Large molecules and their building blocks
The constructions you must state on demand:
| Large molecule | Built from |
|---|---|
| Starch, glycogen, cellulose | Glucose |
| Proteins | Amino acids |
| Fats and oils (lipids) | Fatty acids and glycerol |
| DNA | Nucleotides (two strands coiled into a double helix, with paired bases A–T and C–G) |
Notice that starch, glycogen and cellulose are all made of glucose. The difference is the organism and the job: starch is the plant storage carbohydrate, glycogen the animal storage carbohydrate, cellulose the structural one in plant walls. 'Name the carbohydrate stored in the liver' (glycogen) is a recurring one-marker that students answer with 'starch' at their peril. At Extended level, the DNA base-pairing rule (A with T, C with G) is a free mark whenever it appears.
The four food tests: reagents, conditions, colours
Write each test as reagent → condition → negative colour → positive colour. All four parts can carry marks:
| Test for | Reagent | Method point | Colour change (negative → positive) |
|---|---|---|---|
| Starch | Iodine solution | Add directly | Brown/orange → blue-black |
| Reducing sugars | Benedict's solution | Heat (water bath ~80°C) | Blue → green → orange → brick-red |
| Protein | Biuret solution | Add directly | Blue → purple/violet |
| Fats | Ethanol, then water | Shake with ethanol, pour into water | Clear → white/milky emulsion |
The two most-dropped marks: forgetting that Benedict's needs heating (no heat, no mark) and giving only the end colour. 'Turns brick-red' scores half; 'changes from blue to brick-red' scores full. For the ethanol test, the paying phrase is 'milky/cloudy white emulsion'.
Water as a solvent: the two-sentence Extended point
The syllabus wants water's importance stated as a solvent: substances dissolve in water, so it is the medium for digestion, for transport in blood plasma and in xylem/phloem sap, and for excretion of urea in urine. Questions phrase this as 'explain the importance of water in the transport of substances'. Answer with a named dissolved substance in a named fluid ('glucose dissolves in blood plasma and is carried to respiring cells') rather than the vague 'water carries things around the body', which earns nothing.
How this topic actually appears in papers
Biological molecules rarely gets its own long question. Instead it ambushes you inside other topics: a Paper 6 question gives you an unknown food sample and asks you to plan the tests; an enzyme question asks what amylase breaks starch into (maltose. A reducing sugar, which is why Benedict's appears next); a diet question asks which test confirms protein in a food. So revise it as a toolkit: the four tests, the building blocks, and the storage carbohydrates. When planning a food-test investigation in Paper 6, always include the safety point (care heating / water bath rather than direct flame) and the control comparison. Both are standing marks in the practical mark schemes.