1. Chapter at a Glance
Living things are built from and powered by organic molecules called biomolecules: carbohydrates, proteins, enzymes, vitamins, nucleic acids, lipids and hormones. This chapter looks at their chemistry — structures, classification, important reactions and functions. NEET asks one or two chemistry questions from here (and more from the Biology version of the topic), often on glucose structure, glycosidic linkages, types of protein structure, vitamin deficiencies and DNA/RNA differences.
- Classification of carbohydrates; reducing vs non-reducing sugars.
- Open-chain and cyclic (α/β, pyranose) structures of glucose; reactions that prove the structure.
- Sucrose, maltose, lactose, starch, cellulose and glycogen linkages.
- Amino acids, peptide bond, primary to quaternary protein structure, denaturation.
- Vitamins (fat- vs water-soluble) and nucleic acids (DNA vs RNA).
2. Carbohydrates
- Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that give them on hydrolysis. Many fit Cₓ(H₂O)ᵧ, but this is not a definition (rhamnose, C₆H₁₂O₅, does not fit; acetic acid fits but is not a carbohydrate).
- Monosaccharides cannot be hydrolysed further (glucose, fructose, ribose). They are called aldoses or ketoses according to their carbonyl group.
- Oligosaccharides give 2–10 monosaccharides (disaccharides: sucrose, maltose, lactose).
- Polysaccharides give many monosaccharide units (starch, cellulose, glycogen). They are not sweet.
- Reducing sugars have a free aldehyde or ketone group (or a free hemiacetal) and reduce Tollens' and Fehling's reagents. All monosaccharides, maltose and lactose are reducing; sucrose is non-reducing.
3. Glucose
- Preparation: hydrolysis of sucrose (gives glucose + fructose) or starch (commercially, with dilute H₂SO₄ at 393 K, 2–3 atm).
- Evidence for the open-chain structure: molecular formula C₆H₁₂O₆; prolonged heating with HI gives n-hexane (straight chain of six carbons); forms an oxime with hydroxylamine and a cyanohydrin with HCN (carbonyl group); bromine water oxidises it to gluconic acid (–CHO present); acetylation gives a pentaacetate (five –OH groups); nitric acid oxidises it to saccharic acid (a primary –OH also present).
- Limitations of the open-chain structure: glucose does not give Schiff's test or the NaHSO₃ addition product, its pentaacetate does not react with hydroxylamine, and it exists in two forms (α and β) — all pointing to a cyclic structure.
- Cyclic structure: the –OH at C-5 adds to the –CHO at C-1, forming a six-membered pyranose ring (a hemiacetal). The two forms, α and β, differ only in the configuration at C-1 and are called anomers.
- Mutarotation: in water, α- and β-glucose interconvert through the open chain until an equilibrium specific rotation (+52.5°) is reached.
- Glucose is D-(+)-glucose: "D" describes the configuration (like D-glyceraldehyde), "(+)" describes rotation to the right.
- Fructose is a ketohexose that forms a five-membered furanose ring. It is laevorotatory, D-(−)-fructose.
4. Disaccharides and Polysaccharides
| Sugar | Units | Linkage | Reducing? | NEET Note |
|---|---|---|---|---|
| Sucrose | α-D-glucose + β-D-fructose | C1 (glucose) – C2 (fructose) | No | Hydrolysis gives invert sugar (laevorotatory) |
| Maltose | α-D-glucose × 2 | α(1→4) | Yes | From starch by amylase |
| Lactose | β-D-galactose + β-D-glucose | β(1→4) | Yes | Milk sugar |
| Starch | α-D-glucose | Amylose α(1→4) unbranched (15–20%); amylopectin α(1→4) with α(1→6) branches (80–85%) | No | Plant storage; blue with iodine |
| Cellulose | β-D-glucose | β(1→4), straight chains | No | Plant cell walls; humans cannot digest it |
| Glycogen | α-D-glucose | Like amylopectin but more branched | No | Animal starch in liver and muscle |
5. Amino Acids and Proteins
- α-Amino acids have –NH₂ and –COOH on the same carbon. All except glycine are optically active (L-configuration in proteins).
- Essential amino acids must come from food (e.g. valine, leucine, isoleucine, lysine, threonine, methionine, phenylalanine, tryptophan, histidine, arginine). Non-essential ones are made by the body.
- In solution, amino acids exist as zwitterions (H₃N⁺–CHR–COO⁻), so they have high melting points and behave like salts. At the isoelectric point the net charge is zero.
