IB Biology SL common mistakes
IB Biology SL · Understand the traps and how to avoid them.
When asked to compare two structures or processes, the candidate lists facts about each one separately (e.g., writes one sentence on prokaryotes and one on eukaryotes) instead of pairing the points side by side.
The command term 'compare' is read as 'describe both', and candidates fall back on the linear notes they memorised. Markers award comparison marks only for paired statements, so a strong factual answer can still score zero on that criterion.
What to do instead
Before writing, draw a quick two-column table on the answer sheet and place each feature in the same row for both items. Then convert each row into one sentence using 'whereas' or 'both'. Practise by re-answering past comparison prompts with a hard rule: no sentence may mention only one of the two items unless it is explicitly contrasting.
The candidate writes 'the cell membrane is selectively permeable because it is made of phospholipids' as a complete explanation of membrane transport, without linking the hydrophobic core to the passage of polar and non-polar particles.
The phospholipid bilayer is memorised as a label rather than as a structural reason. Candidates can name the component but not chain it to the behaviour, so the causal link that earns the mark is missing.
What to do instead
For every membrane transport question, write a forced three-step chain: (1) structure of the bilayer region involved, (2) property of the substance (size, charge, polarity), (3) consequence for movement. Use this chain for simple diffusion, facilitated diffusion, osmosis and active transport so the same reasoning is reused rather than re-learned.
When describing an enzyme or metabolic experiment, the candidate states the result ('more gas was produced at 40 °C') but never identifies the independent, dependent and controlled variables or explains the result in terms of the mechanism.
Practical work is treated as something separate from theory, so the candidate reports observations without connecting them to enzyme kinetics or limiting factors. Command terms like 'explain' are answered with 'describe'.
What to do instead
Adopt a fixed answer template for data questions: name the variables, state the trend with a value from the data, then give the mechanism (e.g., increased kinetic energy and more enzyme–substrate collisions up to the optimum, then denaturation of the active site beyond it). Rehearse this on any graph-based question until the mechanism sentence is automatic.
The candidate explains a food web or predator–prey relationship only in terms of 'who eats whom' and omits energy flow, trophic loss and the effect of removing one species on the rest of the community.
Ecology is revised as a set of definitions and named examples. Candidates do not practise the multi-step causal reasoning that questions about disturbance, keystone effects or energy transfer demand.
What to do instead
For any community question, write a numbered causal chain: change to species A, effect on its prey or predator, effect on competition, effect on energy or biomass at the next trophic level. Also memorise the approximate energy loss between trophic levels and use it quantitatively whenever a question gives biomass or energy figures.
In genetics problems, the candidate calculates a ratio or probability but does not state the genotypes and phenotypes explicitly, and does not show the cross (parental genotypes, gametes, offspring grid).
The candidate does the arithmetic mentally and writes only the final answer. Marks are awarded for the method and for correct notation, so a right number with no working or with inconsistent symbols loses most of the credit.
What to do instead
Always write the full working: define alleles with a clear letter convention (dominant capital, recessive lower case), write parental genotypes, list gametes, complete the grid, then state genotype and phenotype ratios separately. Do this even when the answer seems obvious, because the working itself carries marks.
The candidate describes meiosis or DNA replication as a sequence of stages without linking each stage to its outcome, such as crossing over producing genetic variation or semi-conservative replication producing identical copies.
Stage names and order are learned as a list. The candidate can recite prophase, metaphase, anaphase but cannot answer 'why does this matter', which is what the higher-tariff questions ask.
What to do instead
For each stage, attach a one-line consequence at the point of learning: crossing over and independent assortment create variation, separation of homologues halves the chromosome number, semi-conservative replication preserves the base sequence. When revising, cover the stage names and test yourself only on the consequences.
The candidate answers a question on natural selection or speciation by saying 'the organism adapts' or 'it changes to survive', implying directed change within an individual's lifetime.
Everyday language about adaptation is teleological, and candidates carry it into answers. This is a conceptual error that markers penalise, and it undermines any explanation of allele frequency change.
What to do instead
Rewrite every such sentence with a fixed structure: variation already exists in the population, a selection pressure acts, individuals with the advantageous allele survive and reproduce, allele frequency increases over generations. Ban the phrases 'adapts to' and 'tries to' from your written answers and check for them before finishing.
The candidate describes a gas exchange or transport surface in general terms ('it is thin and has a large surface area') without naming the specific structure and quantifying or comparing the feature to something.
Surface-area-to-volume ratio is memorised as a slogan. Candidates do not practise applying it to a named example or calculating it, so answers stay at the level of a definition and miss application marks.
What to do instead
For each exchange surface studied, prepare a four-point profile: name of structure, the feature, the benefit for diffusion, and a comparison (e.g., alveoli versus a single large sphere). Practise one surface-area-to-volume calculation per revision session so the ratio is something you compute, not just quote.
The candidate answers a data-analysis or graph question using only the shape of the curve, ignoring the units, axis labels and the actual values, and does not quote figures in the answer.
Candidates read graphs as pictures rather than as data. Under time pressure they skip the axes and write qualitative statements such as 'it goes up then down', which cannot earn marks tied to numerical evidence.
What to do instead
Use a reading routine for every graph: read both axes with units, identify the trend, quote at least one pair of values, and note any anomaly or plateau. Then state what the quoted value means biologically. Practise by writing one-sentence summaries of graphs that always contain a number with its unit.
When asked to 'evaluate', 'suggest' or 'discuss', the candidate gives a one-sided description with no judgement, no limitation and no alternative explanation.
Command terms are not distinguished during revision, so all questions are treated as 'describe'. The candidate also avoids committing to a judgement because it feels risky.
What to do instead
Build a command-term glossary and practise each term with a fixed scaffold: evaluate means strengths, weaknesses and a conclusion; suggest means a plausible mechanism applied to the novel context; discuss means at least two viewpoints plus a reasoned position. Answer one prompt per term each week and check your answer against the scaffold before marking it.