Biomolecules
Carbohydrates & Glucose Anomers, Reducing vs Non-Reducing Sugars, Amino Acids & Zwitter Ion, Protein Structure & Denaturation, Vitamins & DNA/RNA
Topic 10.1 – 10.1.2.1: Carbohydrates: Classification, Preparation & Open-Chain Structure of Glucose
Picture your morning plate of puttu or boiled rice alongside a ripe banana and a spoonful of cane sugar. Every bit of that energy comes from carbohydrates—literally once thought to be "hydrates of carbon" with formula Cx(H2O)y. Wait: vinegar's acetic acid (CH3COOH) fits C2(H2O)2 yet tastes sour and is NOT a carbohydrate, while rhamnose (C6H12O5) breaks the formula yet IS a true sugar! Chemists solved that riddle by defining carbohydrates by their real functional groups—and cracking glucose's 6-carbon chain one reagent clue at a time, like a forensic detective!
Chemically, carbohydrates (also called saccharides, from Greek sakcharon = sugar) are defined as optically active polyhydroxy aldehydes or ketones, or compounds that produce such units on hydrolysis.
1. Classification of Carbohydrates
- Monosaccharides: Cannot be hydrolysed further into simpler polyhydroxy aldehydes/ketones (~20 in nature; e.g., Glucose, Fructose, Ribose). Classified as aldose (
–CHO) or ketose (>C=O) and by carbon count: Triose (3C), Tetrose (4C), Pentose (5C), Hexose (6C), Heptose (7C). - Oligosaccharides: Yield 2 to 10 monosaccharide units on hydrolysis (e.g., disaccharides: Sucrose → Glucose + Fructose; Maltose → 2 Glucose).
- Polysaccharides: Yield a large number of monosaccharides on hydrolysis; not sweet, hence called non-sugars (Starch, Cellulose, Glycogen, Gums).
- Reducing vs Non-Reducing Sugars: Carbohydrates that reduce Fehling's solution and Tollens' reagent are reducing sugars. ALL monosaccharides (both aldoses and ketoses) are reducing sugars!
2. Preparation of Glucose (C6H12O6)
(ii) Commercially from Starch: (C6H10O5)n + nH2O ⟶H+, 393 K, 2–3 atm n C6H12O6 (Glucose)
3. Six Classic Proofs of Open-Chain Structure of D-(+)-Glucose
| Reaction / Test | Product Formed | Structural Feature Proved |
|---|---|---|
| 1. Molecular Formula | C6H12O6 | Aldohexose composition (Dextrose) |
| 2. Prolonged heating with HI | n-Hexane | All 6 carbons are linked in a straight chain |
| 3. NH2OH / HCN | Glucose oxime / cyanohydrin | Presence of a carbonyl group (>C=O) |
| 4. Br2 water (mild oxidation) | Gluconic acid (C6 monocarboxylic) | Carbonyl group is an aldehyde (–CHO) |
| 5. Acetic anhydride (CH3CO)2O | Glucose pentaacetate (stable) | Presence of 5 –OH groups on different carbons |
| 6. HNO3 (glucose or gluconic acid) | Saccharic acid (dicarboxylic) | Presence of a primary alcoholic (–CH2OH) group |
–OH lies on the right side (in Fischer projection with most oxidised –CHO at top). '(+)' denotes dextrorotatory optical activity. Remember: D/L has NO fixed relation with (+) or (–) rotation!
D-(-) (laevorotatory)! (2) Do NOT confuse Br2 water (mild, oxidises ONLY –CHO to Gluconic acid) with conc. HNO3 (strong, oxidises BOTH –CHO and –CH2OH to Saccharic acid).Topic 10.1.2.1 – 10.1.2.2: Cyclic Pyranose Structure of Glucose (α/β-Anomers) & Fructose Furanose
Just when chemists celebrated glucose's straight-chain formula, three stubborn mysteries refused to fit! Glucose would NOT turn pink with Schiff's reagent, refused to react with sodium hydrogensulphite (NaHSO3), and its pentaacetate ignored hydroxylamine completely—almost as if its aldehyde group were playing hide-and-seek! Even stranger, glucose crystallised in two different forms with different melting points (419 K and 423 K). The brilliant answer: inside water, glucose's own C-5 –OH tail curls around and bites its own C-1 –CHO head to snap shut into a 6-membered ring!
