JEE Mains Biomolecules

Cheatsheet Content

### Carbohydrates (Saccharides) - **Definition:** Polyhydroxy aldehydes or ketones, or compounds that produce such units on hydrolysis. - **General Formula:** $C_x(H_2O)_y$ (not always valid, e.g., deoxyribose $C_5H_{10}O_4$) - **Classification:** - **Monosaccharides:** Cannot be hydrolyzed further. (e.g., Glucose, Fructose, Ribose) - **Oligosaccharides:** Yield 2-10 monosaccharide units on hydrolysis. (e.g., Sucrose, Maltose, Lactose) - **Polysaccharides:** Yield large number of monosaccharide units on hydrolysis. (e.g., Starch, Cellulose, Glycogen) ### Monosaccharide Structures & Reactions #### Glucose ($C_6H_{12}O_6$) - **Aldohexose.** Most abundant organic compound. - **Open Chain Structure:** ``` CHO | H-C-OH | HO-C-H | H-C-OH | H-C-OH | CH2OH ``` - **Fischer Projection:** D-Glucose (OH on C5 on right) - **Haworth Structure (Cyclic):** - Forms hemiacetal with C5-OH and C1-CHO. - **$\alpha$-D-Glucopyranose:** C1-OH is trans to C2-OH (down). - **$\beta$-D-Glucopyranose:** C1-OH is cis to C2-OH (up). - **Mutarotation:** $\alpha$- and $\beta$-forms interconvert in aqueous solution via open-chain form. $$\alpha\text{-D-Glucose} \rightleftharpoons \text{Open chain} \rightleftharpoons \beta\text{-D-Glucose}$$ - **Reactions:** - **Oxidation:** - With Br2 water (mild): Gluconic acid (aldehyde oxidized) - With HNO3 (strong): Saccharic acid (aldehyde and 1° alcohol oxidized) - Tollen's/Fehling's reagent (reducing sugar). - **Reduction:** With Na/Hg or $NaBH_4$: Sorbitol (polyhydric alcohol) - Forms osazones with Phenylhydrazine. #### Fructose ($C_6H_{12}O_6$) - **Ketohexose.** - **Open Chain Structure:** ``` CH2OH | C=O | HO-C-H | H-C-OH | H-C-OH | CH2OH ``` - **Haworth Structure:** Forms hemiketal (C2-ketone with C5-OH or C6-OH). Furanose (5-membered ring) is more common. - **$\alpha$-D-Fructofuranose:** C2-OH down. - **$\beta$-D-Fructofuranose:** C2-OH up. - **Reducing sugar:** Due to isomerisation to aldose in alkaline solution (Lobry-de Bruyn-van Ekenstein rearrangement). ### Disaccharides & Polysaccharides #### Disaccharides ($C_{12}H_{22}O_{11}$) - Formed by two monosaccharide units linked by a **glycosidic linkage** (ether linkage formed by dehydration). 1. **Sucrose (Table Sugar):** $\alpha$-D-Glucose + $\beta$-D-Fructose. - Linkage: C1 of glucose to C2 of fructose. - **Non-reducing sugar:** Anomeric carbons (C1 of glu, C2 of fru) are involved in linkage; no free hemiacetal/hemiketal. - On hydrolysis: "Invert sugar" (dextrorotatory to levorotatory mixture). ``` Glucose(C1)-O-Fructose(C2) ``` 2. **Maltose (Malt Sugar):** $\alpha$-D-Glucose + $\alpha$-D-Glucose. - Linkage: C1 of one glucose to C4 of another glucose ($\alpha$-glycosidic). - **Reducing sugar:** One anomeric carbon is free. ``` Glucose(C1)-O-Glucose(C4) ``` 3. **Lactose (Milk Sugar):** $\beta$-D-Galactose + $\beta$-D-Glucose. - Linkage: C1 of galactose to C4 of glucose ($\beta$-glycosidic). - **Reducing sugar:** One anomeric carbon is free. ``` Galactose(C1)-O-Glucose(C4) ``` #### Polysaccharides 1. **Starch:** Storage carbohydrate in plants. Polymer of $\alpha$-D-glucose. - **Amylose (15-20%):** Unbranched chain, C1-C4 $\alpha$-glycosidic links. - **Amylopectin (80-85%):** Branched chain, C1-C4 $\alpha$-glycosidic links with C1-C6 $\alpha$-glycosidic links at branching points. - **Test:** Gives blue-black colour with iodine. 2. **Cellulose:** Structural carbohydrate in plants. Polymer of $\beta$-D-glucose. - Linear, unbranched chains. C1-C4 $\beta$-glycosidic links. - Humans cannot digest due to lack of enzymes for $\beta$-glycosidic links. - **Test:** No colour with iodine. 3. **Glycogen:** Storage carbohydrate in animals. Highly branched, similar to amylopectin but more branched. ### Proteins - **Definition:** Polymers of $\alpha$-amino acids linked by peptide bonds. Essential for life. - **Amino Acids:** Organic compounds containing both an amino group (-NH2) and a carboxyl group (-COOH). - General structure: ``` R | H2N - CH - COOH ``` - R group determines the identity and properties. - **Zwitterion:** In aqueous solution, $-\text{COOH}$ loses $\text{H}^+$ and $-\text{NH}_2$ gains $\text{H}^+$, forming a dual-charged ion. $${H_3N^+ - CH(R) - COO^-}$$ - **Isoelectric Point (pI):** pH at which amino acid exists as zwitterion and has no net electrical charge (zero migration in electric field). - **Essential Amino Acids:** Cannot be synthesized by the body; must be obtained from diet. (e.g., Valine, Leucine, Isoleucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Histidine, Arginine). ### Peptide Bond & Protein Structure #### Peptide Bond - Formed by condensation reaction between -COOH of one amino acid and -NH2 of another. - $$-CO-NH-$$ linkage. - The C-N bond has partial double bond character (resonance stabilization), making it rigid and planar. #### Levels of Protein Structure 1. **Primary Structure:** Sequence of amino acids in a polypeptide chain. Determines function. 