Carbohydrates
Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones, or compounds that hydrolyse to them. The empirical formula is Cn(H₂O)n for many common sugars, which is why they were originally called "hydrates of carbon."
Classification
| Class | Definition | Examples |
|---|---|---|
| Monosaccharides | Cannot be hydrolysed further | Glucose, fructose, galactose, ribose |
| Disaccharides | Yield 2 monosaccharides on hydrolysis | Sucrose, maltose, lactose |
| Oligosaccharides | 2–10 monosaccharide units | Raffinose (3 units) |
| Polysaccharides | Many monosaccharide units, large MW | Starch, cellulose, glycogen |
Monosaccharides
Classified by number of carbons (triose 3C, pentose 5C, hexose 6C) and by carbonyl type (aldose = aldehyde, ketose = ketone). Glucose is an aldohexose; fructose is a ketohexose.
D and L configuration: based on the orientation of the -OH group on the penultimate carbon (second from the bottom). All naturally occurring sugars in humans are D-sugars.
Glucose
Open-chain formula: CHO-(CHOH)₄-CH₂OH. In solution, glucose exists predominantly as the cyclic (pyranose) ring form. The ring closes when the C-1 aldehyde reacts with the C-5 -OH, forming a hemiacetal. This creates the anomeric carbon at C-1.
Alpha-glucose: -OH at C-1 is below the ring plane (trans to -CH₂OH at C-5). Beta-glucose: -OH at C-1 is above the ring plane (cis to -CH₂OH at C-5). This structural difference is critical: alpha linkages give starch (digestible), beta linkages give cellulose (indigestible by humans).
Reducing and Non-Reducing Sugars
A sugar is reducing if it has a free anomeric carbon (a free -CHO or -CO- group that can be oxidised). Reducing sugars give positive Tollens, Fehling, and Benedict tests.
| Sugar | Reducing? | Reason |
|---|---|---|
| Glucose | Yes | Free C-1 aldehyde group |
| Fructose | Yes | Free C-2 keto group (isomerises to glucose) |
| Galactose | Yes | Free anomeric carbon |
| Maltose | Yes | C-1 of one glucose is free |
| Lactose | Yes | C-1 of glucose unit is free |
| Sucrose | NO | Both anomeric carbons (C-1 of glucose, C-2 of fructose) are involved in the glycosidic bond |
Important Disaccharides
| Disaccharide | Components | Linkage | Reducing? |
|---|---|---|---|
| Sucrose | Glucose + Fructose | alpha-1,2 (C1 of Glc to C2 of Fru) | No |
| Maltose | Glucose + Glucose | alpha-1,4 | Yes |
| Lactose | Galactose + Glucose | beta-1,4 | Yes |
| Cellobiose | Glucose + Glucose | beta-1,4 | Yes |
Polysaccharides
| Polysaccharide | Monomer | Linkage | Structure | Function |
|---|---|---|---|---|
| Amylose (starch) | Glucose | alpha-1,4 | Unbranched helix | Energy storage in plants |
| Amylopectin (starch) | Glucose | alpha-1,4 + alpha-1,6 at branches | Branched | Energy storage in plants |
| Glycogen | Glucose | alpha-1,4 + alpha-1,6 (more branches) | Highly branched | Energy storage in animals (liver, muscle) |
| Cellulose | Glucose | beta-1,4 | Linear, fibrous | Cell wall structure in plants |
Carbohydrate Classifier and Properties Explorer
Click any carbohydrate to see its class, reducing/non-reducing status, glycosidic linkage type, and the NEET-relevant fact. Master sucrose (non-reducing), amylose vs amylopectin (linkage), and cellulose vs starch (alpha vs beta).
Glucose
C₆H₁₂O₆
Proteins and Amino Acids
Proteins are polymers of amino acids linked by peptide bonds. They are the most diverse class of biomolecules and carry out nearly every function in the cell.
Amino Acid Structure
Every amino acid has: (1) a central alpha carbon, (2) an amino group (-NH₂), (3) a carboxyl group (-COOH), (4) a hydrogen atom, (5) a variable side chain (R group). At physiological pH (~7.4), most amino acids exist as zwitterions (dipolar ions): the -NH₂ is protonated to -NH₃⁺ and the -COOH is deprotonated to -COO⁻.
Classification of Amino Acids
| Class | R group character | Examples |
|---|---|---|
| Non-polar (hydrophobic) | Aliphatic or aromatic R, no charge | Glycine, Alanine, Valine, Leucine, Isoleucine, Proline, Phenylalanine |
| Polar, uncharged | -OH, -SH, -CONH₂ in R | Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine |
| Positively charged | Extra -NH₂ or imidazole in R | Lysine, Arginine, Histidine |
| Negatively charged | Extra -COOH in R | Aspartate (Asp), Glutamate (Glu) |
Essential amino acids (9, must come from diet): Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine. Mnemonic: His Is Leu Lys Met Phe Thr Trp Val ("HI! LLL MTT Val").