- Amino acids join by peptide bonds (–CO–NH–), formed with the loss of water. A dipeptide has one peptide bond; a chain of n amino acids has n − 1.
| Level | What it describes | Held by |
|---|---|---|
| Primary | Sequence of amino acids | Peptide (covalent) bonds |
| Secondary | α-helix or β-pleated sheet | Hydrogen bonds between C=O and N–H |
| Tertiary | Overall 3-D folding (fibrous or globular) | H-bonds, disulphide bridges, van der Waals, ionic attractions |
| Quaternary | Arrangement of two or more polypeptide subunits | Same as tertiary (e.g. haemoglobin, 4 subunits) |
- Fibrous proteins (keratin, myosin) are insoluble; globular proteins (insulin, albumin) are soluble.
- Denaturation: heat or change of pH destroys secondary and tertiary structure (primary structure stays). Examples: boiling an egg (albumin coagulates), curdling of milk.
- Enzymes are mostly globular proteins that act as highly specific biological catalysts, lowering activation energy (e.g. maltase, invertase, zymase).
6. Vitamins and Hormones
| Vitamin | Solubility | Deficiency disease |
|---|---|---|
| A (retinol) | Fat | Night blindness, xerophthalmia |
| B₁ (thiamine) | Water | Beri-beri |
| B₂ (riboflavin) | Water | Cheilosis (cracked lips), digestive problems |
| B₆ (pyridoxine) | Water | Convulsions |
| B₁₂ (cyanocobalamin, contains Co) | Water | Pernicious anaemia |
| C (ascorbic acid) | Water | Scurvy (bleeding gums) |
| D | Fat | Rickets (children), osteomalacia (adults) |
| E | Fat | Fragile red blood cells, muscle weakness |
| K | Fat | Slow blood clotting |
Fat-soluble vitamins (A, D, E, K) are stored in the liver and fatty tissue; water-soluble vitamins (B group, C) must be taken regularly because they are excreted in urine (B₁₂ is an exception and can be stored in the liver). Hormones are chemical messengers: steroids (oestrogen, testosterone), polypeptides (insulin, glucagon) and amino-acid derivatives (thyroxine, adrenaline).
7. Nucleic Acids
- Nucleic acids are polymers of nucleotides. Each nucleotide = pentose sugar + nitrogenous base + phosphate. A nucleoside = sugar + base (no phosphate).
- Nucleotides are joined by phosphodiester linkages between C-5′ and C-3′ of the sugars.
- DNA: sugar is 2-deoxyribose; bases A, G, C and T; double helix (Watson and Crick) with A=T (two H-bonds) and G≡C (three H-bonds); carries genetic information and replicates.
- RNA: sugar is ribose; bases A, G, C and U (uracil replaces thymine); usually single-stranded; types mRNA, rRNA and tRNA; involved in protein synthesis.
8. Formula and Fact Master Table
| Item | Formula / fact |
|---|---|
| Glucose, fructose | C₆H₁₂O₆ (aldohexose, ketohexose) |
| Sucrose, maltose, lactose | C₁₂H₂₂O₁₁ |
| Starch, cellulose | (C₆H₁₀O₅)ₙ |
| Peptide bonds | n amino acids ⇒ n − 1 peptide bonds |
| Glucose + HI (heat) | → n-hexane |
| Glucose + Br₂ water | → gluconic acid |
| Glucose + HNO₃ | → saccharic (glucaric) acid |
| Base pairing | A=T (2 H-bonds), G≡C (3 H-bonds) |
9. Master Summary Table
| Topic | Key fact | NEET Note |
|---|---|---|
| Reducing sugars | Free carbonyl/hemiacetal | Sucrose is non-reducing |
| Anomers | Differ at C-1 | α and β glucose |
| Invert sugar | Sucrose hydrolysis | Sign of rotation changes to (−) |
| Starch vs cellulose | α vs β linkage | Cellulose indigestible for humans |
| Protein structure | Four levels | Denaturation keeps primary |
| Vitamins | A, D, E, K fat-soluble | C deficiency ⇒ scurvy |
| DNA vs RNA | Deoxyribose/T vs ribose/U | G≡C has 3 H-bonds |
10. Mind Map
- Glucose structure
- Disaccharides
- Polysaccharides
- Amino acids
- Four levels
- Enzymes
- Fat / water soluble
- Deficiencies
- Nucleotides
- DNA
- RNA
11. Tips and Tricks
- "KEDA" is a quick reminder that vitamins K, E, D and A are fat-soluble.
