The chemistry of life — carbohydrates, proteins, enzymes, vitamins, nucleic acids, and hormones. With interactive biomolecule categorizer, structure diagrams, and 20+ real-life examples.
Before diving in, here's the entire chapter on one page. Biomolecules = building blocks of life. Each type has its own chemistry but all work together.
Biomolecules are the chemical compounds that make up living organisms. Carbohydrates provide energy (glucose, starch, cellulose). Proteins are building blocks and enzymes (amino acids, peptides, DNA-binding). Nucleic acids store and transmit genetic information (DNA, RNA). Vitamins are essential micronutrients. Hormones are chemical messengers. Understanding their chemistry explains life at the molecular level.
Carbohydrates: Polyhydroxy aldehydes or polyhydroxy ketones, or compounds that produce them on hydrolysis.
General formula: C_x(H₂O)_y (apparently "hydrates of carbon" — historically named this way, but the H₂O ratio is not always exact).
Examples: Glucose (C₆H₁₂O₆), sucrose (C₁₂H₂₂O₁₁), starch (C₆H₁₀O₅)_n, cellulose (C₆H₁₀O₅)_n
1. Monosaccharides: Simplest carbohydrates, cannot be hydrolyzed further. Soluble in water, sweet taste.
2. Oligosaccharides: 2-10 monosaccharide units joined by glycosidic bonds.
3. Polysaccharides: Many monosaccharide units (usually thousands).
Reducing sugars: Have a free aldehyde or ketone group (or hemiacetal that can open to aldehyde). They reduce Tollens' and Fehling's reagents.
Non-reducing sugars: No free aldehyde/ketone (both anomeric carbons involved in glycosidic bond).
Glucose (C₆H₁₂O₆, "dextrose"): Simplest most abundant monosaccharide.
Structure: Open chain — aldohexose: CHO-(CHOH)₄-CH₂OH. Cyclic — pyranose (6-membered ring with O), alpha and beta anomers.
Preparation: From sucrose + H₂O/H⁺ → glucose + fructose. Or from starch + H₂O/H⁺ → glucose (enzymatic in industry).
Properties:
Fructose (C₆H₁₂O₆): Ketohexose. Fruit sugar.
Structure: Open chain — CH₂OH-CO-(CHOH)₃-CH₂OH. Cyclic — furanose (5-membered ring), alpha and beta anomers.
Also a reducing sugar. Dextrorotatory in pure form.
Sucrose (C₁₂H₂₂O₁₁): Table sugar. α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside. Non-reducing (both anomeric carbons involved). Dextrorotatory (+66.5°).
Lactose (C₁₂H₂₂O₁₁): Milk sugar. β-D-galactopyranosyl-(1→4)-D-glucopyranose. Reducing. Less sweet than sucrose.
Maltose (C₁₂H₂₂O₁₁): Malt sugar. From starch hydrolysis. α-D-glucopyranosyl-(1→4)-D-glucopyranose. Reducing.
Starch (C₆H₁₀O₅)_n: Mixture of amylose (linear, α-1,4-glucose, 15-20%) and amylopectin (branched, α-1,4 + α-1,6-glucose, 80-85%). Energy storage in plants. Iodine test: blue-black complex.
Cellulose (C₆H₁₀O₅)_n: β-1,4-linked D-glucose. Linear, very strong (H-bonds between chains). Structural in plant cell walls. Humans cannot digest (no β-glucosidase enzyme). Cows/termites can (via gut microbes).
Glycogen: Animal storage form. Similar to amylopectin but more branched.
Carbohydrates = polyhydroxy aldehydes/ketones. Mono (glucose, fructose), oligo (sucrose, lactose, maltose), poly (starch, cellulose, glycogen). Reducing sugars (glucose, fructose, maltose, lactose) reduce Tollens/Fehling. Sucrose is non-reducing. Glucose = aldohexose, fructose = ketohexose. Cellulose is structural (β-linkage); starch is energy storage (α-linkage).
Proteins: High molecular mass polymers of amino acids joined by peptide bonds.
Peptide bond: -CO-NH- (amide bond) — formed between the -COOH of one amino acid and the -NH₂ of another, with loss of water.
