Science Understanding: Core Biological Concepts
What you must understand
Science Understanding in Stage 2 Biology rewards connected knowledge, not isolated definitions. Be fluent across four organising ideas: cells and cellular processes (organelles, membrane transport, enzymes, photosynthesis and cellular respiration, the cell cycle, mitosis and meiosis); genetics and inheritance (DNA structure and replication, protein synthesis, Mendelian and non-Mendelian ratios, pedigrees, mutations); evolution and biodiversity (natural selection, allele frequencies, speciation, evidence for change); and homeostasis and coordination (stimulus–response pathways, feedback loops, nervous and endocrine signalling, and exchange surfaces). You should be able to explain how a structure enables a function and why a process is regulated.
Traps that appear in questions
- Confusing mitosis with meiosis in diagrams: check whether chromosome number halves and whether homologous pairs separate.
- Treating dominant as more common. Dominance describes expression, not frequency in a population.
- Assuming enzymes change the equilibrium of a reaction; they only lower activation energy and speed approach to equilibrium.
- Mixing up transcription and translation locations and templates in eukaryotes.
- Ignoring negative feedback direction: identify the sensor, controller and effector before predicting the response.
- Confusing genotype and phenotype ratios when writing Punnett square answers.
Worked example
Cross two heterozygotes, Aa × Aa. The genotype ratio is 1 AA : 2 Aa : 1 aa, but the phenotype ratio is 3:1 only if A is completely dominant. If the trait shows incomplete dominance, the phenotype ratio becomes 1:2:1. Rule of thumb: always state the inheritance model before converting genotypes to phenotypes.