A Level content: main theoretical concepts, current applications of biology, advanced practical skills
This domain covers the A Level half of Cambridge International AS & A Level Biology (9700): energy and respiration, photosynthesis, homeostasis, nervous and hormonal coordination, inherited change, selection and evolution, biodiversity and conservation, genetic technology and biotechnology, and the advanced practical skills examined through ecological sampling, statistical tests, aseptic technique, chromatography and electrophoresis.
Core rules and applications
Respiration and photosynthesis must be linked to electron transport, chemiosmosis and ATP synthesis, not treated as summarised word equations. Homeostasis answers need a receptor, coordination pathway, effector and negative feedback loop; set point and normal range are not the same. Nervous control requires depolarisation, repolarisation, refractory period and synaptic transmission; hormonal control requires named glands, target cells and second messengers where relevant. In inherited change, use clearly defined alleles, show gametes and state whether genes are linked or independently assorting. Evolution answers should separate variation, selection pressure, differential survival and changed allele frequency. Biodiversity questions need sampling method, species richness or index, and a conservation rationale. Genetic technology answers should name the tool and its purpose: PCR for amplification, restriction enzymes for cutting, ligase for joining, electrophoresis for separation by size, and bioinformatics for sequence analysis.
Worked example: chi-squared
In a test cross, 215 dark and 185 pale seeds are counted; the expected ratio is 1:1, so expected numbers are 200 and 200. χ² = Σ(O−E)²/E = (15²/200) + (−15²/200) = 1.125 + 1.125 = 2.25. Degrees of freedom = number of categories − 1 = 1. The critical value at p = 0.05 is 3.84; 2.25 < 3.84, so the null hypothesis is not rejected. The deviation from 1:1 is not significant. State the null hypothesis, use raw counts, and compare with the correct critical value.
Traps that cost marks
Do not use percentages or ratios in χ²; expected values must be counts. Do not set degrees of freedom as n − 1 unless n means categories. In ecology, avoid biased quadrat placement and remember percentage cover versus frequency. In electrophoresis, DNA moves toward the anode because phosphate groups are negatively charged; smaller fragments move further. In aseptic technique, flaming the loop before and after transfer is required, and the lid is held, not placed on the bench. In homeostasis, 'negative feedback' is not a synonym for 'keeping constant'; explain the correction of a deviation.