A Level Biology: Biological Molecules (Cambridge 9700)

This guide covers Topic 2 Biological molecules, an AS Level topic for Cambridge International AS & A Level Biology 9700, 2025–2027 series. AS Level candidates study Topics 1–11 plus practical skills; A Level candidates study all 19 topics plus practical skills.

Where this fits in 9700

Biological molecules follows directly on from Cell structure, and it supplies the chemistry — carbohydrates, lipids, proteins and water — that every later topic in the course assumes candidates already understand. Enzymes (Topic 3, the next topic) is built entirely around proteins whose structure this topic introduces; cell membranes and transport (Topic 4) depend on the properties of lipids and water covered here; and the biochemical tests introduced in this topic recur throughout the practical assessment components of the course whenever an unknown sample needs identifying.

Syllabus coverage

CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — TOPIC 2 BIOLOGICAL MOLECULES

• 2.1 Testing for biological molecules — the standard biochemical tests used to identify the presence of reducing and non-reducing sugars, starch, proteins and lipids in a sample
• 2.2 Carbohydrates and lipids — the structure and function of monosaccharides, disaccharides and polysaccharides, and the structure and function of triglycerides and phospholipids
• 2.3 Proteins — the structure of amino acids and the primary, secondary, tertiary and quaternary levels of protein structure, and how structure relates to function, including the distinction between globular proteins (such as haemoglobin) and fibrous proteins (such as collagen)
• 2.4 Water — the properties of water (as a solvent, its high specific heat capacity, latent heat of vaporisation and other properties) and their significance for living organisms

How to approach it

The biochemical tests in 2.1 are graded almost entirely on procedural accuracy — knowing exactly which reagent is added, what a positive result looks like, and why a control is needed — so this sub-topic rewards rote practice of the method and expected observation for each test (Benedict’s test for reducing sugars, the iodine test for starch, the biuret test for proteins, and the emulsion test for lipids) rather than a general sense of “there’s a colour change.” Being able to describe what distinguishes a positive result from a negative one in precise, observable terms (a colour, not just “it changes”) is routinely worth more than naming the reagent alone.

For carbohydrates and lipids (2.2), the strongest answers connect structure to function directly: knowing that a polysaccharide such as starch or glycogen is a compact, branched or coiled store of glucose because of its glycosidic bonds, or that a triglyceride’s largely non-polar structure makes it an efficient energy store precisely because it does not attract water, is worth more than simply naming the molecule. Drawing and correctly labelling a glycosidic or ester bond forming through condensation, and recognising the reverse hydrolysis reaction, is a skill worth practising directly rather than only recognising it when shown a diagram.

Proteins (2.3) is often the most conceptually demanding sub-topic here because it requires holding four distinct levels of structure in mind at once and understanding how each depends on the one before it — primary structure (the amino acid sequence) determines secondary structure (alpha helices and beta pleated sheets, held by hydrogen bonding along the backbone), which in turn folds into a tertiary structure stabilised by interactions between R groups (ionic bonds, hydrogen bonds, disulfide bonds and hydrophobic interactions), with quaternary structure applying only where a protein is made of more than one polypeptide chain. Being able to state which specific bonds or interactions stabilise tertiary structure, rather than the vaguer answer “forces between amino acids,” is a common differentiator between mid-band and high-band answers, and this level of structural detail is exactly what Enzymes (Topic 3) builds on when explaining how a change in conditions denatures an enzyme’s active site. Proteins also fall into two broad structural classes worth distinguishing directly: globular proteins, such as haemoglobin, are compact, roughly spherical and generally water-soluble, with a precise tertiary (and, for haemoglobin, quaternary) structure suited to a specific biochemical role such as oxygen transport; fibrous proteins, such as collagen, form long, insoluble strands or fibres in which repeating sequences produce a structure suited to providing mechanical strength, as in connective tissue. Being able to name one example of each class and link its structure to its role, rather than treating “globular” and “fibrous” as unexplained labels, is exactly the kind of structure-to-function reasoning this sub-topic rewards.

Water (2.4) is a short sub-topic but a frequently underprepared one: be ready to explain properties such as water’s high specific heat capacity, its high latent heat of vaporisation, and its effectiveness as a solvent, each in terms of hydrogen bonding between polar water molecules, and to link each property to a concrete biological consequence (thermal stability of aquatic habitats, evaporative cooling by sweating or transpiration, or transport of dissolved substances in blood plasma) rather than describing the property in isolation from any organism.

Because this topic is assessed alongside practical skills as well as written theory, students should expect questions that ask them to interpret the results of an unfamiliar biochemical test or to design a simple experiment applying the tests in 2.1, not just recall the tests’ names and reagents from memory.

A common mistake across the whole topic is answering at the level of naming a molecule or reaction without going the extra step to explain why its structure produces the property being asked about — for instance, stating that cellulose is “strong” without linking that strength to the straight chains of beta-glucose held together by many hydrogen bonds between adjacent chains, forming microfibrils. Building the habit of always pairing a structural feature with its functional consequence in every answer, on every sub-topic in this section, is one of the most transferable exam skills this topic teaches, since the same structure-to-function reasoning recurs in Enzymes, Cell membranes and transport, and well beyond AS Level into topics such as Nucleic acids and protein synthesis.

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027 (Version 1, September 2022) — cambridgeinternational.org.

Related resources

• Cell Structure — the first topic of 9700, a prerequisite for this topic