α-Amino acids: Building blocks of proteins. Have both -NH₂ and -COOH on the SAME carbon (α-carbon).
General formula: H₂N-CH(R)-COOH, where R = side chain (20 different R groups = 20 standard amino acids).
Zwitterion: In aqueous solution, amino acids exist as ⁺H₃N-CH(R)-COO⁻ (both + and − charges — internal salt). This gives them high melting points and water solubility.
By nutrition:
By R group (side chain) polarity:
Peptide: Short chain of amino acids (2-50).
Protein structure (4 levels):
Denaturation: Loss of secondary, tertiary, or quaternary structure of a protein (but not primary). Caused by:
Result: Loss of biological function (e.g., enzyme stops working). Often irreversible.
By shape:
By composition:
1. Biuret test: Protein + NaOH + CuSO₄ → violet color (peptide bonds complex with Cu²⁺). General test for proteins/peptides.
2. Xanthoproteic test: Protein + conc. HNO₃ → yellow color (nitration of aromatic rings in Tyr, Trp, Phe). Positive for proteins containing aromatic amino acids.
3. Ninhydrin test: Amino acid/protein + ninhydrin → purple color (from α-amino acids). Very sensitive test for amino acids.
4. Millon's test: Phenol group in tyrosine → red color with Hg in HNO₂.
5. Sakaguchi test: Arginine (guanidino group) → red color with α-naphthol + NaOCl.
Proteins = polymers of α-amino acids joined by peptide bonds. 20 standard amino acids (10 essential for humans). Zwitterionic. 4 levels of structure: primary (sequence), secondary (α-helix/β-sheet), tertiary (3D fold), quaternary (subunits). Denaturation = loss of 3D structure by heat/pH/urea. Fibrous (structural) vs globular (functional). Biuret test = violet with Cu²⁺ + NaOH.
Enzymes: Proteins that act as highly specific biological catalysts. They speed up biochemical reactions by lowering activation energy (like inorganic catalysts) but are much more efficient and specific.
Key features:
By reaction type (IUB system, 6 classes):
| Enzyme | Substrate | Product | Where |
|---|---|---|---|
| Amylase | Starch | Maltose/glucose | Saliva, pancreas |
| Pepsin | Proteins | Peptides | Stomach |
| Trypsin | Proteins | Peptides | Small intestine |
| Lipase | Fats | Fatty acids + glycerol | Pancreas |
| Lactase | Lactose | Glucose + galactose | Small intestine |
| Urease | Urea | NH₃ + CO₂ | Liver (some bacteria) |
| Zymase | Glucose | Ethanol + CO₂ | Yeast |
| Invertase | Sucrose | Glucose + fructose | Intestine |
| Catalase | H₂O₂ | H₂O + O₂ | Liver, RBCs |
| Carbonic anhydrase | CO₂ + H₂O | H₂CO₃ | RBCs |
1. Lock and Key model: Enzyme has a specific active site that fits only one substrate (like a key fits a lock). Explains enzyme specificity.
2. Induced Fit model: The active site is flexible — it adjusts its shape to fit the substrate as it binds (more accurate).
Steps:
Reaction: E + S ⇌ ES complex → E + P
Michaelis-Menten kinetics: At low [S], rate ∝ [S]. At high [S], rate saturates (Vmax) because all enzyme active sites are occupied.
Competitive inhibition: Inhibitor resembles substrate and binds to active site (e.g., malonate inhibits succinate dehydrogenase). Can be overcome by increasing substrate concentration.
Non-competitive inhibition: Inhibitor binds to a site other than active site (allosteric) — changes enzyme shape. Cannot be overcome by substrate.
Examples:
Enzymes are biological catalysts — proteins (usually). Highly specific (lock-and-key / induced fit). Active site binds substrate, reaction occurs, products released, enzyme regenerated. Classified into 6 types (IUB). Affected by T, pH, [S]. Inhibitors block active site (competitive) or alter enzyme (non-competitive). Denature at high T or extreme pH.
Vitamins: Organic compounds required in small amounts for normal growth and health. Not synthesized by the body (mostly) — must be obtained from diet.
