Biomolecules

Biomolecules

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Biomolecules are the organic compounds synthesised by living cells, grouped into four families: carbohydrates, proteins, lipids, and nucleic acids, plus the accessory micronutrients vitamins.

• Carbohydrates fit the empirical pattern Cₓ(H₂O)ᵧ (e.g., glucose C₆H₁₂O₆, fructose C₆H₁₂O₆, sucrose C₁₂H₂₂O₁₁).
• Proteins are polymers of α-amino acids joined by peptide (amide) bonds; a chain of n residues has (n − 1) peptide bonds.
• Lipids are glycerol esters of fatty acids; oils are unsaturated, fats are largely saturated.
• Nucleic acids (DNA/RNA) are polymers of nucleotides — pentose sugar + phosphate + nitrogenous base.

NEET trap alert: Sucrose is a non-reducing sugar because both anomeric carbons are locked in the glycosidic linkage; do not classify it as reducing.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Carbohydrates

Carbohydrates are polyhydroxy aldehydes (aldoses) or ketones (ketoses). Glucose is an aldohexose; fructose is a ketohexose — both have the molecular formula C₆H₁₂O₆ but differ in the carbonyl position. They show optical activity and mutarotation through the cyclic α- and β-anomers. Monosaccharides join via glycosidic bonds; sucrose (glucose + fructose), maltose, and lactose are common disaccharides.

Mnemonic: “Fructose — Fruity smell” = ketohexose (fructose is a ketone).

Proteins

α-Amino acids have the general formula H₂N–CHR–COOH; the R-group decides whether the side chain is acidic, basic, polar, or non-polar. Polymerisation releases water and forms the peptide bond (–CO–NH–). Structure exists at four levels: primary (covalent sequence), secondary (α-helix / β-pleated sheet held by hydrogen bonds), tertiary (3-D folding driven by R-group interactions), and quaternary (assembly of subunits, e.g., haemoglobin).

Common Detection Tests

Biomolecule / Feature	Reagent	Positive result
Carbohydrates (general)	Molisch’s reagent	Violet ring at junction
Reducing sugars	Benedict’s / Fehling’s / Tollen’s	Red ppt / silver mirror
Peptide bonds	Biuret reagent	Violet / purple colour
Free amino acids	Ninhydrin	Blue-purple colour (Ruhemann’s purple)

NEET Question Patterns

Expect 1–2 assertion-reason or direct MCQs asking students to identify the anomeric carbon (C-1 of glucose), count peptide bonds, distinguish reducing from non-reducing sugars, or name the deficiency disease of a given vitamin (scurvy ← Vit C, rickets ← Vit D, beriberi ← Vit B₁).

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Lipids and Nucleic Acids in Depth

Simple lipids are triesters of glycerol and long-chain fatty acids (triglycerides). Saponification with NaOH yields glycerol and sodium salts of fatty acids (soaps) — the process that defines a lipid as an ester of a fatty acid. Phospholipids carry a polar head and two hydrophobic tails, the basis of membrane bilayers. Cholesterol belongs to sterols.

A nucleotide = pentose sugar (ribose in RNA, 2′-deoxyribose in DNA) + phosphate at C-5′ + a nitrogenous base (A, G, C, and T in DNA; U replaces T in RNA) attached at C-1′. Nucleotides join through 3′→5′ phosphodiester bonds, giving nucleic acids their directional backbone.

Worked Micro-Example

A tripeptide has the sequence Ala–Gly–Ser. Number of amino acid residues n = 3, so peptide bonds = 3 − 1 = 2. Approximate molecular mass ≈ 3 × 110 Da = 330 Da (using the average residue mass), useful for gel-electrophoresis size estimation.

High-Yield Distinctions

Pair	Easy Confusion	Correct Line
Glucose vs Fructose	Same formula, different group	Glucose = aldose (CHO at C-1); Fructose = ketose (C=O at C-2)
Denaturation vs Hydrolysis	Both alter proteins	Denaturation = loss of 3-D shape (no bond cleavage); Hydrolysis = peptide-bond cleavage
Saturated vs Unsaturated fats	Both are triglycerides	Saturated = no C=C, solid (fats); Unsaturated = ≥1 C=C, liquid (oils)
DNA vs RNA bases	A, G, C common to both	DNA has T; RNA has U; both carry A, G, C

Practice Prompt 1: “How many peptide bonds are present in a hexapeptide? State the rule and justify using general formula n − 1.” Practice Prompt 2: “Fructose is a ketohexose, yet it gives a positive Fehling’s test. Reconcile this with the rule that only aldoses are reducing.” Common Mistake: Writing the carbohydrate formula as CₓHᵧO_z instead of the characteristic Cₓ(H₂O)ᵧ form, and forgetting that acetic acid (C₂H₄O₂) also fits Cₓ(H₂O)ᵧ but is not a sugar.

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