Peptide Bond
Formed by a condensation reaction between the -COOH of one amino acid and the -NH₂ of another, with loss of water. The bond is -CO-NH-, which is a partial double bond (resonance) — planar and relatively rigid.
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Levels of Protein Structure
Primary Structure
The specific sequence of amino acids in the polypeptide chain, linked by peptide bonds. Determined entirely by the gene. All other levels of structure follow from this sequence. Changing even one amino acid can alter protein function (e.g., sickle cell haemoglobin: glutamate replaced by valine at position 6 of beta chain).
Secondary Structure
The regular, repeating local folding of the polypeptide backbone. Maintained by hydrogen bonds between the backbone C=O of one peptide bond and the N-H of another.
Alpha-helix: right-handed coil. H-bonds are within the same chain, between residue i and residue i+4. Compact, stable. Found in keratin (hair, nails), muscle proteins.
Beta-pleated sheet: H-bonds between adjacent polypeptide strands (either parallel or antiparallel). Sheet-like structure. Found in silk fibroin.
Tertiary Structure
The overall 3D shape of the entire polypeptide chain. Determined by interactions between R groups: disulfide bonds (covalent, -S-S-), hydrogen bonds (N-H...O), ionic bonds (between charged R groups), hydrophobic interactions (non-polar R groups buried inside). The tertiary structure creates the functional active site.
Quaternary Structure
Assembly of two or more polypeptide subunits. Held by the same non-covalent forces as tertiary structure. Example: haemoglobin (2 alpha + 2 beta subunits), DNA polymerase, collagen (3 chains). Not all proteins have quaternary structure.
Protein Denaturation
Loss of 3D structure (2°, 3°, 4°) without breaking peptide bonds (primary structure is preserved). Causes: high temperature, extreme pH, heavy metal ions, urea, detergents. Active site is destroyed, enzyme becomes non-functional.
Enzymes
Enzymes are biological catalysts, almost all of which are proteins. They speed up reactions by lowering the activation energy (Ea). They are not consumed and do not alter the equilibrium constant (Keq) of the reaction.
Active Site and Specificity
The active site is a specific region on the enzyme surface where the substrate binds. It has a precise shape and chemical environment (amino acid R groups) complementary to the substrate.
Lock-and-key model: the active site is rigid and exactly complementary to the substrate (like a key fitting a specific lock).
Induced fit model: the active site is flexible and changes shape to better fit the substrate when it binds. More accurate for most enzymes.
Enzyme Nomenclature
Enzymes are named by adding -ase to the substrate or reaction type. Examples: Urease (substrate = urea), Protease (substrate = proteins), Lipase (substrate = lipids), Oxidoreductases (catalyse oxidation-reduction reactions), Transferases (transfer groups), Hydrolases (catalyse hydrolysis).
Factors Affecting Enzyme Activity
| Factor | Effect |
|---|---|
| Temperature | Increases up to optimum (~37°C in humans), then drops sharply due to denaturation |
| pH | Each enzyme has an optimum pH; extreme pH denatures. Pepsin: pH 2; trypsin: pH 8 |
| Substrate concentration | Rate increases up to saturation (Vmax); follows Michaelis-Menten kinetics |
| Enzyme concentration | Rate increases proportionally (if substrate excess) |
| Competitive inhibitor | Blocks active site; overcome by excess substrate |
| Non-competitive inhibitor | Binds allosteric site; reduces Vmax; cannot be overcome by substrate |
Cofactors and Coenzymes
Many enzymes require non-protein components (cofactors) for activity. Inorganic cofactors: metal ions (Mg²⁺, Zn²⁺, Fe²⁺). Organic cofactors are called coenzymes: NAD⁺, FAD, coenzyme A (derived from B-vitamins). Holoenzyme = apoenzyme (protein) + cofactor.
Nucleic Acids
Nucleic acids carry and express genetic information. There are two types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
Nucleotide Structure
A nucleotide = pentose sugar + nitrogenous base + phosphate group. The sugar in DNA is deoxyribose (no -OH at C-2'); in RNA it is ribose (-OH at C-2').
Nitrogenous bases:
Purines (double ring): Adenine (A), Guanine (G) — in both DNA and RNA.
Pyrimidines (single ring): Cytosine (C) — in both; Thymine (T) — only in DNA; Uracil (U) — only in RNA.