1. Three Facts Open-Chain Glucose Could NOT Explain
- Despite having an aldehyde group, glucose does not give Schiff's test and does not form a hydrogensulphite addition product with
NaHSO3. - Glucose pentaacetate does NOT react with hydroxylamine (NH2OH), proving the absence of a free
–CHOgroup! - Glucose exists in two distinct crystalline forms:
- α-D-(+)-Glucose (m.p. 419 K): obtained by crystallisation from concentrated glucose solution at 303 K.
- β-D-(+)-Glucose (m.p. 423 K): obtained by crystallisation from hot saturated aqueous glucose solution at 371 K.
2. Cyclic Hemiacetal & Pyranose Haworth Structures
The –OH group at C-5 adds intramolecularly to the –CHO group at C-1 to form a six-membered cyclic hemiacetal called a pyranose ring (analogous to pyran, a 6-membered ring with 5 C atoms and 1 O atom). Both cyclic forms exist in equilibrium with the open-chain structure.
–OH group at C-1 (the Anomeric Carbon).• α-D-(+)-Glucopyranose: C-1
–OH is on the right (Fischer) / pointing DOWN (Haworth).• β-D-(+)-Glucopyranose: C-1
–OH is on the left (Fischer) / pointing UP (Haworth).
3. Fructose — Ketohexose & Five-Membered Furanose Ring
D-(-)-Fructose (C6H12O6) is a natural ketohexose found in fruits, honey, and vegetables (formed with glucose upon sucrose hydrolysis). It contains a ketonic group (>C=O) at C-2, belongs to the D-series, and is laevorotatory (–). Its –OH at C-5 adds to the C-2 >C=O to form a five-membered cyclic hemiketal called a furanose ring (analogous to furan: 4 C + 1 O), giving α- and β-D-(-)-fructofuranose.
–OH groups) and sucrose (8 –OH groups) form extensive intermolecular hydrogen bonds with water, whereas non-polar cyclohexane/benzene cannot! And why doesn't glucose pentaacetate react with NH2OH? Because all 5 –OH (including C-1 hemiacetal –OH) are acetylated in the cyclic ring, leaving no free –CHO!
Topic 10.1.3 – 10.1.5: Disaccharides (Sucrose, Maltose, Lactose), Invert Sugar & Polysaccharides
Why does ordinary table sugar (sucrose) refuse to give a brick-red precipitate in Fehling's test, whereas milk sugar (lactose) and malt sugar (maltose) pass with flying colours? Think of two friends holding hands! In maltose and lactose, one friend holds with C-1 while the other friend leaves their C-1 hand free in solution to open up and reduce Cu2+. In sucrose, BOTH sugars lock their reactive anomeric hands together (C-1 of glucose clasps C-2 of fructose)—leaving zero free reducing hands!
When two monosaccharides condense with loss of a water molecule, they join through an oxygen atom via a glycosidic linkage.
1. Disaccharides & Reducing Behaviour
| Disaccharide | Monosaccharide Units | Glycosidic Linkage | Reducing? |
|---|---|---|---|
| Sucrose (Cane sugar) | α-D-(+)-Glucose + β-D-(-)-Fructose | C-1 of α-glucose to C-2 of β-fructose | NON-REDUCING (both carbonyls locked) |
| Maltose (Malt sugar) | α-D-Glucose + α-D-Glucose | C-1 of I to C-4 of II | REDUCING (free C-1 aldehyde on II) |
| Lactose (Milk sugar) | β-D-Galactose + β-D-Glucose | C-1 of galactose to C-4 of glucose | REDUCING (free C-1 aldehyde on glucose) |
+66.5°), yet upon hydrolysis with dilute acid or enzyme invertase, it yields an equimolar mixture of D-(+)-glucose (+52.5°) and D-(-)-fructose (–92.4°). Because fructose's laevorotation outweighs glucose's dextrorotation, the net optical rotation inverts from (+) to (–) — hence the product mixture is called invert sugar!
