Biomolecules
The Chemistry of Life. Decode the cyclic structures of sugars, the precise pH mechanics of Zwitterions, and the structural bonds holding DNA and proteins together.
Module Focus: Linkages & Monomers
Biomolecules are complex, life-sustaining polymers built from simple monomers. For NEET, you do not need to memorize entire protein chains; instead, you must master the bonds connecting them (Glycosidic, Peptide, Phosphodiester) and the specific chemical reactions that prove the structure of their individual units (like Glucose and Amino Acids).
1. Carbohydrates
Polyhydroxy aldehydes or ketones. They are classified by their behavior upon hydrolysis: Monosaccharides (cannot be hydrolyzed further), Oligosaccharides (yield 2-10 monosaccharides), and Polysaccharides (yield a large number of monosaccharides).
A. Chemical Evidence for the Structure of Glucose
| Reagent Used | Product Formed | What it Proves |
|---|---|---|
| Prolonged heating with $HI$ | n-Hexane | All 6 carbons are linked in a straight chain. |
| $NH_2OH$ (Hydroxylamine) | Oxime | Presence of a carbonyl ($>C=O$) group. |
| Mild Oxidation (Bromine water) | Gluconic acid | The carbonyl group is an Aldehyde ($-CHO$). |
| Strong Oxidation (Conc. $HNO_3$) | Saccharic acid (Glucaric acid) |
Presence of a primary alcohol ($-CH_2OH$) at the bottom of the chain. |
Glucose forms a six-membered pyranose ring via intramolecular hemiacetal formation between C1 and C5.
This creates a new chiral center at C1 (the anomeric carbon). The two isomeric forms ($\alpha$-D-Glucose and $\beta$-D-Glucose) which differ ONLY in the configuration at C1 are called Anomers.
A sugar is Reducing if it can reduce Fehling's/Tollens' reagent. It MUST have a free hemiacetal or hemiketal group (free anomeric $-OH$).
Why is SUCROSE Non-Reducing?
Sucrose is formed by an $\alpha(1\rightarrow2)$ glycosidic linkage between $\alpha$-D-glucose and $\beta$-D-fructose. The anomeric carbons of BOTH monosaccharides are involved in the bond. Because there is no free anomeric carbon, it cannot open into an aldehyde/ketone, making it strictly Non-Reducing.
2. Amino Acids & Proteins
Proteins are polymers of $\alpha$-amino acids linked by peptide bonds (an amide linkage: $-CO-NH-$).
Amino acids contain both an acidic carboxyl group ($-COOH$) and a basic amino group ($-NH_2$). In aqueous solution, the proton transfers from the acid to the base, forming a dipolar, electrically neutral ion called a Zwitterion.
High $H^+$ concentration forces the $COO^-$ to accept a proton. The amino acid becomes a Cation ($H_3N^+-CH(R)-COOH$) and migrates to the cathode.
Base removes $H^+$ from the $NH_3^+$. The amino acid becomes an Anion ($H_2N-CH(R)-COO^-$) and migrates to the anode.
- Primary: The exact sequence of amino acids linked by peptide bonds.
- Secondary: The folding of the chain into $\alpha$-helices or $\beta$-pleated sheets, stabilized strictly by Hydrogen bonds between backbone $-C=O$ and $-N-H$ groups.
- Tertiary: The overall 3D shape, stabilized by H-bonds, disulfide linkages, van der Waals, and electrostatic forces.
When subjected to physical change (heat) or chemical change (pH), the hydrogen bonds are disturbed. Globular proteins unfold and lose their biological activity.
3. Nucleic Acids (DNA & RNA)
Nucleic acids are polymers of nucleotides. They carry genetic information and dictate protein synthesis.
DNA contains $\beta$-D-2-deoxyribose (lacks oxygen at C2). RNA contains $\beta$-D-ribose.
2. Nitrogenous Base- Purines: Adenine (A), Guanine (G).
- Pyrimidines: Cytosine (C), Thymine (T, only in DNA), Uracil (U, only in RNA).
Nucleoside: Base + Sugar (attached at C1 via N-glycosidic linkage).
Nucleotide: Nucleoside + Phosphate group (attached at C5 via phosphoester linkage).
The Backbone: Nucleotides are joined together by Phosphodiester linkages between the 5' and 3' carbon atoms of the pentose sugar.
The two strands of DNA are held together by precise Hydrogen bonding between specific bases:
4. Vitamins (Solubility Traps)
Vitamins A, D, E, and K. They are stored in liver and adipose tissues. Excess intake causes hypervitaminosis.
Vitamins B group and C. Must be supplied regularly in diet because they are readily excreted in urine (Exception: Vitamin $B_{12}$ is stored in the liver).
NEET Grand Test: Biomolecules
15 High-Yield Questions testing specific linkages, reducing sugars, and structural chemistry proofs.
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