DNA Structure (Watson-Crick Model)
DNA is a double helix with two antiparallel strands. The two strands are complementary: A pairs with T (2 hydrogen bonds); G pairs with C (3 hydrogen bonds). The sugar-phosphate backbone is on the outside; bases are on the inside stacked on top of each other.
Chargaff's rules: in any DNA, [A] = [T] and [G] = [C]. So purines always equal pyrimidines.
DNA vs RNA
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose (no 2'-OH) | Ribose (has 2'-OH) |
| Bases | A, T, G, C | A, U, G, C |
| Strands | Double-stranded | Single-stranded (usually) |
| Location | Nucleus (mainly) | Nucleus + cytoplasm |
| Function | Stores genetic information | Protein synthesis (mRNA, tRNA, rRNA) |
| Stability | More stable | Less stable (2'-OH makes it susceptible to hydrolysis) |
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Lipids and Vitamins
Lipids
Lipids are hydrophobic (water-insoluble) biological molecules. They include fats (triglycerides), phospholipids, steroids, and waxes.
Fats (triglycerides): glycerol esterified with three fatty acids. Saturated fats have no C=C bonds; unsaturated fats have one or more C=C bonds. Fats store more than twice as much energy per gram as carbohydrates.
Phospholipids: glycerol + 2 fatty acids + phosphate group + polar head. Form the bilayer of cell membranes.
Steroids: four-ring structure. Cholesterol is the most important. Steroid hormones (testosterone, oestrogen, cortisol) are derived from cholesterol.
Vitamins
Vitamins are essential organic molecules needed in small amounts for normal metabolism. Classified as fat-soluble (A, D, E, K — stored in fatty tissues) or water-soluble (B group, C — not stored, need regular intake).
Vitamin Deficiency Explorer
Click any vitamin to see its chemical name, solubility, deficiency disease, symptoms, and food sources. Covers all 13 vitamins tested in NEET. Fat-soluble (A, D, E, K) are stored in fat; water-soluble (B group, C) are not stored and need regular intake.
Vitamin A
Retinol
Night blindness (nyctalopia) and xerophthalmia
| Vitamin | Name | Deficiency disease |
|---|---|---|
| A | Retinol | Night blindness (nyctalopia) and xerophthalmia |
| D | Calciferol (D₂/D₃) | Rickets (children), osteomalacia (adults) |
| E | Tocopherol | Haemolysis of RBCs (rare in adults |
| K | Phylloquinone (K₁) / Menaquinone (K₂) | Impaired blood clotting (haemorrhage) |
| B1 | Thiamine | Beriberi |
| B2 | Riboflavin | Ariboflavinosis |
| B3 | Niacin (Nicotinic acid) | Pellagra |
| B5 | Pantothenic acid | Burning feet syndrome (rare) |
| B6 | Pyridoxine | Convulsions, peripheral neuritis, dermatitis |
| B7 | Biotin | Dermatitis, hair loss, conjunctivitis |
| B9 | Folic acid (Folate) | Megaloblastic (macrocytic) anaemia |
| B12 | Cobalamin | Pernicious anaemia, subacute combined degeneration of spinal cord |
| C | Ascorbic acid | Scurvy |
Worked NEET Problems
NEET-style problem · Carbohydrates
Question
Solution
By Chargaff's rules: [A] = [T] and [G] = [C].
(a) Thymine = 28% (same as adenine).
(b) A + T = 28 + 28 = 56%. Remaining G + C = 100 - 56 = 44%. Since G = C, guanine = 44/2 = 22%.
(c) Cytosine = 22% (same as guanine).
Check: 28 + 28 + 22 + 22 = 100%. Purines (A+G) = 50%, pyrimidines (T+C) = 50%.
NEET-style problem · Reducing Sugars
Question
Solution
Reducing sugars (have free anomeric carbon): (b) Maltose, (c) Fructose, (d) Lactose.
Non-reducing: (a) Sucrose — both anomeric carbons (C-1 of glucose and C-2 of fructose) are involved in the glycosidic bond. No free -CHO or -CO- group.
Cellulose: the terminal glucose of each chain has a free C-1, but cellulose is a polymer and for NEET purposes is considered non-reducing (its reducing ends are negligible and NCERT classifies it as non-reducing in practice).
NEET-style problem · Protein Structure
Question
Solution
(a) Urea disrupts hydrogen bonds and hydrophobic interactions. These forces maintain tertiary and secondary structure. Urea denatures the protein: tertiary structure (and secondary) is disrupted. Primary structure (peptide bonds) intact.
