GCE A/L Biology common mistakes
GCE A/L Biology · Understand the traps and how to avoid them.
When asked to explain why enzyme rate plateaus, the candidate writes 'the enzyme is used up' or 'the substrate runs out' without linking it to active sites becoming saturated, and does not distinguish the two curves (rate vs substrate, rate vs time).
The rate-versus-substrate graph and the rate-versus-time graph look similar, and candidates memorise one stock sentence. They also conflate 'limiting factor' with 'denaturation', so they miss that at high substrate concentration all active sites are occupied and adding more substrate cannot increase collisions per unit time.
What to do instead
Before writing, label the x-axis explicitly: if it is substrate concentration, the plateau means Vmax with active sites saturated; if it is time, the plateau means substrate depleted or product inhibition. Practise writing the two explanations side by side and force yourself to name the limiting factor in each case.
In an osmosis question the candidate writes 'water moves from high water potential to low water potential' but then assigns the wrong sign or direction to a plant cell in a solution, or states that solutes move across the membrane during osmosis.
Water potential is expressed as a negative number, and candidates treat 'high' and 'low' as magnitude rather than value, so they say water moves from -2000 kPa to -500 kPa. They also carry over the idea that the solute is what moves, because diffusion of solute is the more familiar process.
What to do instead
Write water potentials as numbers on a number line every time, and state the rule as 'water moves from the less negative to the more negative value'. Add the sentence 'osmosis is movement of water only; the solute is not crossing' and check the direction against the actual numbers before finalising.
When describing mitosis or meiosis, the candidate lists stage names in order but misplaces the key events: crossing over placed in metaphase, independent assortment placed in anaphase II, or DNA replication placed in prophase.
Stage names are learned as a sequence of labels, while the events are learned separately, so the two lists are not linked. Exam figures and photomicrographs reinforce naming over event placement, and candidates rarely practise matching a named event to its stage under time pressure.
What to do instead
Build a one-page table with columns for stage, chromosomes present, key event, and ploidy, and fill it from memory weekly. For each event, practise writing the stage in a single sentence ('crossing over occurs during prophase I at the chiasmata'), so the pairing is retrieved as a unit rather than reconstructed.
In a monohybrid or dihybrid cross the candidate writes genotypes without defining symbols, mixes up which phenotype belongs to which genotype, and gives a ratio that does not follow from the gametes shown.
Candidates copy the ratio (3:1 or 9:3:3:1) from memory instead of deriving it, and they skip the step of stating which allele is dominant. Under time pressure the Punnett square is drawn but the gametes along the edges are not written, so errors go unchecked.
What to do instead
Always start with a key ('let A = dominant allele for X, a = recessive'), write parental genotypes, list gametes, then draw the square with gametes on both edges. Derive the ratio by counting the square rather than recalling it, and finish with a phenotype statement that maps each class to its genotype.
When explaining transpiration or translocation, the candidate describes the mechanism as a single cause ('water is pulled up by transpiration') and omits cohesion-tension, adhesion, and the role of the Casparian strip, or claims that phloem sap moves by mass flow driven by transpiration.
Both transport processes are taught as stories, and the vivid transpiration pull dominates memory. The phloem mechanism (pressure flow from source to sink) is easily overwritten by the xylem mechanism because both involve water and both are described as 'flow'.
What to do instead
Write the two mechanisms as separate numbered chains: xylem as transpiration pull, cohesion, adhesion, and root pressure; phloem as active loading at the source, water entry, pressure gradient, unloading at the sink. State explicitly at the top of each answer which tissue and which direction, then check that no sentence from the other chain has crept in.
In a question on the cardiac cycle or control of breathing, the candidate gives the sequence of events but attaches the wrong valve state or pressure relationship to a phase, for example saying the atrioventricular valves open during ventricular systole.
The cycle is learned as a list of phase names (atrial systole, ventricular systole, diastole) without the pressure changes that drive valve opening and closing. Candidates do not connect 'valve opens' to 'pressure behind exceeds pressure ahead', so the valve states are guessed.
What to do instead
Learn the cycle as pressure-driven: draw a pressure-time graph for atrium, ventricle, and aorta, and mark each valve opening and closing at the crossing points. For each phase write the sentence 'blood flows from X to Y because pressure in X is greater than in Y', so valve state is derived rather than recalled.
When answering on immunity, the candidate uses 'antibody' and 'antigen' interchangeably, describes phagocytes as producing antibodies, or states that vaccination gives immediate passive immunity.
The vocabulary is dense and the terms are visually similar, and the distinction between humoral and cell-mediated responses is often taught as two lists rather than as two coordinated systems. Passive versus active immunity is confused because both are described as 'protection'.
What to do instead
Maintain a glossary card for antigen, antibody, phagocyte, B cell, T cell, plasma cell, memory cell, and use each in one sentence about its role. For immunity type, write a two-row table (active/passive crossed with natural/artificial) and place vaccination and colostrum in it, noting time to protection and duration.
In ecology questions on energy flow, the candidate writes that energy is 'lost as heat' but then calculates transfer efficiency using biomass or numbers rather than energy, or claims energy is recycled in the ecosystem.
The carbon and nitrogen cycles genuinely recycle matter, and candidates transfer that idea to energy. Efficiency calculations are attempted from whatever data are given, without checking that the units are energy units.
What to do instead
State the rule at the start of every energy-flow answer: matter cycles, energy flows one way and is lost as heat at each trophic level. When calculating efficiency, check the units of the given figures first and convert biomass to energy only if a conversion is supplied; if not, say the calculation cannot be made from the data.
In a required-practical-style question, the candidate describes a method without stating the independent variable, dependent variable, and at least two controlled variables, and reports a conclusion that goes beyond the data shown.
Candidates treat practical questions as recall of a protocol they once followed, so they narrate steps and omit the experimental design frame. The habit of writing a confident conclusion is carried over from essay questions, and the word 'proves' appears where 'suggests' is required.
What to do instead
Use a fixed four-line scaffold before writing any method: independent variable, dependent variable, controlled variables, and how the dependent variable is measured. End with a conclusion that names the trend and its limits ('within the range tested, rate increased with temperature up to 40 °C'), and replace 'proves' with 'indicates' or 'supports'.
When asked to interpret a graph or evaluate a claim about a biotechnology or applied-biology context, the candidate describes the shape of the graph ('it goes up then down') without extracting the biological meaning, and accepts a correlation as evidence of causation.
Graph description is taught as a skill in isolation from biological explanation, and candidates are anxious to write something, so they describe the line. Biotechnology contexts are unfamiliar, so candidates defer to the presented claim rather than interrogating it.
What to do instead
Use the pattern 'as X increases, Y does A, which is consistent with B because C' for every graph, forcing a mechanism into the answer. For evaluation, write one sentence on what the data show and one on what they cannot show, and check whether a controlled variable or sample size could explain the pattern before accepting causation.