2. Polysaccharides & Importance of Carbohydrates
- Starch (Plant food storage): Polymer of α-D-glucose made of two components:
- Amylose (15–20%): Water-soluble, long unbranched chain of 200–1000 α-D-(+)-glucose units via C1–C4 glycosidic linkages.
- Amylopectin (80–85%): Water-insoluble, branched polymer of α-D-glucose with C1–C4 chains and C1–C6 branching.
- Cellulose (Most abundant plant organic compound): Straight-chain polymer of β-D-glucose units joined exclusively by C1–C4 β-glycosidic linkages (cell walls of plants, wood, cotton fibre).
- Glycogen (Animal Starch): Stored in liver, muscles, and brain (and yeast/fungi); structure resembles amylopectin but is more highly branched.
- Biological Roles: Instant energy (honey), storage (starch/glycogen), structural cell walls, textiles/paper/breweries, and aldopentoses (D-ribose & 2-deoxy-D-ribose) in nucleic acids!
Topic 10.2.1 – 10.2.2: Amino Acids: Classification, Essential Amino Acids, Zwitter Ion & Optical Activity
Why do tiny crystals of glycine dissolve easily in water and refuse to melt until a scorching 290 °C—behaving far more like table salt than like a typical organic carboxylic acid or amine? Look inside a single amino acid molecule in water: its acidic –COOH group actually hands a proton straight over to its own basic –NH2 group! The molecule becomes a Zwitter Ion (from German zwitter = hybrid), holding BOTH a positive –NH3+ and negative –COO- charge at once, turning amino acids into tiny internal ionic salts!
Proteins (from Greek proteios = of prime importance) are polymers of α-amino acids (R–CH(NH2)–COOH). Only α-amino acids are obtained on hydrolysis of proteins (about 20 natural ones, represented by 3-letter and 1-letter codes—e.g., Glycine/Gly/G = sweet glykos; Tyrosine/Tyr/Y = cheese tyros).
1. Two Key Classifications of Amino Acids
- Acidic, Basic, or Neutral (by relative count of
–NH2vs–COOH):- Neutral: Equal
–NH2&–COOH(e.g., Glycine R = H, Alanine R = CH3, Valine). - Acidic: More
–COOHthan–NH2(e.g., Aspartic acid, Glutamic acid). - Basic: More
–NH2than–COOH(e.g., Lysine, Arginine, Histidine).
- Neutral: Equal
- Essential vs Non-Essential:
- Non-essential (10): Synthesised inside the human body (Glycine, Alanine, Glutamic acid, Serine, Cysteine, Tyrosine, Proline, etc.).
- Essential (10): CANNOT be synthesised in the body and MUST be supplied in diet — remember PVT TIM HALL: Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine.
2. Zwitter Ion, Amphoteric Nature & Optical Activity
- Dipolar Salt-like Behaviour (NCERT Intext 10.4): Because amino acids exist as zwitter ions with strong electrostatic attractions, they are colourless crystalline water-soluble solids with much higher melting points and water solubility than corresponding haloacids!
- Amphoteric Behaviour: In zwitter ionic form, the
–COO–group accepts protons from acids while–NH3+donates protons to bases—reacting with BOTH acids and bases! - Optical Activity & L-Configuration: Except Glycine (where
R = H, so α-carbon has two identical H atoms), ALL naturally occurring α-amino acids are optically active and predominantly possess the L-configuration (written with–NH2on the left side).
