SEC G3 Combined Science Biology component K327/K328
B3: Biological Molecules
Connect nutrient roles, food tests, biological synthesis, and enzyme action to evidence.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Biological molecules support energy supply, storage, insulation, growth, and repair, while enzymes control reactions by binding specific substrates.
Roles and food tests
Carbohydrates provide an immediate energy source, fats provide insulation and long-term energy storage, and proteins support cell growth and repair.
For starch, add iodine in potassium iodide solution: a positive sample changes from yellow-brown to blue-black. For reducing sugar, add Benedict's solution and heat in a water bath: a positive sample changes from blue through green, yellow, or orange to a brick-red precipitate as concentration increases.
For protein, add Biuret solution: a positive sample changes from blue to purple. For fat, shake the sample with ethanol, then add water: a positive sample forms a cloudy white emulsion. Include a suitable negative control when carrying out the tests.
Building large molecules
Glucose units build cellulose, glycogen, and starch. Amino acids build polypeptides and proteins, while glycerol and fatty acids build lipids such as fats.
Name both the smaller units and the resulting large molecule. Do not treat all carbohydrates as though they have the same structure or role.
Enzyme specificity, temperature, and pH
In the lock-and-key hypothesis, an enzyme's active site is complementary to its substrate. Binding produces an enzyme-substrate complex, allowing product formation while the enzyme remains available for reuse.
Rate rises with temperature until an optimum, then falls as active-site shape is disrupted. Each enzyme also has a suitable pH range; extreme pH can alter the active site. To investigate either factor, use equal enzyme and substrate amounts, vary only temperature or pH, control the other factor, and measure a defined endpoint or product rate over equal intervals.
Formulae and relationships
This chapter is assessed mainly through models, field patterns and explanations. Build the causal chain before adding any calculation.
Worked examples
Example 1: Why does an enzyme-controlled reaction slow sharply above its optimum temperature?
- High temperature disrupts bonds maintaining enzyme shape.
- The active site is no longer complementary to the substrate.
- Fewer enzyme-substrate complexes form.
Answer: Denaturation changes the active site and reduces successful binding.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Explain Roles and food tests with the named terms, evidence, and causal links kept distinct.
- Use Building large molecules to interpret the evidence given and justify each conclusion.
- Apply Enzyme specificity, temperature, and pH to a new example, then check the conclusion against the information given.
Official outcome coverage
K327 B3: 5 mapped outcomes, references B3(a), B3(b), B3(c), B3(d), B3(e). Check the official K327 syllabus.
K328 B3: 5 mapped outcomes, references B3(a), B3(b), B3(c), B3(d), B3(e). Check the official K328 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Carry out Benedict's, iodine, biuret and ethanol-emulsion food tests with appropriate controls, or investigate an enzyme while changing one factor. Record reagent, condition and observation before inference and handle heating and ethanol safely.
Exam traps and retrieval check
Avoid these traps
- Using Benedict's solution without heating.
- Calling an enzyme a reactant that is consumed.
- Saying low temperature denatures the enzyme.
Check from memory
What positive result identifies starch?
A blue-black colour with iodine solution.
What are proteins built from?
Amino acids.
Why is an enzyme specific?
Its active site is complementary to particular substrate molecules.
Official Combined Science scope
This shared Combined Biology owner serves both K327 and K328. Roles of carbohydrates, fats and proteins, four food tests, molecule assembly, enzyme specificity, temperature, and pH.