Classification by solubility:
| Vitamin | Chemical Name | Deficiency Disease | Sources |
|---|---|---|---|
| A | Retinol | Night blindness, xerophthalmia | Carrots, liver, green vegetables |
| D | Calciferol | Rickets (children), osteomalacia | Sunlight, fish oil, egg yolk |
| E | Tocopherol | Rare — hemolysis, sterility | Vegetable oils, nuts, wheat germ |
| K | Phylloquinone (K₁), menaquinone (K₂) | Prolonged bleeding (no clotting) | Green vegetables, gut bacteria |
| Vitamin | Chemical Name | Deficiency | Function |
|---|---|---|---|
| B₁ | Thiamine | Beri-beri | Coenzyme in carbohydrate metabolism |
| B₂ | Riboflavin | Cheilosis, glossitis | Coenzyme (FAD, FMN) in redox |
| B₃ | Niacin (nicotinic acid) | Pellagra | Coenzyme (NAD⁺, NADP⁺) |
| B₅ | Pantothenic acid | Rare — fatigue | Coenzyme A |
| B₆ | Pyridoxine | Anemia, dermatitis | Amino acid metabolism |
| B₇ | Biotin | Rare — dermatitis | Carboxylation reactions |
| B₉ | Folic acid | Anemia, neural tube defects | DNA synthesis |
| B₁₂ | Cobalamin | Pernicious anemia | Contains Co, RBC formation |
| C | Ascorbic acid | Scurvy | Antioxidant, collagen synthesis |
Vitamin A (Retinol): Fat-soluble. Prevents night blindness. Retinal is part of rhodopsin (visual pigment). Deficiency → xerophthalmia (dry eyes), night blindness.
Vitamin D (Calciferol): Fat-soluble. Synthesized in skin from sunlight (UV) on 7-dehydrocholesterol. Promotes Ca²⁺ absorption. Deficiency → rickets (bowed legs) in children, osteomalacia (soft bones) in adults.
Vitamin K (Phylloquinone): Fat-soluble. Coenzyme in blood clotting (prothrombin synthesis). Deficiency → prolonged bleeding.
Vitamin C (Ascorbic acid): Water-soluble. Antioxidant, cofactor for collagen synthesis. Deficiency → scurvy (bleeding gums, poor wound healing).
Vitamin B₁ (Thiamine): Coenzyme in carbohydrate metabolism. Deficiency → beri-beri (nerve damage).
Vitamin B₁₂ (Cobalamin): Contains cobalt. Necessary for RBC formation. Deficiency → pernicious anemia. Only from animal sources (or supplements).
Vitamins = organic micronutrients. Fat-soluble: A, D, E, K — stored in body, toxic in excess. Water-soluble: B-complex (8 vitamins) + C — not stored, need daily intake. Key deficiencies: A = night blindness, D = rickets, K = bleeding, C = scurvy, B₁ = beri-beri, B₁₂ = pernicious anemia.
Nucleic acids: Polymers of nucleotides. Store and transmit genetic information.
Two types:
Nucleotide = base + sugar + phosphate
1. Nitrogenous bases (purines and pyrimidines):
2. Sugar (pentose):
3. Phosphate: Phosphoric acid linking sugars (forming the sugar-phosphate backbone)
Nucleoside = base + sugar (no phosphate)
Nucleotide = nucleoside + phosphate (base + sugar + phosphate)
Naming: nucleosides — adenosine, guanosine, cytidine, uridine, thymidine.
Nucleotides — adenylic acid (AMP), guanylic acid (GMP), etc.
Watson-Crick model (1953):
Chargaff's rules:
| Feature | DNA | RNA |
|---|---|---|
| Full name | Deoxyribonucleic acid | Ribonucleic acid |
| Sugar | Deoxyribose (no 2'-OH) | Ribose (2'-OH) |
| Bases | A, G, C, T | A, G, C, U |
| Strands | Double-stranded (helix) | Usually single-stranded |
| Function | Stores genetic info | Transmits & translates |
| Stability | More stable (no 2'-OH) | Less stable (2'-OH) |
| Location | Nucleus, mitochondria | Nucleus + cytoplasm |
DNA → RNA → Protein
Nucleic acids = polymers of nucleotides (base + sugar + phosphate). DNA (deoxyribose, A/T/G/C, double helix) stores information. RNA (ribose, A/U/G/C, single-stranded) transmits and translates. Watson-Crick model: antiparallel double helix, complementary base pairing (A=T, G≡C). Chargaff's rules. Central dogma: DNA → RNA → Protein. Types of RNA: mRNA, tRNA, rRNA.