H2N–CH2–COOH), because its α-carbon carries two identical hydrogens and lacks chirality! And note the stereochem contrast: natural carbohydrates belong to the D-series, whereas natural α-amino acids belong to the L-series!Topic 10.2.3 – 10.2.4: Peptide Linkage, Fibrous vs Globular Proteins, Four Levels of Structure & Denaturation
Crack a raw egg into a frying pan: transparent, runny liquid albumin turns within seconds into a firm opaque white solid—and no matter how long you cool it afterwards, it NEVER turns runny again! Why did cooking transform a liquid into a solid without breaking a single amino-acid bond? Because heat broke only the delicate hydrogen bonds holding the protein's 3D coiled ball together, causing chains to unfold and tangle (denaturation), while leaving the covalent peptide chain completely untouched!
1. Peptide Linkage (–CO–NH–)
Proteins are condensation polymers of α-amino acids joined by peptide bonds (peptide linkages), chemically an amide bond (–CO–NH–) formed between –COOH of one amino acid and –NH2 of the next with loss of H2O (e.g., Glycine + Alanine → Glycylalanine, a dipeptide).
- A tripeptide has 3 amino acids joined by 2 peptide linkages.
- Polypeptide: >10 amino acids; Protein: typically >100 amino acids and molecular mass >10,000 u (notable exception: Insulin has 51 amino acids yet has a well-defined protein conformation).
2. Classification by Molecular Shape: Fibrous vs Globular
| Property | Fibrous Proteins | Globular Proteins |
|---|---|---|
| Shape & Bonding | Parallel polypeptide chains held by H-bonds & disulphide bonds (fibre-like) | Polypeptide chains coil around into a compact spherical ball |
| Water Solubility | Generally insoluble in water | Usually soluble in water |
| Examples | Keratin (hair, wool, silk), Myosin (muscles) | Insulin, Albumins, Haemoglobin, Enzymes |
3. Four Structural Levels of Proteins
- Primary (1°) Structure: Specific linear sequence of amino acids in each polypeptide chain. Changing even one amino acid creates a different protein!
- Secondary (2°) Structure: Regular folding of the polypeptide backbone via hydrogen bonding between
C=Oand–NH–groups of peptide bonds:- α-Helix: Polypeptide twists into a right-handed screw with
–NHhydrogen-bonded toC=Oof an adjacent turn (intramolecular H-bonds). - β-Pleated Sheet: Polypeptide chains stretch nearly fully and lie side-by-side held together by intermolecular H-bonds (like pleated folds of drapery).
- α-Helix: Polypeptide twists into a right-handed screw with
- Tertiary (3°) Structure: Overall 3D folding of secondary structures producing fibrous or globular shapes. Stabilised by hydrogen bonds, disulphide linkages (–S–S–), van der Waals, and electrostatic forces.
- Quaternary (4°) Structure: Spatial arrangement of two or more polypeptide subunits relative to each other (e.g., Haemoglobin has 4 subunits).
4. Denaturation of Proteins
When a biologically active native protein faces a physical change (temperature rise) or chemical change (pH change), hydrogen bonds are disturbed: globules unfold, helices uncoil, and the protein loses its biological activity. Crucial rule: During denaturation, 2° and 3° structures are destroyed, while 1° structure remains intact! Examples: coagulation of egg white on boiling (Intext 10.5: water molecules get absorbed/hydrogen-bonded into the coagulated protein!) and curdling of milk (lactic acid formed by bacteria).
n - 1 peptide bonds for a linear chain of n amino acids!). And during denaturation: 1° structure NEVER breaks—only 2° and 3° structures unfold!Topic 10.3 – 10.4: Enzymes (Biocatalysts) & Vitamins (Classification & Deficiency Diseases)
Every day, your stomach digests complex starches and proteins effortlessly at gentle body temperature (37 °C) and mild pH—reactions that would need boiling strong acid in a glass beaker! That biological magic comes from enzymes, super-specific globular proteins that cut activation energy dramatically. Meanwhile, tiny micronutrients called vitamins act as essential co-factors: eat a slice of orange today and your body flushes excess water-soluble Vitamin C out in urine by evening—which is why sailors once got scurvy on long ocean voyages without fresh fruit!
