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Playbook

Biochemistry

4 q · 0% HARD in 10 years. Food Spoilage — Rancidity and Browning (2), Anaerobic Respiration and Fermentation (1), Protein Structure (1 · peptide-bond sequence). Verify strand because the recurring question shape is 'which of the following statements about X is/are correct?'. Tiny chapter — read once in 20 min, don't over-invest.

questions in the bank
4
tagged HARD
0%
subtopic(s)
3
worked examples
2

When you’ll see it

A rancidity / browning question (fat-and-oil spoilage), an anaerobic-respiration / fermentation question (ethanol + CO₂ from yeast), or a protein-structure question (peptide bonds, primary/secondary/tertiary structure).

How this chapter is tested

4 q in 10 years, ZERO HARD. The smallest NDA Biology chapter — tied with Genetics and Evolution. Verify strand because the recurring shape is 'which of the following statements about X is/are correct?'. Food Spoilage — Rancidity and Browning (2 q): rancidity = OXIDATION of fats + oils in food → off-smell + off-taste. Caused by air exposure + light + heat. Prevention: refrigeration, packaging in opaque containers, antioxidants (BHA, BHT), packaging in inert gas (N₂). Browning = Maillard reaction (sugars + amino acids react when heated — that's why baked bread + grilled meat brown).

Anaerobic Respiration and Fermentation (1 q): cellular respiration without oxygen. In yeast (Saccharomyces) → ethanol + CO₂ (alcoholic fermentation, used in baking + brewing). In animal muscle under O₂-starvation → lactic acid (causes muscle fatigue). Net ATP: anaerobic = 2 ATP per glucose; aerobic = 36–38 ATP per glucose. So anaerobic is much less efficient — that's why organisms switch to aerobic when O₂ is available.

Protein Structure (1 q): primary structure = linear sequence of amino acids joined by PEPTIDE BONDS (C-N bond between -COOH of one and -NH₂ of next). Secondary = α-helix or β-sheet (H-bonded). Tertiary = 3D folding (R-group interactions). Quaternary = multi-subunit assembly (haemoglobin = 4 subunits). The 2025 PYQ tests 'linear sequence of amino acids linked by peptide bonds best represents the' → answer = primary structure of protein.

The sub-skills

The rules and habits that decide whether you get a question right.

  • Rancidity mechanism + prevention

    Rancidity = oxidation of fats/oils → off-smell + taste. Causes: O₂ exposure + light + heat + moisture. Prevention: refrigeration, opaque/airtight packaging, antioxidants (BHA, BHT, vitamin E), inert gas (N₂) flushing. The 2025 PYQ tests this: 'what happens when fat-and-oil-containing foods are left out long' = rancidity.

  • Anaerobic respiration types

    Yeast / fungi: glucose → ethanol + CO₂ + 2 ATP (alcoholic fermentation, used in baking + brewing + alcohol production). Animal muscle: glucose → lactic acid + 2 ATP (lactic fermentation, builds up during intense exercise → muscle fatigue). Both produce only 2 ATP vs aerobic's 36–38.

  • Protein structural hierarchy

    PRIMARY: linear amino acid sequence joined by PEPTIDE BONDS (C-N covalent bonds). SECONDARY: α-helix or β-sheet, held by H-bonds between backbone atoms. TERTIARY: overall 3D fold from R-group interactions (H-bonds, ionic bonds, disulfide bridges, hydrophobic). QUATERNARY: multi-subunit assembly (haemoglobin = 4 subunits).

2 worked examples from the bank

Real past-year questions illustrating the playbook. Click to reveal options + solution.

Example 1BiochemistryEASY
What happens when the fat and oil containing food materials are left outside for a long time?

[Q109 · Apr · 2025]

Example 2BiochemistryEASY
A chain of peptide containing linear sequences of amino acid linked by peptide bonds best represent the

[Q107 · Apr · 2025]

Traps to expect

Distractor shapes specific to this chapter. The page-wide Traps section covers the bank-level patterns.

  • Rancidity is microbial spoilage

    Rancidity is CHEMICAL spoilage (oxidation of fats), NOT microbial. Microbial spoilage is what happens to milk (souring by Lactobacillus) or bread (mould). Distractor conflates the two types of food spoilage. Refrigeration slows both but the mechanisms are different.

  • Anaerobic respiration produces more ATP

    Anaerobic respiration produces ONLY 2 ATP per glucose. Aerobic produces 36–38 ATP. Distractor swaps them or says 'anaerobic is more efficient'. Aerobic is much more efficient — that's evolutionary why organisms with O₂ access use it.

Drill every biochemistry question

4 questions from the bank, scoped to 3 bundled subtopics.

Drill the 4 questions

Related playbooks

Often paired with this one — drill these next if you found the worked examples above tractable.