Biomolecules — NEET notes

Carbohydrates

Carbohydrates are polyhydroxy aldehydes/ketones or compounds that produce them on hydrolysis. Classified as monosaccharides (can't be hydrolyzed further), disaccharides (give 2 monosaccharides), and polysaccharides (give many monosaccharides). The reducing/non-reducing distinction depends on whether a free anomeric –OH is available to open into the aldehyde/ketone form.

C_x(H_2O)_y (general formula — hence 'hydrates of carbon')

Classification of Carbohydrates

Carbohydrates are grouped by how many sugar units they contain. This classification is the backbone of every NEET question on carbohydrates.

Carbohydrates are classified based on how many monosaccharide units they yield on hydrolysis. A monosaccharide cannot be broken down further. A disaccharide gives exactly 2 monosaccharides. A polysaccharide gives many (hundreds to thousands).

• Monosaccharides: Cannot be hydrolyzed further. Examples: glucose ( C_6H_ 12 O_6 ), fructose, galactose, ribose ( C_5H_ 10 O_5 )
• Disaccharides: Hydrolysis gives 2 monosaccharides. Examples: sucrose (glucose + fructose), maltose (glucose + glucose), lactose (glucose + galactose)
• Oligosaccharides: 2–10 monosaccharide units (disaccharides are a subclass). Raffinose (trisaccharide) is an example
• Polysaccharides: Hundreds to thousands of monosaccharide units. Examples: starch, cellulose, glycogen

Aldoses vs Ketoses

Monosaccharides are further classified based on whether they contain an aldehyde group (aldoses) or a ketone group (ketoses), and by the number of carbon atoms.

Every monosaccharide has either an aldehyde group (making it an aldose) or a ketone group (making it a ketose). The number of carbon atoms adds a prefix: triose (3C), tetrose (4C), pentose (5C), hexose (6C).

• Glucose = an aldo-hexose (aldehyde + 6 carbons)
• Fructose = a keto-hexose (ketone + 6 carbons)
• Ribose = an aldo-pentose (aldehyde + 5 carbons)
• Ribulose = a keto-pentose (ketone + 5 carbons)
• Glyceraldehyde = the simplest aldose (aldo-triose, 3C)
• Dihydroxyacetone = the simplest ketose (keto-triose, 3C)

Aldose: R–CHO | Ketose: R–CO–R'

Reducing vs Non-Reducing Sugars

A reducing sugar has a free anomeric hydroxyl that can open to the aldehyde/ketone form, allowing it to reduce Tollens' and Fehling's reagents. Non-reducing sugars have their anomeric carbons locked in a glycosidic bond.

A reducing sugar can reduce oxidizing agents like Tollens' reagent ( Ag^+ → Ag mirror) and Fehling's solution ( Cu^ 2+ → red Cu_2O ). This happens because the sugar has a free anomeric –OH that can open to the free aldehyde (or ketone → aldehyde via tautomerism).

• All monosaccharides are reducing sugars (glucose, fructose, galactose, ribose — ALL)
• Maltose and lactose are reducing disaccharides (one anomeric carbon is free)
• Sucrose is a NON-reducing sugar — both anomeric carbons (C-1 of glucose and C-2 of fructose) are involved in the glycosidic bond
• Polysaccharides (starch, cellulose) are essentially non-reducing (only one free end among thousands of units)

Starch, Cellulose & Glycogen

All three are polymers of glucose but differ in the type of glycosidic linkage, branching, and biological function. Starch stores energy in plants, glycogen stores energy in animals, and cellulose provides structural support.

Starch has two components: amylose (linear, -1,4 linkages, 15–20%) and amylopectin (branched, -1,4 + -1,6 linkages, 80–85%). Starch gives a blue color with iodine (iodine fits into the amylose helix).

Cellulose is a linear polymer of -D-glucose units joined by -1,4 glycosidic bonds. Humans lack the enzyme cellulase → we cannot digest cellulose (but cows can, thanks to rumen bacteria). Cellulose does NOT give a color with iodine.

