protein synthesis.

  • DNA (Deoxyribonucleic Acid): Contains sugar (\(\beta\)-D-2-deoxyribose), a phosphate group, and nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). Has a double-helical structure.
  • RNA (Ribonucleic Acid): Contains sugar (\(\beta\)-D-ribose). Nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), and Uracil (U). Usually short, single-stranded.

Competency-Based Questions (CBQs)

Q1. (CBSE 2024 Pattern) Two samples of carbohydrates (A and B) are analyzed. Sample A reduces Fehling’s solution, whereas Sample B does not. Furthermore, upon hydrolysis in an acidic medium, Sample B yields an equimolar mixture of a dextrorotatory sugar and a laevorotatory sugar, which when combined has a net laevoritatory rotation. Identify sugars A and B with proper justification.


Answer: Sample A: is a reducing sugar. It could be any monosaccharide (like glucose or fructose) or a reducing disaccharide (like maltose or lactose) since they have a free aldehyde/ketone group. Sample B: is a non-reducing sugar. Since Sample B does not reduce Fehling’s solution, it strongly suggests Sucrose. Justification: Sucrose (A dextrorotatory disaccharide, \(+66.5^\circ\)) on hydrolysis gives an equimolar mixture of D-(+)-glucose (\(+52.5^\circ\)) and D-(-)-fructose (\(-92.4^\circ\)). Since the laevorotation of fructose \(-92.4^\circ\) is greater in magnitude than the dextrorotation of glucose \(+52.5^\circ\), the resulting hydrolysed mixture is laevorotatory. This change in specific rotation from dextro to laevo is called inversion of sugar, and the mixture is called invert sugar, exactly matching the description of Sample B.

Q2. (Sample Paper 2023) Hard-boiled eggs and curd are typical examples of protein chemistry in everyday life. Explain the biochemical process involved when an egg is boiled in water.


Answer: When an egg is boiled in water, the heat causes a physical change in the protein present in the egg white (albumin). This process is known as Denaturation of Protein.

  1. Due to the high temperature, the hydrogen bonds, disulphide linkages, and other stabilizing forces holding the intricate 3D structure of the protein are disrupted.
  2. The globules unfold (tertiary and quaternary structure break down) and the helices uncoil (secondary structure is lost).
  3. The primary structure (the sequence of amino acids linked by peptide bonds) remains completely intact.
  4. The uncoiled protein chains get entangled and form a massive network via new intermolecular cross-links, causing the water soluble, translucent egg white to coagulate into a firm, water-insoluble opaque white mass. Denaturation renders the protein biologically inactive.

Q3. (CBSE 2020) Distinguish between the following pairs on the basis of their chemical structure/composition: (i) DNA and RNA (ii) Amylose and Amylopectin


Answer: (i) DNA and RNA:

  • Sugar: DNA contains \(\beta\)-D-2-deoxyribose sugar; RNA contains \(\beta\)-D-ribose sugar.
  • Nitrogenous Bases: DNA contains Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). RNA contains Adenine, Guanine, Cytosine, and Uracil (U) instead of Thymine.
  • Structure: DNA usually exists as a very long double-stranded helix. RNA usually exists as a shorter single-stranded molecule (though it can fold onto itself).

(ii) Amylose and Amylopectin: Both are components of starch and are polymers of \(\alpha\)-D-glucose.

  • Amylose: Water-soluble component of starch (approx 15-20%). It is a long unbranched chain consisting of 200-1000 \(\alpha\)-D-(+)-glucose units held together by C1-C4 glycosidic linkages.
  • Amylopectin: Water-insoluble fraction of starch (approx 80-85%). It is a highly branched chain polymer. The straight chains are formed by C1-C4 glycosidic linkages, whereas the branching occurs by C1-C6 glycosidic linkages.