1. Enzymes — Nature's Biocatalysts
- Almost all enzymes are globular proteins; needed in very small quantities and highly specific for a particular reaction and substrate.
- Named after the substrate or reaction ending in -ase (e.g., maltase hydrolyses maltose → 2 glucose; oxidoreductase catalyses simultaneous oxidation & reduction).
- Mechanism: Like chemical catalysts, enzymes provide an alternate reaction pathway that lowers Activation Energy (Ea)!
• Acid-catalysed hydrolysis: Ea = 6.22 kJ mol-1 | • Catalysed by enzyme Sucrase: Ea = 2.15 kJ mol-1!
2. Vitamins — Classification & NCERT Table 10.3
Originally named vitamine (vital + amine), the 'e' was dropped when most proved not to contain amino groups. Most cannot be synthesised by humans (plants make nearly all; gut bacteria make some) and are classified by solubility:
- Fat-Soluble Vitamins (A, D, E, K): Soluble in fats/oils, insoluble in water; stored in liver and adipose (fat-storing) tissues (excess intake can cause hypervitaminosis).
- Water-Soluble Vitamins (B-group & C): Soluble in water; readily excreted in urine and CANNOT be stored in the body—EXCEPT Vitamin B12! Hence must be supplied regularly in diet (Intext 10.6).
| Vitamin | Rich Sources | Deficiency Disease(s) |
|---|---|---|
| Vitamin A (Fat) | Fish liver oil, carrots, butter, milk | Xerophthalmia (hardening of cornea), Night blindness |
| Vitamin B1 (Thiamine) | Yeast, milk, green vegetables, cereals | Beri beri (loss of appetite, retarded growth) |
| Vitamin B2 (Riboflavin) | Milk, eggwhite, liver, kidney | Cheilosis (fissuring at corners of mouth/lips), digestive disorders, burning skin |
| Vitamin B6 (Pyridoxine) | Yeast, milk, egg yolk, cereals, grams | Convulsions |
| Vitamin B12 (Water*) | Meat, fish, egg, curd | Pernicious anaemia (RBC deficient in haemoglobin) |
| Vitamin C (Ascorbic acid) | Citrus fruits, amla, green leafy vegetables | Scurvy (bleeding gums) |
| Vitamin D (Fat) | Exposure to sunlight, fish, egg yolk | Rickets (bone deformities in children) & Osteomalacia (soft bones/joint pain in adults) |
| Vitamin E (Fat) | Wheat germ oil, sunflower oil | Increased fragility of RBCs & muscular weakness |
| Vitamin K (Fat) | Green leafy vegetables | Increased blood clotting time |
Topic 10.5 – 10.5.3: Nucleic Acids: Nucleosides, Nucleotides, DNA Double Helix, RNA Types & DNA Fingerprinting
How does a mango seed always grow into a mango tree and never a coconut palm, passing exact instructions across millions of years? Inside every cell nucleus sit chromosomes made of proteins and nucleic acids (DNA & RNA). Imagine DNA as a twisted spiral staircase whose side rails are alternating sugar-phosphate links and whose steps are complementary chemical code letters locked by hydrogen bonds—a master blueprint so unique to every person that forensic scientists can match a single drop of blood by DNA fingerprinting!
Nucleic acids (DNA = deoxyribonucleic acid; RNA = ribonucleic acid) are long-chain polymers of nucleotides (polynucleotides).