2. **Secondary Structure:** Local folding of the polypeptide chain due to hydrogen bonding between peptide backbone atoms. - **$\alpha$-Helix:** Coiled, right-handed spiral. H-bonds between N-H of one AA and C=O of AA four residues away. (e.g., Keratin) - **$\beta$-Pleated Sheet:** Extended, zig-zag arrangement. H-bonds between adjacent polypeptide chains (inter-chain) or segments of the same chain (intra-chain). (e.g., Silk fibroin) 3. **Tertiary Structure:** Overall three-dimensional folding of the polypeptide chain, stabilized by interactions between R-groups. - **Forces:** H-bonds, disulfide bridges (-S-S- from two Cysteine residues), ionic bonds (salt bridges), hydrophobic interactions, van der Waals forces. - Gives rise to globular or fibrous proteins. 4. **Quaternary Structure:** Arrangement of two or more polypeptide subunits (monomers) to form a functional protein complex. (e.g., Hemoglobin - 4 subunits). #### Denaturation of Proteins - Loss of 2°, 3°, or 4° structure (sometimes 1°) due to disruption of intramolecular bonds. - Caused by changes in pH, temperature, presence of heavy metal salts, alcohol, etc. - Often irreversible (e.g., coagulation of egg white on heating). ### Enzymes & Vitamins #### Enzymes - **Definition:** Biocatalysts. Globular proteins that catalyze biochemical reactions. - **Properties:** - Highly specific (substrate and reaction). - Highly efficient (increase reaction rates significantly). - Operate under mild conditions (optimum pH and temperature). - Activity destroyed at high temperatures (denaturation). - **Mechanism:** Lower activation energy. - **Lock and Key Model:** Active site perfectly matches substrate. - **Induced Fit Model:** Active site changes shape slightly to fit substrate precisely. #### Vitamins - **Definition:** Organic compounds required in small amounts in diet for normal growth and health. - **Deficiency:** Leads to specific diseases. - **Classification:** - **Fat-soluble Vitamins:** A, D, E, K (stored in liver and adipose tissue). - **Water-soluble Vitamins:** B-complex, C (excreted in urine, generally not stored). | Vitamin | Source | Deficiency Disease | |---------|--------|--------------------| | A (Retinol) | Fish liver oil, Carrots | Xerophthalmia, Night blindness | | D (Calciferol) | Sunlight, Milk, Eggs | Rickets (children), Osteomalacia (adults) | | E (Tocopherol) | Vegetable oils | Muscular weakness, Increased fragility of RBCs | | K (Phylloquinone) | Green leafy vegetables | Increased blood clotting time | | B1 (Thiamine) | Yeast, Milk, Cereals | Beri-beri | | B2 (Riboflavin) | Milk, Egg white | Cheilosis, Digestive disorders | | B6 (Pyridoxine) | Yeast, Cereals | Convulsions | | B12 (Cyanocobalamin) | Meat, Fish, Eggs | Pernicious anaemia | | C (Ascorbic acid) | Citrus fruits, Amla | Scurvy | ### Nucleic Acids (DNA & RNA) - **Definition:** Biopolymers responsible for heredity and protein synthesis. - **Monomers:** Nucleotides. - **Components of a Nucleotide:** 1. **Pentose Sugar:** Ribose (in RNA) or 2'-deoxyribose (in DNA). ``` CH2OH | O---C1' / \ C4' C2' \ / O---C3' ``` - In DNA, C2' has H instead of OH. 2. **Nitrogenous Base:** - **Purines:** Adenine (A), Guanine (G) (two rings). - **Pyrimidines:** Cytosine (C), Thymine (T - in DNA), Uracil (U - in RNA) (one ring). 3. **Phosphate Group:** $H_3PO_4$ - **Nucleosides:** Base + Sugar (e.g., Adenosine, Guanosine, Cytidine, Thymidine, Uridine). - **Nucleotides:** Base + Sugar + Phosphate (e.g., Adenosine Monophosphate - AMP). - Phosphate group linked to 5'-OH of sugar. Base linked to 1'-C of sugar via N-glycosidic link. #### DNA (Deoxyribonucleic Acid) - **Structure:** Double helix (Watson-Crick model). - Two polynucleotide strands run anti-parallel (5' to 3' and 3' to 5'). - Sugar-phosphate backbone on outside, bases on inside. - Bases pair via **hydrogen bonds:** - A always pairs with T (A=T, two H-bonds). - G always pairs with C (G$\equiv$C, three H-bonds). (Chargaff's Rule: A=T and G=C). - One complete turn of helix is $\approx 3.4$ nm, with $\approx 10$ base pairs per turn. - **Function:** Storage and transfer of genetic information (heredity). - **Replication:** DNA makes copies of itself. #### RNA (Ribonucleic Acid) - **Structure:** Single-stranded, usually. Contains Uracil (U) instead of Thymine (T). Contains Ribose sugar. - **Types:** - **mRNA (messenger RNA):** Carries genetic code from DNA to ribosomes. - **tRNA (transfer RNA):** Carries specific amino acids to ribosomes for protein synthesis. - **rRNA (ribosomal RNA):** Constituent of ribosomes, involved in protein synthesis. - **Function:** Protein synthesis.