Madrid PAU Biology common mistakes
Madrid PAU Biology · Understand the traps and how to avoid them.
When asked to identify cell structures in a described or schematic cell, the candidate labels the Golgi apparatus as smooth endoplasmic reticulum (or vice versa) and assigns protein synthesis to the Golgi instead of the ribosomes.
Both are membrane systems of flattened sacs, and revision notes often list them together under 'endomembranas'. Candidates memorise the list without anchoring each organelle to its function and its position relative to the nucleus and plasma membrane.
What to do instead
Build a functional map: ribosomes (free or on rough ER) synthesise proteins; rough ER folds and glycosylates them; Golgi modifies, sorts and packages them in vesicles; smooth ER synthesises lipids and detoxifies. For each organelle write one 'input → output' sentence and one positional clue (rough ER continuous with the nuclear envelope; Golgi on the secretory side). Practise by describing the route of a secreted protein step by step.
On enzyme kinetics questions, the candidate states that a competitive inhibitor lowers Vmax, or that a non-competitive inhibitor raises Km, instead of distinguishing the two effects on the Michaelis-Menten parameters.
Candidates memorise 'competitive = competes for the active site' but never connect that to the graph. They do not rehearse how the Lineweaver-Burk plot or the saturation curve changes, so the Km/Vmax distinction collapses under exam pressure.
What to do instead
Learn the two signatures as pictures: competitive inhibitor - same Vmax, higher apparent Km (lines cross on the y-axis of the double-reciprocal plot, because high substrate can outcompete the inhibitor); non-competitive inhibitor - same Km, lower Vmax (lines cross on the x-axis). For each, write the mechanism in one sentence (competes for active site vs binds elsewhere and distorts the site) and sketch both curves from memory.
In a Mendelian genetics problem the candidate calculates phenotypic ratios correctly but ignores linkage, or assumes independent assortment when the problem states two genes are on the same chromosome, and then fails to interpret the recombinant (minority) classes.
Textbook practice is dominated by 9:3:3:1 dihybrid crosses, so candidates apply independent assortment by default. They do not read the phrase 'genes linked on the same chromosome' or do not know that parental classes are the most frequent and recombinants the least frequent.
What to do instead
Before calculating, scan the stem for linkage cues and, if given progeny counts, rank the classes by frequency: the two largest are parental, the two smallest are recombinants. Compute recombination frequency as (recombinants/total) × 100 to get map distance in centimorgans. Practise classifying a table of four progeny classes as parental vs recombinant before doing any ratio maths.
When asked whether a population is in Hardy-Weinberg equilibrium, the candidate calculates allele frequencies and then stops, declaring equilibrium without comparing observed and expected genotype frequencies.
The formula p² + 2pq + q² = 1 is drilled as a calculation exercise, and candidates treat reaching it as the answer. They forget that the test is a comparison between observed counts and expectations under random mating.
What to do instead
Use a fixed four-step routine: (1) count alleles to get p and q; (2) compute expected genotype frequencies p², 2pq, q²; (3) multiply by total N to get expected numbers; (4) compare with observed (chi-square or simple inspection). State the conclusion in terms of the comparison, e.g. 'observed heterozygotes far exceed 2pq, so the population is not in equilibrium - possibly due to selection or non-random mating'.
The candidate describes an antibiotic resistance mechanism as 'the bacterium becomes immune to the antibiotic' or 'the antibiotic makes the bacterium mutate', instead of framing resistance as selection acting on pre-existing variation.
Everyday language ('immunity', 'the drug creates resistance') leaks into answers, and the Lamarckian phrasing is common in the media. Candidates also confuse the immune system with bacterial defence mechanisms.
What to do instead
Write resistance answers with the strict Darwinian sequence: random mutation (or horizontal gene transfer) exists before exposure → antibiotic kills susceptible cells → resistant variants survive and reproduce → resistance allele frequency rises. Ban the words 'immune' and 'create resistance' from your notes; use 'selected for'. Contrast this explicitly with the immune system, which is about antibodies and lymphocytes, not bacterial genetics.