1. Three Hydrolysis Components + Nucleoside vs Nucleotide
Nucleotide = Nucleoside + Phosphoric Acid (linked at C-5' position of sugar)
Polynucleotide Chain: Nucleotides join by Phosphodiester linkages between 5' and 3' carbons of pentose sugars!
2. Structural & Functional Differences: DNA vs RNA
| Feature | DNA (Deoxyribonucleic Acid) | RNA (Ribonucleic Acid) |
|---|---|---|
| Pentose Sugar | β-D-2-deoxyribose (H at C-2') | β-D-ribose (OH at C-2') |
| Nitrogenous Bases | Purines: A, G | Pyrimidines: C, Thymine (T) | Purines: A, G | Pyrimidines: C, Uracil (U) |
| Secondary Structure | Double-stranded α-helix (Watson & Crick, 1962 Nobel) | Single-stranded helix (sometimes folds back on itself) |
| Base Pairing | A = T (2 H-bonds), G ≡ C (3 H-bonds) → complementary strands! | No fixed 1:1 base ratio on hydrolysis (Intext 10.8) |
| Biological Role | Chemical basis of heredity; self-duplication (replication); genetic blueprint | Protein synthesis via 3 types: m-RNA, r-RNA, t-RNA |
• Hydrolysis of a DNA nucleotide containing thymine yields: 2-deoxy-D-ribose + Thymine + Phosphoric acid.
• The two strands of DNA are NOT identical—they are COMPLEMENTARY (because A pairs only with T and G only with C).
• DNA Fingerprinting: Sequence of DNA bases is unique for every person, identical in every cell, and unalterable by surgery—used in forensics, paternity testing, identifying accident victims, and evolutionary studies (Genetic code cracked by Har Gobind Khorana, Nirenberg & Holley, 1968 Nobel Prize).
Topic 10.6: Hormones: Intercellular Messengers, Chemical Classes & Physiological Regulation
Picture your body as a bustling city of trillions of cells. When you eat a sweet payasam, blood glucose shoots up within minutes—how do muscle and liver cells far away instantly know they should absorb that extra sugar? Endocrine glands release tiny chemical text-messages called hormones directly into the bloodstream! Within moments, insulin lowers blood glucose, while during fasting its partner glucagon lifts glucose back up—proving how delicate hormonal balance keeps us alive every second.
Hormones are molecules that act as intercellular messengers. They are produced by endocrine glands and poured directly into the bloodstream, which transports them to their site of action.
1. Chemical Classification of Hormones
| Chemical Class | Hormone Examples | Key Gland / Role |
|---|---|---|
| 1. Steroids | Estrogens (Estradiol), Androgens (Testosterone), Progesterone, Glucocorticoids, Mineralocorticoids | Gonads (testes/ovaries) & Adrenal cortex |
| 2. Polypeptides | Insulin, Glucagon, Endorphins | Pancreas (glucose balance) & nervous signalling |
| 3. Amino Acid Derivatives | Epinephrine, Norepinephrine, Thyroxine | Adrenal medulla (stress) & Thyroid gland |
2. Essential Physiological Functions & Deficiency Conditions
- Blood Glucose Regulation: Rapid rise in blood glucose triggers Insulin (lowers blood glucose); Glucagon increases blood glucose. Both together keep blood sugar within narrow limits.
- Stress & External Stimuli: Epinephrine and Norepinephrine mediate rapid responses to external stimuli.
- Thyroxine (Thyroid Gland): An iodinated derivative of amino acid tyrosine.
- Low thyroxine → Hypothyroidism (lethargy & obesity). Dietary iodine deficiency causes hypothyroidism and enlarged thyroid gland—prevented by adding sodium iodide (
NaI) to table salt ("Iodised" salt). - Excess thyroxine → Hyperthyroidism.
- Low thyroxine → Hypothyroidism (lethargy & obesity). Dietary iodine deficiency causes hypothyroidism and enlarged thyroid gland—prevented by adding sodium iodide (
- Adrenal Cortex Hormones:
- Glucocorticoids: Control carbohydrate metabolism, modulate inflammatory reactions, and mediate stress response.