Hormones: Chemical messengers secreted by endocrine glands, transported through blood to target organs, where they act to regulate metabolism, growth, reproduction, etc.
Key features:
1. Peptide/protein hormones: Chains of amino acids. Most hormones.
2. Steroid hormones: Derived from cholesterol (4-ring structure).
3. Amino acid derivatives:
| Hormone | Gland | Function |
|---|---|---|
| Insulin | Pancreas (β cells) | Lowers blood glucose |
| Glucagon | Pancreas (α cells) | Raises blood glucose |
| Thyroxine | Thyroid | Regulates metabolism |
| Adrenaline | Adrenal medulla | Fight or flight |
| Cortisol | Adrenal cortex | Stress response, anti-inflammatory |
| Testosterone | Testes | Male secondary sex characteristics |
| Estrogen | Ovaries | Female secondary sex characteristics |
| Progesterone | Ovaries | Prepares uterus for pregnancy |
| ADH (vasopressin) | Pituitary (posterior) | Water retention by kidneys |
| Oxytocin | Pituitary (posterior) | Uterine contraction, milk let-down |
| GH (growth hormone) | Pituitary (anterior) | Body growth |
Hormones = chemical messengers from endocrine glands, act on specific target cells. Types: peptide/protein (insulin, glucagon, ADH, GH), steroid (cortisol, testosterone, estrogen), amino acid derivatives (thyroxine, adrenaline, melatonin). Essential for regulating metabolism, growth, reproduction, stress response. Deficiency/excess causes specific diseases (diabetes, hypothyroidism, gigantism).
Choose a biomolecule to see its type, function, and key information.
"A, D, E, K" — "All Dogs Eat Kitty"
Stored in fat/liver. Toxic in excess. Deficiencies: A = night blindness, D = rickets, E = hemolysis, K = bleeding.
B-complex + C — not stored, need daily intake.
Excess excreted in urine. B₁ = beri-beri, B₃ = pellagra, B₁₂ = pernicious anemia, C = scurvy.
"PVT TIM HALL"
Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine.
10 essential amino acids — cannot be made by body.
"PURines = PURe As Gold (A, G)" — Pure = two rings (Adenine, Guanine)
"PYRimidines = CUT" — Cytosine, Uracil, Thymine (single ring)
"DNA has T, RNA has U"
DNA: A, T, G, C + deoxyribose (no 2'-OH)
RNA: A, U, G, C + ribose (with 2'-OH)
"Apples in the Tree, Cars in the Garage"
A pairs with T (2 H-bonds)
G pairs with C (3 H-bonds)
G≡C has 3 bonds → more stable → GC-rich DNA has higher melting point.
"Primary, Secondary, Tertiary, Quaternary" = "PS, TQ"
P = sequence (primary), S = α-helix/β-sheet (secondary), T = 3D fold (tertiary), Q = multi-subunit (quaternary).
"OTHLIL"
Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases.
"BXNMS"
Biuret (violet), Xanthoproteic (yellow, aromatic AA), Ninhydrin (purple, amino acids), Millon's (red, tyrosine), Sakaguchi (red, arginine).
"Insulin Lowers Sugar, Glucagon Glows it Up"
Insulin: ↓ blood glucose (promotes uptake, storage)
Glucagon: ↑ blood glucose (promotes breakdown of glycogen)
Use "All Dogs Eat Kitty" for fat-soluble vitamins, "PVT TIM HALL" for essential amino acids, "PURe As Gold" for purines, "Apples in Tree, Cars in Garage" for A-T, G-C.
Q1. Why is glucose called a reducing sugar?
Glucose has a free aldehyde group (CHO) in its open-chain form. Even though the cyclic form has a hemiacetal, it can open to the aldehyde in solution (mutarotation). The aldehyde can be oxidized to carboxylate.