In metabolism questions the candidate states that glycolysis produces 'a lot of ATP' or places the Krebs cycle in the cytoplasm, and cannot say where each stage occurs or how many ATP/NADH/FADH2 it yields.
The pathway is memorised as a list of names without compartmentalisation or stoichiometry. Because the overall equation is often quoted only at the level of glucose oxidation, candidates lose track of which stage happens where.
What to do instead
Memorise a one-line table per stage: glycolysis (cytoplasm; glucose → 2 pyruvate; net 2 ATP, 2 NADH); pyruvate oxidation (mitochondrial matrix; 2 NADH, 2 CO2); Krebs cycle (matrix; per turn 3 NADH, 1 FADH2, 1 ATP/GTP, 2 CO2); oxidative phosphorylation (inner mitochondrial membrane; ~26–28 ATP via chemiosmosis). Recite the table, then use it to answer 'where' and 'how much' questions in one step.
When describing meiosis the candidate says 'chromosomes separate' without distinguishing meiosis I (homologous chromosomes separate, crossing over occurs) from meiosis II (sister chromatids separate), and so misplaces the source of genetic variation.
Mitosis and meiosis are learned as parallel lists, and the phrase 'chromosomes separate' is used loosely for both divisions. Candidates do not connect the reductional division to the halving of chromosome number.
What to do instead
Anchor each division to one event: meiosis I = homologous chromosomes pair, cross over at chiasmata, then separate → chromosome number halves (2n → n); meiosis II = sister chromatids separate, like mitosis. Attribute variation to two sources only: crossing over in prophase I and independent assortment of homologues in metaphase I. Draw the two divisions once, labelling ploidy at each stage.
In genetic engineering questions the candidate confuses a restriction enzyme with a DNA ligase, or says the plasmid 'cuts itself' to accept the insert, rather than describing the cut-and-paste sequence in the right order.
The tools are introduced together as 'molecular scissors and glue', and candidates remember the metaphor but not which enzyme does which job or the order of steps.
What to do instead
Write the workflow as an ordered recipe: restriction endonuclease cuts vector and insert at the same recognition site (sticky ends) → ligase seals the phosphodiester backbone → transformation into host → selection (e.g. antibiotic resistance marker) → screening. Learn one fact per enzyme: restriction enzymes hydrolyse phosphodiester bonds at specific palindromic sequences; ligases form them. State the order explicitly in every answer.
On experimental data questions the candidate describes the trend ('the line goes up') but never identifies the variables, the control, or a conclusion tied to the hypothesis, and does not comment on sources of error.
Candidates treat data questions as reading comprehension and stop at the shape of the graph. They have not practised the standard structure of a scientific conclusion, so they omit the control group and the uncertainty.
What to do instead
Answer with a fixed four-part template: (1) name the independent and dependent variables and units; (2) state the trend quantitatively (e.g. 'absorbance rises from 0.2 to 0.8 between 10 and 30 °C, then falls'); (3) link to the hypothesis and identify the control; (4) name one plausible error or limitation and its direction of effect. Rehearse this template on any table or graph until it is automatic.
When asked to explain a result in immunology, the candidate describes the innate and adaptive responses as a single sequence and attributes antibody production to macrophages or cytotoxic T cells rather than to plasma cells derived from B lymphocytes.
The two systems are taught as a list of cell types, and the names (macrophage, lymphocyte, plasma cell) are memorised without a lineage map. Candidates do not rehearse which cell does which job.
What to do instead
Build a lineage-and-function chart: innate (macrophages, neutrophils, dendritic cells - phagocytosis, inflammation, no memory) vs adaptive (B lymphocytes → plasma cells → antibodies; T lymphocytes → helper cells coordinate, cytotoxic cells kill infected cells; memory cells). For each cell write one verb only (phagocytose, produce antibodies, kill infected cells, present antigen). Use the chart to answer 'which cell' questions in one step.