- Mineralocorticoids: Control level of water and salt excretion by the kidneys.
- Malfunction of adrenal cortex causes Addison's disease (hypoglycemia, weakness, increased susceptibility to stress; fatal unless treated with glucocorticoids & mineralocorticoids).
- Sex Hormones (Gonads): Testosterone (male secondary traits: deep voice, facial hair), Estradiol (female secondary traits & menstrual cycle control), Progesterone (prepares uterus for implantation of fertilised egg).
Solved Previous Year Questions (Kerala DHSE & NCERT)
Q1 (Kerala HSE March (Repeated Favorite) • 3 Marks): What happens when D-glucose is treated with the following reagents? State what each reaction proves about glucose structure: (a) Prolonged heating with HI (b) Bromine water (c) Concentrated HNO3
- (a) Prolonged heating with HI: Forms n-hexane → proves all 6 carbon atoms are linked in a straight chain.
- (b) Bromine water: Oxidises glucose to gluconic acid (6-carbon monocarboxylic acid) → proves the carbonyl group is present as an aldehydic (–CHO) group.
- (c) Concentrated HNO3: Oxidises glucose (or gluconic acid) to saccharic acid (dicarboxylic acid) → proves the presence of a primary alcoholic (–CH2OH) group.
Q2 (Kerala HSE SAY / NCERT Exercise • 2 Marks): Enumerate any two experimental facts or reactions of D-glucose that cannot be explained by its open-chain structure.
- Despite having an aldehyde group, glucose does not give Schiff's test and does not form the hydrogensulphite addition product with NaHSO3.
- Glucose pentaacetate does not react with hydroxylamine (NH2OH), indicating the absence of a free
–CHOgroup. - Glucose exists in two distinct crystalline anomeric forms: α-form (m.p. 419 K) and β-form (m.p. 423 K).
Q3 (Kerala HSE March (Most Asked) • 2 Marks): Sucrose is dextrorotatory, yet its hydrolysis mixture is laevorotatory. Explain why it is called 'invert sugar' and state whether sucrose is a reducing or non-reducing sugar.
Sucrose (+66.5°) on hydrolysis yields an equimolar mixture of D-(+)-glucose (+52.5°) and D-(-)-fructose (–92.4°). Because the laevorotation of fructose exceeds the dextrorotation of glucose, the net optical rotation of the product changes sign from dextro (+) to laevo (–), so it is called invert sugar.
Sucrose is a non-reducing sugar because the reducing groups of glucose (C-1) and fructose (C-2) are involved in forming the glycosidic bond.
Q4 (Kerala HSE Model / NCERT • 2 Marks): Write the hydrolysis products of: (a) Sucrose (b) Lactose (c) Maltose.
- (a) Sucrose → α-D-(+)-Glucose + β-D-(-)-Fructose
- (b) Lactose (Milk sugar) → β-D-Galactose + β-D-Glucose
- (c) Maltose → Two molecules of α-D-Glucose
Q5 (Kerala HSE March • 2 Marks): Differentiate between: (a) Amylose and Amylopectin (b) Starch and Cellulose.
(a) Amylose vs Amylopectin: Amylose (15–20% of starch) is water-soluble and an unbranched polymer of α-D-glucose with C1–C4 linkages. Amylopectin (80–85% of starch) is water-insoluble and a branched polymer of α-D-glucose with C1–C4 chains and C1–C6 branching.
(b) Starch vs Cellulose: Starch is a storage polymer of α-D-glucose, whereas cellulose is a straight-chain structural polymer of β-D-glucose linked by C1–C4 glycosidic bonds.
Q6 (Kerala HSE SAY / NCERT Intext 10.4 • 2 Marks): Why are the melting points and water solubility of α-amino acids generally higher than those of corresponding haloacids? How do they show amphoteric behaviour?