Glucose + Tollens reagent → silver mirror (Ag↓)
Glucose + Fehling's solution → red Cu₂O precipitate
Q2. Why is sucrose called a non-reducing sugar while maltose is reducing?
Sucrose = glucose + fructose linked via α-1,2-glycosidic bond. Both anomeric carbons (C1 of glucose, C2 of fructose) are involved in the glycosidic linkage. No free aldehyde/ketone → cannot open to reducing form → non-reducing.
Maltose = glucose + glucose linked via α-1,4-glycosidic bond. Only one anomeric carbon involved; the other (C1 of second glucose) has a free hemiacetal group → can open to aldehyde → reducing sugar.
Q3. What are the four levels of protein structure? Explain each briefly.
Sequence of amino acids in the polypeptide chain (encoded by DNA). Determines all higher levels.
Local folding via H-bonds between backbone N-H and C=O groups. Two main types: α-helix (coiled, keratin) and β-pleated sheet (extended, silk fibroin).
Overall 3D folding of the entire chain. Driven by disulfide bonds (S-S), hydrophobic interactions, salt bridges, H-bonds. Determines function.
Arrangement of multiple polypeptide subunits. Example: hemoglobin = 2α + 2β subunits, each with a heme group.
Q4. Write the differences between DNA and RNA.
DNA: deoxyribose (2'-H), bases A, T, G, C. RNA: ribose (2'-OH), bases A, U, G, C.
DNA: double-stranded helix (Watson-Crick). RNA: usually single-stranded (can form hairpins).
DNA: stores genetic information (stable). RNA: transmits and translates information (less stable due to 2'-OH → more reactive). Types: mRNA, tRNA, rRNA.
Q5. Discuss the classification of carbohydrates with examples.
Carbohydrates = polyhydroxy aldehydes or polyhydroxy ketones, or compounds that produce them on hydrolysis. General formula C_x(H₂O)_y.
Simplest, cannot be hydrolyzed. Examples:
All are reducing sugars.
2-10 monosaccharide units joined by glycosidic bonds.
Reducing: maltose, lactose. Non-reducing: sucrose.
Many monosaccharide units (thousands). Examples:
Reducing sugars: free aldehyde/ketone group. Reduce Tollens/Fehling. Examples: glucose, fructose, maltose, lactose.
Non-reducing: no free anomeric carbon. Example: sucrose (both anomeric carbons linked).
Q6. Describe the structure of DNA. Why is DNA more stable than RNA?
Nucleotide = nitrogenous base + deoxyribose sugar + phosphate group.
Bases: purines (A, G) and pyrimidines (C, T).
• Double-stranded helix, right-handed.
• Two antiparallel strands (5'→3' and 3'→5').
• Sugar-phosphate backbone outside, bases inside.
• Complementary base pairing: A=T (2 H-bonds), G≡C (3 H-bonds).
• Diameter 2 nm, pitch 3.4 nm, ~10 bp per turn.
In natural DNA: %A = %T, %G = %C. Purines = pyrimidines.
(1) Deoxyribose has no 2'-OH → less prone to hydrolysis and nucleophilic attack. Ribose's 2'-OH makes RNA more reactive and easier to degrade.
(2) Double-stranded structure → bases protected inside helix, H-bonding between strands → more stable.
(3) DNA has thymine (methylated) instead of uracil — T's methyl group gives extra protection against nucleases.
(4) DNA is repaired by cellular machinery (proofreading) → high fidelity.
These features make DNA the ideal long-term storage molecule, while RNA is short-lived and used for transient messages.
Q7. What are enzymes? Discuss their mechanism of action and inhibition.
Enzymes = biological catalysts, usually proteins, that speed up biochemical reactions by lowering activation energy. Highly specific — each enzyme catalyzes a particular reaction.
Enzyme has a specific active site that only the substrate fits. Like a key fits a lock. Explains specificity.
More accurate: the active site is flexible and changes shape to fit the substrate as it binds. Better fits the actual observations.
E + S ⇌ ES complex → E + P
Rate ∝ [S] at low substrate; saturates at high [S] (all active sites occupied).