Glycogen is the animal equivalent of starch — stored mainly in liver and muscles. It's even more branched than amylopectin ( -1,4 and -1,6 linkages with branching every 8–12 glucose units vs every 24–30 in amylopectin).

• Amylose: water-soluble, forms helical structure, gives blue with I_2
• Amylopectin: water-insoluble, branched, gives violet with I_2
• Glycogen = animal starch, more compact and more branched than amylopectin
• Cellulose is the most abundant organic substance on Earth
• Cotton is almost pure cellulose (~90%)

Glucose Structure

Glucose (C₆H₁₂O₆) is the most important monosaccharide. Its open-chain (Fischer projection) has an aldehyde at C-1 and hydroxyl groups on C-2 to C-6. In solution, it predominantly exists as a six-membered cyclic hemiacetal (pyranose form) in two anomers: α-D-glucose (–OH at C-1 axial/down) and β-D-glucose (–OH at C-1 equatorial/up).

C_6H_ 12 O_6 (Molecular formula of glucose)

Open-Chain Structure

The open-chain (acyclic) structure of glucose was established through a series of reactions. The Fischer projection shows a straight chain of 6 carbons with specific –OH orientations.

Glucose has the molecular formula C_6H_ 12 O_6 . The open-chain structure was deduced from chemical evidence:

• Reaction with HI : Gives n-hexane → all 6 carbons are in a straight chain (no branching)
• Reaction with NH_2OH : Forms an oxime → a >C=O (carbonyl) group is present
• Reaction with Br_2 water (mild oxidation): Gives gluconic acid (a 6C acid) → the carbonyl is an aldehyde ( –CHO ), not a ketone
• Acetylation with acetic anhydride: Forms a pentaacetate → 5 –OH groups are present
• Reaction with HCN : Forms cyanohydrin → confirms the C=O group

CH_2OH–(CHOH)_4–CHO

In D-glucose, the –OH on the penultimate carbon (C-5) is on the right in the Fischer projection. This makes it a D-sugar. L-glucose has it on the left. Naturally occurring glucose is D-glucose.

Cyclic Structure

In aqueous solution, glucose exists predominantly in cyclic form. The C-1 aldehyde reacts with the C-5 hydroxyl to form a six-membered hemiacetal ring called a pyranose.

Two observations prove glucose is NOT a simple open-chain aldehyde in solution: (1) it does not give the 2,4-DNP test or Schiff's test (reactions typical of free aldehydes), and (2) it exists as two crystalline forms ( and ) with different melting points and specific rotations.

The explanation: the –CHO at C-1 reacts intramolecularly with the –OH at C-5 to form a six-membered hemiacetal ring. This ring is called a pyranose (named after the six-membered heterocycle pyran).

C-1 (–CHO) + C-5 (–OH) arrow six-membered hemiacetal (pyranose ring)

• Ring formation creates a new chiral center at C-1 (the anomeric carbon)
• Two configurations at C-1 → two anomers: and
• -D-glucose: –OH at C-1 is below the ring plane (axial in Haworth projection)
• -D-glucose: –OH at C-1 is above the ring plane (equatorial in Haworth projection)
• The Haworth projection draws the ring as a flat hexagon with substituents above/below

Mutarotation

When either pure α or β form of glucose is dissolved in water, its specific rotation gradually changes until it reaches an equilibrium value of +52.7°. This is mutarotation — the anomers interconvert via the open-chain form.

Freshly dissolved -D-glucose has a specific rotation of +112° . Freshly dissolved -D-glucose has +18.7° . Over time, both solutions reach the same equilibrium value of +52.7° . This phenomenon is called mutarotation.

-D-glucose (+112°) leftharpoons open-chain leftharpoons -D-glucose (+18.7°)

• Equilibrium rotation = +52.7° (a weighted average: ~36% + ~64% )
• Mechanism: ring opens → free aldehyde → ring closes to either or
• Catalyzed by both acid and base
• Only occurs in sugars that can form a hemiacetal/hemiketal (i.e., with a free anomeric –OH)

Glucose–Fructose Relationship

Glucose and fructose are functional group isomers (aldose vs ketose), not epimers. They are interconverted by base-catalyzed tautomerism and are linked together in sucrose through a glycosidic bond.