(b) DTT reduces disulfide bonds (-S-S- → -SH HS-). Disulfide bonds are part of tertiary structure (and quaternary, if between subunits). DTT disrupts tertiary (and possibly quaternary) structure.
(c) A point mutation changes the amino acid sequence — this is the primary structure. Any subsequent change in 2°, 3°, and 4° structure follows from the changed primary structure.
NEET-style problem · Vitamins
Question
Solution
1. Vitamin A — Night blindness (and xerophthalmia). Vitamin A (retinol) is needed for rhodopsin synthesis in rod cells.
2. Vitamin B1 (thiamine) — Beriberi. Thiamine is required for carbohydrate metabolism; deficiency affects nervous system and heart.
3. Vitamin B12 (cobalamin) — Pernicious anaemia. Required for RBC maturation and nerve function.
4. Vitamin C (ascorbic acid) — Scurvy. Required for collagen synthesis.
5. Vitamin D (calciferol) — Rickets (children), osteomalacia (adults). Required for calcium absorption.
6. Vitamin K (phylloquinone) — Impaired blood clotting. Required for synthesis of clotting factors (prothrombin, etc.).
Summary Cheat Sheet
Carbohydrate Quick Reference
| Compound | Type | Reducing? | Linkage |
|---|---|---|---|
| Glucose / Fructose / Galactose | Monosaccharide | Yes | N/A |
| Sucrose | Disaccharide | NO | alpha-1,2 (Glc-Fru) |
| Maltose | Disaccharide | Yes | alpha-1,4 (Glc-Glc) |
| Lactose | Disaccharide | Yes | beta-1,4 (Gal-Glc) |
| Amylose | Polysaccharide | No (negligible) | alpha-1,4 unbranched |
| Amylopectin | Polysaccharide | No | alpha-1,4 + alpha-1,6 branches |
| Cellulose | Polysaccharide | No | beta-1,4 |
Protein Structure Summary
| Level | Description | Bonds |
|---|---|---|
| Primary | Amino acid sequence | Peptide bonds (covalent) |
| Secondary | Alpha-helix / beta-sheet | H-bonds (backbone to backbone) |
| Tertiary | 3D folding of full chain | Disulfide, H-bond, ionic, hydrophobic |
| Quaternary | Multiple subunit assembly | Same as tertiary (non-covalent mostly) |
DNA vs RNA Key Differences
| DNA | RNA |
|---|---|
| Deoxyribose sugar | Ribose sugar |
| Thymine (T) base | Uracil (U) base (no thymine) |
| Double-stranded | Single-stranded |
| A-T: 2 H-bonds; G-C: 3 H-bonds | A-U: 2 H-bonds (in double-helical regions) |
Frequently asked questions
What makes a sugar a "reducing sugar"? Which common sugars are reducing and which are not?
A reducing sugar is one that has a free aldehyde (-CHO) or ketone (-CO-) group that can be oxidised by Tollens reagent, Fehling solution, or Benedict reagent. The free anomeric carbon (C-1 for aldoses, C-2 for ketoses) must be unsubstituted — not involved in a glycosidic bond. Reducing sugars: glucose, fructose, galactose, maltose (its C-1 is free), and lactose (its C-1 of the glucose unit is free). Non-reducing sugars: sucrose — in sucrose, both the C-1 of glucose AND the C-2 of fructose are involved in the glycosidic bond, leaving no free anomeric carbon. Sucrose cannot reduce Fehling solution. NEET tests this directly: "Which of the following is a non-reducing sugar? Answer: Sucrose."
What are essential amino acids and why are they essential?
Essential amino acids are those that cannot be synthesised by the human body at all, or not in adequate amounts, and must therefore be obtained from the diet. There are 20 standard amino acids in human proteins, of which 9 are essential: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. A helpful mnemonic: PVT TIM HaLL (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, arginine (conditionally), Leucine, Lysine). The remaining amino acids are non-essential — the body can synthesise them from metabolic intermediates. NEET often asks to identify essential vs non-essential amino acids from a list.
What are the four levels of protein structure? How does each level arise?
Primary structure: the sequence of amino acids in the polypeptide chain. It is determined by the gene (DNA sequence). Peptide bonds (-CO-NH-) link the amino acids. Secondary structure: the local folding of the polypeptide chain into regular repeating patterns. The two common patterns are alpha-helix (hydrogen bonds within one chain, along the backbone) and beta-pleated sheet (hydrogen bonds between two adjacent parallel or antiparallel chains). Both are held by hydrogen bonds between the C=O of one peptide bond and the N-H of another. Tertiary structure: the overall 3D shape of a single polypeptide chain. Maintained by interactions between R groups: disulfide bonds (cysteine-cysteine), hydrophobic interactions, hydrogen bonds, ionic interactions. This gives the functional protein shape with an active site. Quaternary structure: the arrangement of multiple polypeptide subunits (chains). Example: haemoglobin has 4 subunits (2 alpha + 2 beta). Held by the same forces as tertiary structure. Not all proteins have quaternary structure.
