Biomolecules

Biomolecules

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Biomolecules are organic compounds synthesised by living cells, classified into four families: carbohydrates, proteins, lipids, and nucleic acids. Glucose, the reference monosaccharide, has molecular formula C₆H₁₂O₆; sucrose (a disaccharide) is C₁₂H₂₂O₁₁; amino acids share the general form H₂N–CHR–COOH and join via a peptide bond (–CO–NH–) with loss of water. For MDCAT, focus on (1) identifying mono-, di-, and polysaccharides, (2) knowing the four levels of protein structure and the difference between denaturation and hydrolysis, and (3) distinguishing DNA (deoxyribose, A-T-G-C, double-stranded) from RNA (ribose, A-U-G-C, single-stranded).

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Carbohydrates

Polyhydroxy aldehydes or ketones with formula Cₙ(H₂O)ₙ. Monosaccharides (glucose, fructose, galactose) are the monomers. Disaccharides form by glycosidic linkage between two monosaccharides: sucrose = glucose + fructose, maltose = glucose + glucose, lactose = glucose + galactose. Polysaccharides are long chains: starch and glycogen use α-1,4 glycosidic bonds (energy storage), cellulose uses β-1,4 bonds (structural). Tests: Benedict’s reagent (reducing sugars → brick-red Cu₂O); iodine (starch → blue-black).

Proteins

Polymers of α-amino acids linked by peptide bonds (–CO–NH–). Four structural levels:

• Primary – linear sequence of amino acids, held by covalent peptide bonds.
• Secondary – α-helix or β-pleated sheet, stabilised by hydrogen bonds between backbone C=O and N–H.
• Tertiary – 3D folding held by H-bonds, ionic bonds, hydrophobic interactions, disulfide bridges.
• Quaternary – assembly of multiple polypeptide subunits (e.g., haemoglobin, 4 chains).

Denaturation disrupts H-bonds, ionic interactions, and disulfide bridges but does not break peptide bonds; the primary sequence is preserved. Biuret test (Cu²⁺ in alkali) → violet/purple confirms peptide bonds. Ninhydrin test detects free amino groups.

Lipids

Hydrophobic biomolecules. Simple lipids are triglycerides = glycerol + 3 fatty acids, joined by ester bonds. Saponification (NaOH/KOH hydrolysis) yields glycerol and soap (sodium salts of fatty acids). Phospholipids (phosphate group + 2 fatty acids + glycerol) form bilayers in cell membranes. Sterols such as cholesterol are rigid, ring-based lipids.

Nucleic Acids

Polymers of nucleotides (phosphate + pentose sugar + nitrogenous base) joined by phosphodiester bonds (sugar–O–PO₂–O–sugar). DNA: deoxyribose, bases A, T, G, C, double helix. RNA: ribose, bases A, U, G, C, single-stranded.

Enzymes

Globular protein biocatalysts lowering activation energy. Affected by temperature, pH, substrate concentration, and inhibitors (competitive blocks active site; non-competitive binds elsewhere).

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Edge Cases and Common Confusions

α vs β glycosidic linkages are the most-tested distinction in MDCAT Biomolecules. α-1,4 (maltose, starch, glycogen) is digestible by human enzymes because salivary and pancreatic amylase recognise the α geometry; β-1,4 (cellulose) is not hydrolysable by humans — we lack cellulase, which is why dietary fibre passes undigested. Confusing these is a frequent MCQ trap.

Denaturation vs hydrolysis: denaturation is reversible for many proteins once the denaturing agent (heat, pH change, heavy metals, urea) is removed because only non-covalent interactions are disrupted. Hydrolysis breaks the peptide bond itself and is permanent, producing free amino acids — this is irreversible. Exam questions often phrase both as “protein is broken down”; only hydrolysis actually cleaves bonds.

Saturated vs unsaturated fatty acids: saturated (no C=C, e.g., palmitic acid C₁₅H₃₁COOH) are solid at room temperature and come mainly from animal fats; unsaturated (one or more C=C, e.g., oleic acid) are typically liquid plant oils and lower melting points due to kinks preventing tight packing.

Worked Quick Example

How many peptide bonds in a tetrapeptide (4 amino acids)? Each linkage between consecutive residues = 1 bond, so a tetrapeptide has 3 peptide bonds and 3 water molecules released during synthesis (n–1 rule). Conversely, hydrolysis of a decapeptide requires 9 water molecules to free all 10 amino acids.

Exam Strategy for MDCAT

Biomolecules contributes ~3% of the chemistry paper — usually 2–3 MCQs. Highest-yield topics: (a) Benedict’s, Biuret, and iodine tests and the functional group each detects; (b) DNA/RNA base-pairing (A=T, G≡C in DNA; A=U in RNA); (c) identifying levels of protein structure from a description; (d) glycogen (animal) vs starch (plant). Time per question: ~60 seconds.

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