Glucose (an aldohexose) and fructose (a ketohexose) share the molecular formula C_6H_ 12 O_6 but differ in their functional group — they are functional group isomers (not epimers). In alkaline solution, glucose and fructose interconvert through an enediol intermediate.

• Glucose + Fructose: functional group isomers (aldo vs keto), same molecular formula
• Glucose + Galactose: epimers — differ only at C-4 configuration
• Glucose + Mannose: epimers — differ only at C-2 configuration
• -D-glucose + -D-glucose: anomers — differ only at C-1 (anomeric carbon)
• In sucrose, C-1 of -D-glucose is linked to C-2 of -D-fructose

Inversion of sucrose: Sucrose ( [ ] = +66.5° ) on hydrolysis gives an equimolar mixture of glucose ( +52.7° ) and fructose ( -92° ). The net rotation becomes negative ( -39.3° ). This sign change is why the mixture is called invert sugar and the process is called inversion.

+66.5° Sucrose H^+/invertase +52.7° Glucose + -92° Fructose

Amino Acids & Proteins

Amino acids are the building blocks of proteins. Each has an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a side chain (R group) attached to the α-carbon. Twenty standard amino acids combine via peptide bonds to form proteins whose shape (1° → 2° → 3° → 4° structure) dictates their function.

H_2N–CHR–COOH (general structure of an -amino acid)

Amino Acid Structure

Every α-amino acid has the same core: a central carbon bonded to –NH₂, –COOH, –H, and a variable R group. The R group determines the amino acid's identity, properties, and behavior.

In an -amino acid, both the amino ( –NH_2 ) and carboxyl ( –COOH ) groups are attached to the same carbon — the -carbon. The 20 standard amino acids differ only in their R group (side chain).

• All amino acids except glycine ( R = H ) have a chiral -carbon → optically active
• Naturally occurring amino acids are L-amino acids (similar to L-glyceraldehyde configuration)
• At physiological pH (~7.4), amino acids exist as zwitterions: ^+H_3N–CHR–COO^-
• The isoelectric point (pI) is the pH at which the amino acid has zero net charge
• Amino acids are amphoteric — they can act as both acids and bases

Amino acids are classified based on R group properties: non-polar (Ala, Val, Leu, Ile, Pro), polar uncharged (Ser, Thr, Asn, Gln), positively charged (Lys, Arg, His), and negatively charged (Asp, Glu).

Peptide Bond

A peptide bond forms when the –COOH of one amino acid reacts with the –NH₂ of another, releasing water. This is a condensation reaction that creates the C–N amide bond — the backbone of every protein.

When two amino acids combine, the –COOH of one reacts with the –NH_2 of the other, releasing H_2O and forming a peptide bond ( –CO–NH– ). The product is a dipeptide. Repeating this gives polypeptides and eventually proteins.

H_2N–CHR–COOH + H_2N–CHR'–COOH -H_2O H_2N–CHR–CO–NH–CHR'–COOH

• The peptide bond has partial double bond character (due to resonance: C=O rightarrow C–O^-, C=N^+ )
• This makes the –CO–NH– unit planar and rigid — no free rotation around the C–N bond
• A dipeptide has 1 peptide bond, a tripeptide has 2, a polypeptide with n residues has (n-1) peptide bonds
• Proteins are polypeptides with 50 amino acid residues (rough cutoff)

Protein Structure Levels

Proteins have four hierarchical levels of structure. Primary = amino acid sequence. Secondary = local folding (α-helix, β-sheet). Tertiary = overall 3D shape. Quaternary = multi-subunit assembly.

The primary structure is the linear sequence of amino acids in a polypeptide chain, held by peptide bonds. This sequence is genetically determined and unique for each protein. Changing even one amino acid can alter function (e.g., sickle cell anemia: Glu → Val at position 6 of -hemoglobin).

The secondary structure describes local folding patterns stabilized by hydrogen bonds between backbone C=O and N–H groups. Two main types: the -helix (a right-handed coil) and the -pleated sheet (parallel or antiparallel strands connected by H-bonds).