- Glycogen is more branched than amylopectin, which is more branched than amylose (unbranched).
- A nucleotide has a phosphate; a nucleoside does not.
- Only glycine lacks a chiral carbon among the common α-amino acids.
- Calling sucrose a reducing sugar.
- Thinking denaturation breaks peptide bonds.
- Putting thymine in RNA or uracil in DNA.
- Calling α- and β-glucose enantiomers (they are anomers).
12. Important One-liners
- Glucose is the most abundant monosaccharide; cellulose is the most abundant organic compound.
- Invert sugar is an equimolar mixture of glucose and fructose.
- Starch gives a blue colour with iodine.
- Insulin is a protein hormone.
- Vitamin B₁₂ contains cobalt.
- Deficiency of vitamin C causes scurvy.
- In DNA, adenine always pairs with thymine.
13. NEET-pattern Questions with Detailed Answers
These questions are original and based on concepts that NEET UG asks again and again.
Which of the following is a non-reducing sugar?
- Glucose
- Maltose
- Sucrose
- Lactose
Show solution
In sucrose, the glycosidic bond joins C-1 of glucose to C-2 of fructose. Both reducing groups are used up, so no free hemiacetal is left. Glucose, maltose and lactose all have a free reducing group.
Answer: (C)
Deficiency of vitamin D causes:
- Scurvy
- Rickets
- Beri-beri
- Night blindness
Show solution
Vitamin D is needed to absorb calcium, so its deficiency softens the bones, causing rickets in children. Scurvy is caused by lack of vitamin C, beri-beri by lack of B₁, and night blindness by lack of vitamin A.
Answer: (B)
Glucose on prolonged heating with HI gives:
- Gluconic acid
- n-Hexane
- Saccharic acid
- Sorbitol
Show solution
HI reduces all the oxygen-containing groups, giving n-hexane. This shows that glucose has a straight chain of six carbons. Gluconic acid comes from bromine water and saccharic acid from HNO₃.
Answer: (B)
α-D-glucose and β-D-glucose differ in configuration at:
- C-1
- C-2
- C-4
- C-5
Show solution
They are anomers, which differ only at the anomeric carbon C-1 formed when the ring closes. Glucose and mannose differ at C-2, and glucose and galactose differ at C-4.
Answer: (A)
The α-helix of proteins is an example of:
- Primary structure
- Secondary structure
- Tertiary structure
- Quaternary structure
Show solution
The α-helix and the β-pleated sheet are secondary structures, held by hydrogen bonds between the C=O and N–H groups of the backbone. The primary structure is the sequence of amino acids.
Answer: (B)
Which base is present in RNA but not in DNA?
- Adenine
- Guanine
- Uracil
- Thymine
Show solution
RNA uses uracil in place of thymine. Adenine and guanine are found in both DNA and RNA, and thymine is found only in DNA.
Answer: (C)
Amylose and cellulose are both polymers of D-glucose. They differ in that:
- Amylose has β(1→4) and cellulose α(1→4) links
- Amylose has α(1→4) and cellulose β(1→4) links
- Both are branched
- Cellulose is soluble in water
Show solution
Amylose is an α(1→4) unbranched chain. Cellulose has β(1→4) links, which give straight chains that pack into fibres. Human enzymes cannot break β links. (A) reverses the two linkages. Neither polymer is branched (amylopectin is), and cellulose is insoluble in water.
Answer: (B)
A tripeptide made of glycine, alanine and valine contains how many peptide bonds, and how many different linear tripeptides are possible if each amino acid is used once?
- 2 bonds; 6 tripeptides
- 3 bonds; 6 tripeptides
- 2 bonds; 3 tripeptides
- 3 bonds; 9 tripeptides
Show solution
Three amino acids are joined by 3 − 1 = 2 peptide bonds. The sequence matters (the N-terminal end differs from the C-terminal end), so there are 3! = 6 possible orders.
Answer: (A)
14. Quick Revision Summary
- Mono-, oligo-, polysaccharides; sucrose non-reducing; invert sugar laevorotatory.
- Glucose: open chain (HI → hexane) and pyranose ring; α/β anomers at C-1; mutarotation.
- Starch α (amylose + amylopectin), cellulose β(1→4), glycogen highly branched.
- Proteins: peptide bonds; 1°–4° structure; denaturation keeps sequence; enzymes are catalysts.
- Vitamins A, D, E, K fat-soluble; DNA (deoxyribose, T) vs RNA (ribose, U); A=T, G≡C.