In aqueous solution, the –COOH group loses a proton and –NH2 accepts it to form a dipolar zwitter ion (+H3N–CH(R)–COO–). Due to strong dipolar salt-like electrostatic attractions, amino acids have higher melting points and water solubility than haloacids.
In zwitter ionic form, they act as amphoteric species because –COO– reacts with acids (accepts H+) while –NH3+ reacts with bases (donates H+).
Q7 (Kerala HSE March (High Yield) • 3 Marks): Distinguish between fibrous and globular proteins with one example each. What is denaturation of proteins, and how does it affect the 1°, 2°, and 3° structures?
Fibrous vs Globular: Fibrous proteins have parallel polypeptide chains held by H-bonds and disulphide bonds, are fibre-like and water-insoluble (e.g., keratin, myosin). Globular proteins coil into spherical shapes and are water-soluble (e.g., insulin, albumin).
Denaturation: When a native protein is subjected to heat or pH change, hydrogen bonds are disturbed, globules unfold, helices uncoil, and biological activity is lost (e.g., coagulation of egg white, curdling of milk). Secondary (2°) and tertiary (3°) structures are destroyed while primary (1°) structure remains intact.
Q8 (Kerala HSE March • 2 Marks): Name the two common types of secondary structure of proteins. What type of bonding stabilises the α-helix structure?
The two common secondary structures are α-helix and β-pleated sheet.
The α-helix is stabilised by intramolecular hydrogen bonding between the –NH– group of one amino acid residue and the C=O group of an adjacent turn of the right-handed helix.
Q9 (Kerala HSE March (Repeated) • 3 Marks): How are vitamins classified? Why cannot Vitamin C be stored in our body? Name the vitamins whose deficiency causes: (a) Scurvy (b) Rickets (c) Pernicious anaemia (d) Increased blood clotting time.
Vitamins are classified as Fat-soluble (A, D, E, K) and Water-soluble (B-group and C). Vitamin C cannot be stored in our body because it is water-soluble and readily excreted in urine (exception among water-soluble vitamins: Vitamin B12).
- (a) Scurvy → Vitamin C
- (b) Rickets → Vitamin D
- (c) Pernicious anaemia → Vitamin B12
- (d) Increased blood clotting time → Vitamin K
Q10 (Kerala HSE SAY • 2 Marks): Write two structural differences between DNA and RNA. Distinguish between a nucleoside and a nucleotide.
DNA vs RNA: (1) DNA contains β-D-2-deoxyribose sugar and is double-stranded; RNA contains β-D-ribose and is single-stranded. (2) DNA has pyrimidine base Thymine (T), whereas RNA has Uracil (U).
Nucleoside vs Nucleotide: A nucleoside contains only a pentose sugar + nitrogenous base (at 1'), whereas a nucleotide contains a nucleoside + phosphoric acid attached at the 5' position.
Q11 (Kerala HSE / NCERT Intext 10.7 & 10.8 • 2 Marks): (a) The two strands in DNA are not identical but are complementary. Explain. (b) What products form when a DNA nucleotide containing thymine is completely hydrolysed?
(a) Complementary strands: Specific hydrogen bonds hold the double helix together only between Adenine (A) and Thymine (T), and between Guanine (G) and Cytosine (C). Thus the base sequence of one strand automatically dictates (is complementary to) the other.
(b) Hydrolysis products: Complete hydrolysis yields β-D-2-deoxyribose, Thymine, and Phosphoric acid (H3PO4).
Q12 (Kerala HSE Improvement • 1 Marks): Name: (a) The enzyme that lowers the activation energy of sucrose hydrolysis from 6.22 to 2.15 kJ mol-1, and (b) The disease caused by malfunction of the adrenal cortex.
(a) Enzyme: Sucrase | (b) Adrenal cortex disorder: Addison's disease (characterised by hypoglycemia, weakness, and increased susceptibility to stress).