The tertiary structure is the overall 3D shape of a single polypeptide chain, stabilized by hydrophobic interactions, disulfide bonds ( –S–S– ), ionic bonds, and hydrogen bonds between R groups. This level determines the protein's biological activity.

The quaternary structure exists only in proteins with multiple polypeptide subunits. Example: hemoglobin has 4 subunits ( 2 + 2 ). The subunits are held together by the same non-covalent forces as in tertiary structure.

• 1° structure: amino acid sequence → peptide bonds
• 2° structure: -helix, -sheet → backbone H-bonds
• 3° structure: overall 3D folding → hydrophobic, disulfide, ionic, H-bonds between R groups
• 4° structure: multi-subunit assembly → same forces as 3°
• Fibrous proteins (keratin, collagen) — mostly 2° structure, insoluble, structural role
• Globular proteins (enzymes, hemoglobin) — complex 3° structure, soluble, functional role

Denaturation

Denaturation is the disruption of secondary, tertiary, and quaternary structures of a protein without breaking peptide bonds. The primary structure remains intact, but the biological activity is lost.

When a protein is heated, treated with acid/base, urea, or organic solvents, its higher-order structure (2°, 3°, 4°) unfolds. The protein loses its biological activity but the primary structure (peptide bonds) remains intact. This is denaturation.

• Agents of denaturation: heat, pH change, urea, detergents, heavy metal salts, organic solvents
• What breaks: H-bonds, hydrophobic interactions, ionic bonds, disulfide bonds
• What survives: peptide bonds (covalent, strong) → primary structure stays
• Denatured protein = unfolded, biologically inactive, often insoluble (coagulation)
• Renaturation is sometimes possible if conditions are gently restored (Anfinsen's experiment with ribonuclease)

Enzymes

Enzymes are biological catalysts — mostly proteins — that speed up biochemical reactions by lowering activation energy. They are highly specific (lock-and-key model), classified into 6 classes by IUBMB, and their activity depends on temperature, pH, substrate concentration, and the presence of inhibitors or activators.

E + S leftharpoons ES arrow E + P

Enzyme Basics

Enzymes are protein catalysts that accelerate reactions by factors of 10⁶ to 10¹² without being consumed. Each enzyme typically catalyzes only one type of reaction on one type of substrate.

Enzymes are globular proteins (with a few exceptions — some RNA molecules called ribozymes also catalyze reactions). They lower the activation energy ( E_a ) of a reaction, making it proceed much faster at body temperature (37°C). The enzyme is not consumed and can be reused.

• Enzymes typically accelerate reactions by a factor of 10^6 to 10^ 12
• The region where substrate binds is called the active site — a small 3D pocket
• Lock-and-key model (Fischer, 1894): substrate fits the active site exactly, like a key in a lock
• Induced-fit model (Koshland, 1958): active site slightly changes shape to accommodate the substrate — more accurate
• Enzymes are highly specific: urease acts only on urea, maltase only on maltose

Some enzymes require non-protein components for activity. A cofactor is an inorganic ion ( Zn^ 2+ , Mg^ 2+ , Fe^ 2+ ). A coenzyme is an organic molecule (often derived from vitamins — NAD⁺ from niacin, FAD from riboflavin, coenzyme A from pantothenic acid). The protein part alone is called the apoenzyme; protein + cofactor/coenzyme = holoenzyme.

Apoenzyme (protein) + Cofactor/Coenzyme = Holoenzyme (active)

Enzyme Classification

The International Union of Biochemistry classifies enzymes into 6 major classes based on the type of reaction they catalyze. Each class has a specific name and reaction type.

Enzymes are classified into 6 classes by IUBMB (International Union of Biochemistry and Molecular Biology) based on the type of chemical reaction they catalyze:

• 1. Oxidoreductases: catalyze oxidation-reduction reactions. Example: alcohol dehydrogenase, lactate dehydrogenase
• 2. Transferases: transfer a functional group from one molecule to another. Example: hexokinase (transfers phosphate), transaminases
• 3. Hydrolases: catalyze hydrolysis (bond cleavage using water). Example: lipase, amylase, pepsin, esterases
• 4. Lyases: remove groups by non-hydrolytic mechanisms (or add groups to double bonds). Example: aldolase, decarboxylases
• 5. Isomerases: catalyze isomerization (rearrangement within one molecule). Example: phosphoglucose isomerase, racemases
• 6. Ligases (Synthetases): join two molecules using ATP energy. Example: DNA ligase, glutamine synthetase

Factors Affecting Enzyme Activity

Enzyme activity depends on temperature, pH, substrate concentration, and the presence of inhibitors. Each enzyme has an optimum temperature and pH where activity is maximum.

