Biomolecules
Complete Chemistry Revision for CBSE Class 12
1. Carbohydrates
Carbohydrates are optically active polyhydroxy aldehydes or ketones, or substances that produce these on hydrolysis.
Classification of Sugars
- Monosaccharides: Simplest, cannot be hydrolyzed further. (e.g., Glucose, Fructose, Ribose).
- Disaccharides: On hydrolysis give two units of monosaccharides. (e.g., Sucrose, Lactose, Maltose).
- Polysaccharides: Give large number of monosaccharide units. (e.g., Starch, Cellulose, Glycogen).
Key Concepts
- Reducing vs Non-Reducing: Sugars that reduce Tollens' and Fehling's reagent are reducing sugars. Sucrose is a non-reducing sugar because its anomeric carbons are involved in the glycosidic bond.
- Glucose ($C_6H_{12}O_6$): Shows cyclic structure. Formation of $\alpha$-D-glucopyranose and $\beta$-D-glucopyranose.
2. Proteins & Amino Acids
Proteins are polymers of $\alpha$-amino acids. Amino acids contain both amino ($-NH_2$) and carboxyl ($-COOH$) groups.
The Zwitter Ion
In aqueous solution, the carboxyl group can lose a proton and the amino group can accept a proton, giving rise to a dipolar ion known as a zwitter ion. This ion is electrically neutral but contains both positive and negative charges.
$$\text{H}_2\text{N-CH(R)-COOH} \rightleftharpoons \text{H}_3\text{N}^+\text{-CH(R)-COO}^-$$Protein Structure
- Primary: Specific sequence of amino acids in a polypeptide chain.
- Secondary: $\alpha$-helix (stabilized by H-bonds) or $\beta$-pleated sheet.
- Tertiary: Three-dimensional folding, giving rise to globular or fibrous shapes.
- Quaternary: Arrangement of two or more polypeptide chains (subunits).
Denaturation
When a protein in its native state is subjected to heat or pH change, the hydrogen bonds are disrupted. The protein uncoils, loses its 3D structure, and consequently loses its biological activity. Example: Boiling an egg causes coagulation of albumin protein.
3. Nucleic Acids
Biopolymers responsible for heredity. There are two types: DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid).
- Composition: Pentose sugar + Phosphoric acid + Nitrogenous Base.
- Bases: Purines (Adenine, Guanine) and Pyrimidines (Cytosine, Thymine, Uracil). DNA has Thymine; RNA has Uracil.
- Double Helix: DNA exists as a double strand held together by hydrogen bonds between base pairs ($A=T$, $G\equiv C$).
Board Exam Question Bank
Q1. Why is sucrose a non-reducing sugar? 1 Mark
Q2. What is the difference between $\alpha$-helix and $\beta$-pleated sheet structures? 2 Marks
Q3. What are the products of hydrolysis of Lactose? 1 Mark
Q4. List the essential differences between DNA and RNA. 3 Marks
- DNA contains 2-deoxyribose sugar; RNA contains ribose sugar.
- DNA has Thymine base; RNA has Uracil base.
- DNA is usually double-stranded; RNA is usually single-stranded.
Exam Prep Strategy
- Structure Focus: Ensure you can draw the Haworth structures of glucose and fructose.
- Definitions: Memorize exact definitions for Denaturation, Zwitter ion, and Glycosidic linkage; CBSE marking schemes are precise here.
- DNA/RNA Comparison: This is a very high-frequency question. Memorize the three key differences provided above.
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