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.
Key terms to connect with this topic
Use these definitions inside explanations and questions rather than memorising them as isolated sentences.
- Amino acid: A small organic molecule that is the monomer of proteins. Twenty different amino acids exist, and they are joined together by peptide bonds during protein synthesis at ribosomes. The sequence of amino acids determines the shape and function of the protein.
- Cellulose: A complex carbohydrate (polysaccharide) made of long chains of beta-glucose molecules. It forms the main structural component of plant cell walls, providing rigidity and strength.
- Fatty acid: One of the two components of a lipid (fat or oil) molecule. Fatty acids are long hydrocarbon chains with a carboxyl group at one end. Three fatty acid molecules combine with one glycerol molecule to form a lipid.
- Glucose: A simple sugar (monosaccharide) with the formula C6H12O6 that serves as the primary substrate for respiration in cells. It is produced during photosynthesis and digestion of carbohydrates, and is transported in the blood to supply energy.
- Glycerol: A small organic molecule with three hydroxyl groups that forms the backbone of lipid molecules. One glycerol molecule combines with three fatty acid molecules to form a fat or oil.
- Glycogen: A complex carbohydrate (polysaccharide) used as the main energy storage molecule in animals and fungi. It is stored in the liver and muscles and can be rapidly broken down into glucose when energy is needed.
- Starch: A complex carbohydrate (polysaccharide) used as the main energy storage molecule in plants. It is made of long chains of alpha-glucose molecules and is insoluble in water, making it ideal for storage because it does not affect the water potential of cells.
Browse the full IGCSE Biology glossary when a related term needs checking.
Apply the topic in practical work
These practical guides show how the Biology appears in methods, variables, measurements, graphs and evaluation.
- Food Tests: Reagents, Methods and Colour Changes: Use Benedict’s plus heat for reducing sugar, iodine for starch, biuret for protein, ethanol then water for fat, and DCPIP decolourisation for vitamin C. State both the reagent and the positive colour change.
- Enzyme Rate Investigation With Amylase and Starch: Mix amylase and starch under controlled conditions, sample at timed intervals into iodine and record when starch is no longer detected. Change one factor, repeat each value and calculate rate as 1 ÷ time.