Competitive: Inhibitor resembles substrate, binds to active site. Can be overcome by excess substrate. E.g., malonate inhibits succinate dehydrogenase.
Non-competitive: Inhibitor binds elsewhere (allosteric site), changes enzyme shape. Cannot be overcome by substrate. E.g., cyanide on cytochrome oxidase.
Applications: Penicillin (inhibits cell wall synthesis), aspirin (COX inhibitor), statins (HMG-CoA reductase).
Selected exercise questions from NCERT Class 12 Chemistry Chapter 10. Click each to reveal the solution.
Monosaccharides: Simplest carbohydrates that cannot be hydrolyzed to smaller carbohydrate units. They are polyhydroxy aldehydes (aldoses) or polyhydroxy ketones (ketoses). General formula: (CH₂O)_n, usually n=3-7.
Examples:
• Glucose (C₆H₁₂O₆) — aldohexose, main blood sugar
• Fructose (C₆H₁₂O₆) — ketohexose, fruit sugar
• Ribose (C₅H₁₀O₅) — pentose in RNA
(a) Br₂ water: Mild oxidation of aldehyde to carboxylic acid → gluconic acid. (The cyclic structure is also opened; only aldehyde oxidized.)
(b) HNO₃: Strong oxidation — both aldehyde and primary OH (C6) oxidized → saccharic acid (glucaric acid).
(c) HCN: Nucleophilic addition to aldehyde → cyanohydrin (glucose cyanohydrin).
(d) Tollens reagent: Aldehyde oxidized to carboxylic acid, Ag⁺ reduced to Ag (silver mirror). Confirms glucose is a reducing sugar.
Answer: Glucose has the open-chain structure:
CHO-CHOH-CHOH-CHOH-CHOH-CH₂OH
All six carbons are in a straight chain (aldohexose). The following evidence supports this:
• Glucose reacts with HCN to form cyanohydrin (1 mol HCN per mol glucose) → one aldehyde group (C1)
• Glucose + Br₂ water → gluconic acid (only aldehyde oxidized) → one aldehyde
• Glucose + HNO₃ → saccharic acid (both CHO and CH₂OH oxidized) → one aldehyde + one primary alcohol (C6)
• Glucose + acetic anhydride → pentaacetate → 5 OH groups
• Glucose + NaBH₄ → sorbitol (also from fructose) → confirms straight chain
These reactions confirm all 6 carbons are in a single chain with one aldehyde (C1), four secondary OH (C2-C5), and one primary OH (C6).
Difference: They are anomers — differ only in the configuration at the anomeric carbon (C1):
• α-D-glucose: The -OH at C1 is on the same side as the ring oxygen (below the ring when drawn in the standard orientation). Specific rotation +112°.
• β-D-glucose: The -OH at C1 is on the opposite side from the ring oxygen. Specific rotation +19°.
Significance:
• In solution, α and β interconvert via the open-chain aldehyde form (mutarotation) until equilibrium is reached (~36% α, ~64% β, trace open chain).
• The two forms are important in disaccharide and polysaccharide structure: starch and glycogen use α-linkages (digestible by humans), while cellulose uses β-linkages (not digestible by humans, gives dietary fiber).
• The anomeric form determines the linkage type and biological function.
Peptide bond: The amide linkage -CO-NH- formed between the carboxyl group of one amino acid and the amino group of another, with elimination of water.
Reaction: H₂N-CH(R₁)-COOH + H₂N-CH(R₂)-COOH → H₂N-CH(R₁)-CO-NH-CH(R₂)-COOH + H₂O
The bond is a partial double bond (resonance between C=O and C-N) → planar, rigid. This restricts rotation around the peptide bond, contributing to protein secondary structure.
Peptide bonds are stable (like amides) — require strong acid/base or enzymes (proteases) to hydrolyze.
Enzymes: Biological catalysts (usually proteins) that increase the rate of biochemical reactions without being consumed.
(a) Oxidoreductase: Catalyzes oxidation-reduction reactions. Example: dehydrogenase (lactate dehydrogenase oxidizes lactate to pyruvate with NAD⁺ → NADH).