Enzyme activity is measured as the rate of product formation (or substrate disappearance). Several factors control this rate:

• Temperature: Activity increases with temperature up to the optimum (usually 25–40°C for human enzymes). Above this, the enzyme denatures and activity drops sharply
• pH: Each enzyme has an optimum pH. Pepsin: pH 2 (stomach). Trypsin: pH 7.8 (intestine). Extreme pH causes denaturation
• Substrate concentration [ S ]: At low [ S ], rate increases linearly. At high [ S ], rate plateaus → all active sites are saturated ( V_ )
• Enzyme concentration: If substrate is in excess, rate is directly proportional to enzyme concentration

Enzyme inhibition can be reversible or irreversible. Reversible inhibition has two main types: competitive (inhibitor competes with substrate for the active site — increases apparent K_m , V_ unchanged) and non-competitive (inhibitor binds elsewhere, changes enzyme shape — K_m unchanged, V_ decreases).

Vitamins

Vitamins are organic compounds required in small amounts for normal physiological function. They cannot be synthesized in adequate quantities by the body and must come from the diet. Classified as fat-soluble (A, D, E, K — stored in body fat) and water-soluble (B-complex, C — excreted in urine, not stored).

Fat-soluble: A, D, E, K | Water-soluble: B-complex, C

Fat-Soluble Vitamins (A, D, E, K)

These vitamins dissolve in fats and oils, are absorbed with dietary fat, and can accumulate in the body. Excess intake can cause toxicity (hypervitaminosis). Deficiency develops slowly because the body has reserves.

Fat-soluble vitamins are absorbed along with dietary fats in the small intestine and stored in the liver and adipose tissue. Because they are stored, deficiency takes longer to develop — but excess intake can also cause hypervitaminosis (toxicity).

• Vitamin A (Retinol): Source — fish liver oil, carrots ( -carotene is a precursor), milk. Deficiency → xerophthalmia (dry eyes), night blindness. Chemical name: retinol
• Vitamin D (Calciferol): Source — sunlight (skin synthesis), fish liver oil, egg yolk. Deficiency → rickets (children: soft bones), osteomalacia (adults). Chemical name: calciferol (D₂ = ergocalciferol, D₃ = cholecalciferol)
• Vitamin E (Tocopherol): Source — vegetable oils, wheat germ, nuts. Deficiency → increased oxidative damage, sterility in rats. Acts as an antioxidant — protects cell membranes from free radical damage
• Vitamin K (Phylloquinone): Source — green leafy vegetables, intestinal bacteria synthesis. Deficiency → delayed blood clotting, hemorrhage. Essential for synthesis of prothrombin (clotting factor)

Water-Soluble Vitamins (B-complex, C)

Water-soluble vitamins dissolve in water, are readily excreted in urine, and cannot accumulate in the body. This means they must be consumed regularly. B-complex vitamins serve mainly as coenzyme precursors.

Water-soluble vitamins cannot be stored in significant amounts — excess is excreted in urine. This makes daily dietary intake essential. Most B vitamins function as coenzymes (helpers for enzyme-catalyzed reactions).