What is enzyme denaturation and what conditions cause it?
Denaturation is the loss of the three-dimensional (3D) shape of an enzyme (or any protein) without breaking the primary structure (peptide bonds). The active site is disrupted, so the enzyme loses catalytic activity. Conditions that cause denaturation: (1) High temperature: breaks weak non-covalent bonds (H-bonds, hydrophobic interactions). Above the optimum temperature (~37°C for most human enzymes), activity drops sharply. (2) Extreme pH: changes the ionisation state of amino acid R groups in and around the active site, altering the shape. Optimum pH is usually 6–8 but varies (pepsin: pH 2, trypsin: pH 8). (3) Heavy metal ions: e.g., Hg²⁺, Pb²⁺ bind to the protein and alter the shape. (4) Organic solvents and detergents: disrupt hydrophobic interactions. Denaturation is usually irreversible for severe conditions (hard-boiled egg) but can be reversible if mild (some enzymes refold when the denaturing agent is removed).
What are Chargaff's rules and why are they important?
Chargaff's rules: in any double-stranded DNA, the molar ratio of adenine (A) equals thymine (T), and the ratio of guanine (G) equals cytosine (C). In other words: [A] = [T] and [G] = [C], so [A]+[G] = [C]+[T] (purines = pyrimidines). Also, [A]+[T] / [G]+[C] is constant for a given species. These rules were crucial evidence for the Watson-Crick double helix model: A pairs with T (2 hydrogen bonds) and G pairs with C (3 hydrogen bonds). NEET application: if %A = 30%, then %T = 30%, and %G + %C = 40%, so %G = %C = 20%. These calculations appear directly in NEET.
What are fat-soluble vs water-soluble vitamins? Which diseases result from deficiency?
Fat-soluble vitamins are stored in body fat and liver; toxicity is possible with excess. Water-soluble vitamins are not stored (excreted in urine); must be taken regularly. Fat-soluble vitamins (A, D, E, K): A (retinol) — night blindness, xerophthalmia. D (calciferol) — rickets (children), osteomalacia (adults). E (tocopherol) — haemolysis of RBCs. K (phylloquinone) — impaired blood clotting. Water-soluble vitamins (C and B group): C (ascorbic acid) — scurvy. B1 (thiamine) — beriberi. B2 (riboflavin) — cheilosis, glossitis. B3 (niacin) — pellagra. B5 (pantothenic acid) — burning feet syndrome. B6 (pyridoxine) — convulsions. B7 (biotin) — dermatitis, hair loss. B9 (folic acid) — megaloblastic anaemia. B12 (cobalamin) — pernicious anaemia. NEET questions typically test which vitamin and which disease are correctly paired.
What is the difference between DNA and RNA?
DNA (deoxyribonucleic acid): double-stranded helix, sugar = deoxyribose (no -OH at 2' carbon), bases = A, T, G, C, base pair T with A. Stable. Found mainly in the nucleus. Carries genetic information. RNA (ribonucleic acid): usually single-stranded, sugar = ribose (-OH at 2' carbon), bases = A, U (uracil, not T), G, C. Uracil pairs with A. Less stable than DNA. Found in nucleus and cytoplasm. Three types: mRNA (carries genetic code from DNA to ribosome), tRNA (carries specific amino acids to ribosome), rRNA (structural component of ribosome). Key NEET distinction: RNA has uracil instead of thymine, and ribose instead of deoxyribose.
How many NEET questions come from Biomolecules and what topics are most tested?
Chemistry Biomolecules contributes 2–4 NEET questions per year from Class 12. The most frequently tested topics are: (1) Reducing vs non-reducing sugars — sucrose is non-reducing; all monosaccharides are reducing. (2) Levels of protein structure — know which bonds hold each level (H-bonds: secondary; disulfide, hydrophobic: tertiary; same forces: quaternary). (3) Enzyme characteristics — specificity, lock-and-key model, effect of temperature and pH, enzyme inhibition. (4) Chargaff rules and DNA/RNA differences — nucleotide percentage calculations. (5) Vitamin deficiency diseases — fat vs water soluble, specific disease pairs. Note: Biology (Zoology/Botany) Biomolecules chapter has more weightage in NEET than the Chemistry chapter, so coordinate study with the zoology biomolecules content.
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