(b) Hydrolase: Catalyzes hydrolysis reactions. Example: amylase (hydrolyzes starch → maltose/glucose), lipase (fat → fatty acids + glycerol), protease (protein → peptides).
(c) Transferase: Transfers functional groups from one molecule to another. Example: kinase (transfers phosphate from ATP to substrate), transaminase (transfers amino group).
Vitamins: Organic compounds required in small quantities for normal growth and health. Most cannot be synthesized by the body → must come from diet.
Classification by solubility:
1. Fat-soluble vitamins: A, D, E, K.
• Stored in liver/fat, excess can be toxic
• Example: Vitamin A (retinol) — prevents night blindness; Vitamin D (calciferol) — prevents rickets
2. Water-soluble vitamins: B-complex + C.
• Not stored, excess excreted in urine
• Example: Vitamin C (ascorbic acid) — prevents scurvy; Vitamin B₁ (thiamine) — prevents beri-beri
DNA structure: Double helix (Watson-Crick, 1953)
• Two antiparallel polynucleotide strands
• Sugar-phosphate backbone on outside, bases inside
• Complementary base pairing: A=T (2 H-bonds), G≡C (3 H-bonds)
• Right-handed helix, diameter 2 nm, pitch 3.4 nm, 10 bp per turn
Differences:
1. Sugar: DNA = deoxyribose; RNA = ribose
2. Bases: DNA has A, T, G, C; RNA has A, U, G, C (U instead of T)
3. Strands: DNA = double-stranded; RNA = usually single-stranded
4. Function: DNA stores genetic information; RNA transmits and translates it
5. Stability: DNA is more stable (no 2'-OH, double-strand); RNA is less stable
6. Location: DNA mainly in nucleus; RNA in nucleus + cytoplasm
1. mRNA (messenger RNA): Carries the genetic message from DNA in the nucleus to ribosomes in the cytoplasm. Transcribed from DNA. Each 3-base codon codes for one amino acid.
2. tRNA (transfer RNA): Carries specific amino acids to the ribosome and matches them to the mRNA codon via its anticodon. Small (~76 nucleotides), cloverleaf structure.
3. rRNA (ribosomal RNA): Structural and catalytic component of ribosomes. Most abundant RNA (~80% of total RNA). Helps align mRNA and tRNA for protein synthesis.
Others: snRNA (splicing), miRNA (gene regulation), siRNA (RNA interference).
1. Peptide/protein hormones: Chains of amino acids. Examples:
• Insulin (51 aa, 2 chains A & B) — lowers blood glucose
• Glucagon (29 aa) — raises blood glucose
• ADH (9 aa) — water retention
• Growth hormone (191 aa) — promotes growth
2. Steroid hormones: Derived from cholesterol (4-ring structure). Examples:
• Cortisol — stress response, anti-inflammatory
• Testosterone — male sex characteristics
• Estrogen, progesterone — female sex hormones
• Aldosterone — Na⁺ retention
3. Amino acid derivatives: Modified single amino acids. Examples:
• Thyroxine (T₄) — from tyrosine + iodine, regulates metabolism
• Adrenaline (epinephrine) — from tyrosine, "fight or flight"
• Melatonin — from tryptophan, sleep-wake cycle
Structural features of DNA:
1. Nucleotides: Basic units — each has a nitrogenous base, deoxyribose sugar, and phosphate.
2. Bases: Purines (A, G) and pyrimidines (T, C). A pairs with T (2 H-bonds); G pairs with C (3 H-bonds).
3. Strands: Two antiparallel strands. One runs 5'→3', the other 3'→5'.
4. Double helix: Right-handed helix, diameter 2 nm, pitch 3.4 nm, ~10 bp per turn.
5. Backbone: Sugar-phosphate backbone on the outside; bases point inward.
6. Chargaff's rules: %A=%T, %G=%C; purines = pyrimidines.
7. Major/minor grooves: Present on the surface of the helix.
8. Complementary pairing: Enables faithful replication (each strand serves as a template).
(A) Both true, R explains A. (B) Both true, R does NOT explain A. (C) A true, R false. (D) A false, R true.
All key facts at one place — perfect for quick revision before exams.