• Vitamin B₁ (Thiamine): Deficiency → beriberi (nerve damage, muscle weakness). Coenzyme: TPP (thiamine pyrophosphate)
• Vitamin B₂ (Riboflavin): Deficiency → cheilosis (cracking of lips/corners of mouth), glossitis. Coenzyme: FAD, FMN
• Vitamin B₃ (Niacin/Nicotinic acid): Deficiency → pellagra (the 3 D's: dermatitis, diarrhea, dementia). Coenzyme: NAD⁺, NADP⁺
• Vitamin B₅ (Pantothenic acid): Component of Coenzyme A (central to metabolism). Deficiency is rare
• Vitamin B₆ (Pyridoxine): Deficiency → convulsions, anemia. Coenzyme: PLP (pyridoxal phosphate) — crucial for amino acid metabolism
• Vitamin B₇ (Biotin/Vitamin H): Deficiency → dermatitis, hair loss. Coenzyme for carboxylases
• Vitamin B₉ (Folic acid): Deficiency → megaloblastic anemia, neural tube defects in fetus. Essential for DNA synthesis
• Vitamin B₁₂ (Cyanocobalamin): Deficiency → pernicious anemia. The only vitamin containing a metal ion (cobalt). Requires intrinsic factor for absorption
• Vitamin C (Ascorbic acid): Deficiency → scurvy (bleeding gums, loose teeth, poor wound healing). Powerful antioxidant. Source: citrus fruits, amla

Nucleic Acids

Nucleic acids (DNA and RNA) are polymers of nucleotides that store and transmit genetic information. Each nucleotide has three components: a nitrogenous base, a pentose sugar, and a phosphate group. DNA uses deoxyribose and has the bases A, T, G, C in a double helix; RNA uses ribose and has A, U, G, C in a single strand.

Nucleotide = Base + Sugar + Phosphate

Nucleotide Structure

A nucleotide is made of three parts: a nitrogenous base (purine or pyrimidine), a pentose sugar (ribose or deoxyribose), and a phosphate group. Without the phosphate, it's called a nucleoside.

The monomer of nucleic acids is a nucleotide. It has three components joined together: (1) a nitrogenous base, (2) a pentose sugar, and (3) a phosphate group. Remove the phosphate → you get a nucleoside.

Base + Sugar = Nucleoside | Nucleoside + Phosphate = Nucleotide

• Purines (double-ring): Adenine (A) and Guanine (G) — present in both DNA and RNA
• Pyrimidines (single-ring): Cytosine (C) (in both), Thymine (T) (DNA only), Uracil (U) (RNA only)
• Pentose sugars: -D-2-deoxyribose (in DNA — missing –OH at C-2), -D-ribose (in RNA — –OH at both C-2 and C-3)
• Base attaches to sugar at C-1' (N-glycosidic bond). Phosphate attaches at C-5' (ester bond)
• Nucleotides link together via 3'→5' phosphodiester bonds to form the polynucleotide chain

DNA Double Helix

DNA is a double-stranded helix with two antiparallel polynucleotide chains held together by hydrogen bonds between complementary base pairs: A=T (2 H-bonds) and G≡C (3 H-bonds). This is the basis of genetic information storage and replication.

Watson and Crick (1953) proposed the double helix model of DNA, building on X-ray data from Rosalind Franklin and Maurice Wilkins. Two polynucleotide strands wind around each other in a right-handed helix, with the sugar-phosphate backbones on the outside and the bases pointing inward.

• Two strands are antiparallel: one runs 5'→3', the other 3'→5'
• Complementary base pairing: A pairs with T (2 H-bonds), G pairs with C (3 H-bonds)
• Chargaff's rule: In any DNA, [A] = [T] and [G] = [C] ; therefore [A] + [G] [T] + [C] = 1
• The helix has a pitch of 3.4 nm (one complete turn) with 10 base pairs per turn
• Distance between adjacent base pairs = 0.34 nm
• The double helix has a major groove and a minor groove (where proteins interact with DNA)

A = T (2 H-bonds) | G C (3 H-bonds)

DNA vs RNA

DNA and RNA differ in sugar (deoxyribose vs ribose), one base (thymine vs uracil), strandedness (double vs single), stability, and biological role (information storage vs protein synthesis).

There are three main types of RNA: (1) mRNA (messenger RNA) — carries the genetic code from DNA to ribosomes, (2) rRNA (ribosomal RNA) — structural and catalytic component of ribosomes (~80% of total RNA), (3) tRNA (transfer RNA) — brings amino acids to the ribosome during translation.

