A Level Biology: Biological Molecules — Revision Notes (Cambridge 9700)

Related: Topic 2 study guide

Condensed, exam-focused notes for Topic 2 of Cambridge AS & A Level Biology (9700), 2025-2027 series.

2.1 Testing for biological molecules

Test	Substance	Positive result
Benedict’s	Reducing sugars	Brick-red precipitate on heating
Iodine	Starch	Blue-black colour
Biuret	Proteins	Purple/lilac colour
Emulsion (ethanol)	Lipids	White emulsion forms

• Always describe the positive result as a specific colour or observable change, not “it changes colour.”
• A control is needed to confirm the observed change is due to the substance, not the reagent alone.

2.2 Carbohydrates and lipids

• Monosaccharides join via glycosidic bonds (condensation reaction) to form disaccharides and polysaccharides; hydrolysis reverses this.
• Polysaccharides (e.g. starch, glycogen) are compact, branched/coiled glucose stores — structure directly explains function.
• Triglycerides: largely non-polar structure makes them efficient energy stores because they don’t attract water.
• Practise drawing and labelling a glycosidic bond and an ester bond forming through condensation.

2.3 Proteins — four levels of structure

• Primary: amino acid sequence.
• Secondary: alpha helices/beta pleated sheets, held by hydrogen bonding along the backbone.
• Tertiary: further folding, stabilised by ionic bonds, hydrogen bonds, disulfide bonds, and hydrophobic interactions between R groups.
• Quaternary: applies only where a protein has more than one polypeptide chain.
• Name the specific bonds stabilising tertiary structure — “forces between amino acids” is too vague for top marks.
• Globular proteins (e.g. haemoglobin) — compact, roughly spherical, generally soluble, precise structure suited to a specific role such as oxygen transport.
• Fibrous proteins (e.g. collagen) — long, insoluble strands/fibres, repeating structure suited to mechanical strength.

2.4 Water

• High specific heat capacity → thermal stability of aquatic habitats.
• High latent heat of vaporisation → evaporative cooling by sweating or transpiration.
• Effective solvent → transport of dissolved substances in blood plasma.
• Every property traces back to hydrogen bonding between polar water molecules — always link property → hydrogen bonding → biological consequence.

Exam technique for this topic

The single highest-value habit in this topic is always pairing a structural feature with its functional consequence, never stopping at naming a molecule or reaction alone. For example, don’t just state that cellulose is “strong” — link that strength to straight chains of beta-glucose held together by many hydrogen bonds between adjacent chains, forming microfibrils. This structure-to-function reasoning is not unique to this topic: it recurs directly in Enzymes (how a denatured active site loses function) and Cell membranes and transport (how lipid structure enables selective permeability), so building the habit here pays off well beyond this topic’s own questions. Expect practical-style questions asking you to interpret an unfamiliar biochemical test result or design a simple experiment applying the tests in 2.1, not just recall reagents and colours from memory.

Worked example: interpreting an unfamiliar test result

A student tests an unknown solution and observes a blue-black colour with iodine solution, but no colour change with Benedict’s test even after heating. A strong interpretation reasons through both results together rather than reporting them in isolation: the blue-black colour with iodine confirms the presence of starch, while the absence of a colour change with Benedict’s test confirms the absence of reducing sugars — together these results are consistent with the sample containing starch (a polysaccharide) without any free reducing sugar present, since starch itself does not react with Benedict’s reagent unless first hydrolysed into its constituent glucose units. This kind of two-result-together reasoning, rather than treating each test as a separate fact, is exactly the skill practical-style questions on this topic are designed to assess.

Structure-to-function: a worked comparison

Compare a triglyceride and a phospholipid to see structure-to-function reasoning in practice. Both share a glycerol backbone, but a triglyceride has three fatty acid tails (making it wholly non-polar, hydrophobic, and therefore an efficient, compact energy store that doesn’t interact with water), while a phospholipid replaces one fatty acid tail with a phosphate group (creating a molecule with both a polar, hydrophilic head and non-polar, hydrophobic tails). This dual nature is precisely why phospholipids, and not triglycerides, form the bilayer structure of cell membranes: in water, the hydrophilic heads naturally orient outward and the hydrophobic tails inward, spontaneously forming the bilayer that underlies membrane structure — a direct example of how a small structural difference (one tail replaced by a phosphate group) produces a completely different biological function.

Self-test

1. What is the positive result for the Biuret test, and what does it test for?
2. Why is a control needed alongside a biochemical test?
3. Name the bonds/interactions that stabilise tertiary protein structure.
4. When does quaternary structure apply?
5. Give one property of water and the biological consequence it produces.

Answers: 1. Purple/lilac colour; tests for proteins. 2. To confirm the observed change is due to the substance being tested, not the reagent reacting alone. 3. Ionic bonds, hydrogen bonds, disulfide bonds, hydrophobic interactions. 4. Only when a protein consists of more than one polypeptide chain. 5. E.g. high specific heat capacity → thermal stability of aquatic habitats (or latent heat of vaporisation → evaporative cooling by sweating/transpiration; solvent property → transport of dissolved substances in blood).

Why this topic underpins the rest of the course

Because Biological molecules supplies the chemistry every later topic assumes is already secure, gaps left here tend to resurface unexpectedly much later in the course — a shaky grasp of hydrogen bonding, for instance, makes both protein structure (Topic 2) and DNA base pairing (Nucleic acids, much later in the syllabus) harder to understand properly. Treating this topic as a one-off unit to revise once and move past, rather than as foundational vocabulary the rest of AS and A Level Biology depends on, is one of the more common and avoidable long-term mistakes students make early in the course.