• mRNA: linear, carries codons, template for protein synthesis
• tRNA: cloverleaf shape, has anticodon loop and amino acid attachment site (3'-CCA end)
• rRNA: most abundant RNA, forms the structural core of ribosomes
• Central dogma: DNA transcription RNA translation Protein

Central Dogma

The central dogma of molecular biology describes the flow of genetic information: DNA is transcribed into RNA, which is translated into protein. This is the fundamental principle governing gene expression.

The central dogma (proposed by Francis Crick, 1958) states that genetic information flows in one direction: DNA → RNA → Protein. DNA replication copies the information. Transcription converts DNA to mRNA. Translation converts mRNA to protein.

DNA [replication] DNA ; DNA [transcription] RNA [translation] Protein

• Replication: DNA → DNA (occurs before cell division, semi-conservative)
• Transcription: DNA → mRNA (enzyme: RNA polymerase, occurs in nucleus)
• Translation: mRNA → Protein (occurs on ribosomes in cytoplasm)
• Reverse transcription: RNA → DNA (exception to central dogma, occurs in retroviruses like HIV, enzyme: reverse transcriptase)
• The genetic code is a triplet code: every 3 bases (codon) on mRNA code for one amino acid

Hormones

Hormones are chemical messengers produced by endocrine glands that travel through the blood to target organs, regulating physiological processes. They can be peptide/protein-based (insulin), steroid-based (testosterone, estrogen), or amino acid derivatives (adrenaline, thyroxine). Even tiny amounts produce significant biological effects.

Endocrine gland secretion Hormone blood Target organ arrow Response

Hormone Basics

Hormones are secreted directly into the blood by ductless (endocrine) glands. They act on specific target organs/cells that have the appropriate receptors, even at very low concentrations.

Hormones are biomolecules produced by endocrine (ductless) glands and secreted directly into the bloodstream. They travel to distant target organs and regulate processes like metabolism, growth, reproduction, and stress response. Even nanogram ( 10^ -9 g) quantities can trigger significant effects.

• Peptide/Protein hormones: insulin (pancreas), growth hormone (pituitary), oxytocin (pituitary). Water-soluble → bind receptors on cell surface
• Steroid hormones: testosterone, estrogen, progesterone, cortisol. Derived from cholesterol. Lipid-soluble → enter cells and bind intracellular receptors
• Amino acid derivatives: adrenaline/epinephrine (adrenal medulla), thyroxine (thyroid). Derived from tyrosine
• Hormones act in very low concentrations — they are effective at 10^ -8 to 10^ -12 M
• Excess or deficiency of any hormone causes disease (e.g., excess growth hormone → gigantism, deficiency → dwarfism)

Key Hormones

Three hormones NCERT specifically discusses in the biomolecules context: insulin (blood glucose regulation), adrenaline (fight-or-flight response), and thyroxine (metabolic rate regulation).

Insulin is a peptide hormone (51 amino acids, 2 chains: A-chain and B-chain linked by disulfide bonds) secreted by -cells of the islets of Langerhans in the pancreas. It lowers blood glucose by promoting glucose uptake into cells and glycogen synthesis in the liver.

Adrenaline (Epinephrine) is secreted by the adrenal medulla during stress. It's the 'fight-or-flight' hormone — it increases heart rate, dilates airways, raises blood glucose (by promoting glycogen breakdown), and redirects blood flow to muscles.

Thyroxine ( T_4 ) is an iodine-containing amino acid derivative produced by the thyroid gland. It regulates the basal metabolic rate (BMR). Deficiency → hypothyroidism (low BMR, weight gain; in children: cretinism). Excess → hyperthyroidism (high BMR, weight loss, Graves' disease).

• Insulin = blood glucose ↓ (hypoglycemic). Deficiency → diabetes mellitus
• Glucagon (from -cells of pancreas) = blood glucose ↑ (hyperglycemic). Insulin and glucagon are antagonistic
• Adrenaline = emergency hormone. Derived from tyrosine
• Thyroxine contains 4 iodine atoms (T₄). T₃ (triiodothyronine) is more active with 3 iodine atoms
• Iodine deficiency → goiter (enlarged thyroid) — prevented by iodized salt