Canary PAU Biology common mistakes
Canary PAU Biology · Understand the traps and how to avoid them.
Explaining enzyme kinetics by saying the enzyme 'se destruye' or 'se gasta' during the reaction, and drawing the product curve as if it stopped at Vmax without distinguishing initial rate from total product.
Textbook diagrams of Michaelis-Menten show a plateau, and candidates conflate the saturation of active sites (velocity plateau) with a supposed limit on product formed. They also memorise 'los enzimas no se consumen' as a slogan without connecting it to the catalytic cycle.
What to do instead
Always write the cycle explicitly: E + S ⇄ ES → E + P, and state that E is regenerated unchanged, so a fixed amount of enzyme converts unlimited substrate, only slower as [S] falls. Separate the two graphs in your answer: velocity vs [S] (hyperbolic, Vmax when all active sites are occupied) and product vs time (keeps rising, slope decreasing). Practise explaining what happens when you double [E] (Vmax doubles, Km unchanged) versus double [S] at saturating levels (no change in rate).
Treating water's properties as a list to recite (alto calor específico, capilaridad, disolvente universal) without linking each property to its molecular cause and to a concrete biological consequence.
The topic is taught as a bullet list and exam questions often start with 'razona', which requires the causal chain, not the label. Candidates run out of marks because they name the property but not the hydrogen bond behind it.
What to do instead
For each property, build a three-step chain: structure (polaridad, puentes de hidrógeno) → physical property (cohesión, alta temperatura de vaporización, elevada constante dieléctrica) → biological role (ascenso de savia en el xilema, termorregulación por sudoración, transporte de iones y metabolitos). Write these chains out from memory for at least five properties and rehearse a 'razona por qué el agua es líquida a temperatura ambiente' answer.
Describing membrane transport by labelling any protein-mediated movement as 'transporte activo' and forgetting that facilitated diffusion uses a carrier or channel but still goes down the gradient without ATP.
The word 'proteína' is mentally paired with 'bomba', and the syllabus diagrams show pumps prominently. Candidates also miss that active transport is defined by movement against the electrochemical gradient, not by the presence of a protein.
What to do instead
Classify every example on two axes before answering: (1) ¿atraviesa la bicapa o usa proteína? (2) ¿a favor o en contra del gradiente? ¿gasta ATP directamente o aprovecha un gradiente iónico? Place osmosis, difusión simple, difusión facilitada (GLUT), bomba Na+/K+ and cotransporte glucosa-Na+ in a 2x2 table and justify each placement in one sentence.
Writing the overall equation of aerobic respiration or photosynthesis correctly but then placing ATP yields, proton pumping and electron flow in the wrong compartment, e.g. saying the Krebs cycle occurs in the cytoplasm or that the Calvin cycle produces ATP.
Candidates memorise equations as strings and never anchor them to the mitochondrial matrix, inner membrane, stroma or thylakoid lumen. The compartment names look interchangeable in notes, so they get shuffled under exam pressure.
What to do instead
Draw a single annotated diagram of a mitochondrion and a chloroplast side by side, labelling where each stage occurs and which way protons move. Then state the energy logic in one sentence each: in respiration, substrate oxidation pumps H+ and the gradient drives ATP synthase; in photosynthesis, light excites electrons, the resulting H+ gradient drives ATP synthase, and the ATP and NADPH are consumed in the stroma to reduce CO2. Redo the diagram from blank paper until it is exact.
Confusing replication, transcription and translation in terms of template, enzyme, direction and product, for example saying that DNA polymerase synthesises RNA or that translation uses DNA as a direct template.
The three processes share vocabulary (cadena molde, polimerasa, complementariedad) and are studied close together, so the details blur into one generic 'dogma' narrative.
What to do instead
Build a comparison table with fixed rows: molde, enzima principal, sustrato, producto, sentido, localización en eucariotas, y qué garantiza la fidelidad. Force yourself to state the base-pairing rule used in each case (A-T in replication, A-U in transcription, codón-anticodón in translation). Answer a short '¿qué enzima actúa y sobre qué molde?' question for each process until it is automatic.
Solving Mendelian genetics problems by intuition or by writing phenotypes in the Punnett square, instead of defining alleles, genotypes of parents and gametes before crossing, and failing to test the result against expected ratios.
Candidates rush to fill a square because it looks like a familiar exercise, and they forget that the square only works once gamete frequencies are known. Linked genes, sex linkage and epistasis then produce wrong ratios that go unchecked.
What to do instead
Use a fixed protocol for every problem: (1) assign symbols and state dominance; (2) write parental genotypes; (3) list gametes with their proportions; (4) draw the square or use probabilities; (5) express the result as both genotypes and phenotypes with fractions; (6) sanity-check against 3:1, 1:1, 9:3:3:1 or the modified ratios. If the observed ratio deviates, explicitly consider linkage, lethality or sex linkage before concluding.
Describing a mutation as always harmful and always changing the phenotype, without distinguishing silent, missense, nonsense and frameshift consequences, or where in the gene the change occurs.
Everyday language equates 'mutación' with 'defecto', and the redundancy of the genetic code is memorised as a fact but not applied to examples. Candidates also forget that mutations in non-coding or regulatory regions can matter.
What to do instead
For any given change, work through three questions: does it alter the codon, does it alter the amino acid, does it alter protein length or reading frame? Classify the outcome explicitly (silenciosa, con cambio de sentido, sin sentido, de cambio de marco) and say what happens to the protein and hence the phenotype. Add the qualifier that effect depends on location (somática vs germinal, codificante vs reguladora) and that some mutations are neutral or advantageous, which is the raw material of evolution.
Explaining natural selection as a process that creates the variation needed or that 'the organism adapts by mutating in response to a need', mixing up Lamarckian and Darwinian causation.
School-level phrasing ('se adaptan para sobrevivir') slips into teleology, and the distinction between the origin of variation (mutation, recombination, random) and its sorting (selection, non-random) is never stated sharply.
What to do instead
Write the Darwinian argument in four fixed steps: existe variación previa de origen aleatorio; se hereda; hay competencia y recursos limitados; los individuos con variantes más favorables dejan más descendencia, de modo que la frecuencia alélica cambia entre generaciones. Contrast it explicitly with Lamarck in one sentence. Practise applying it to antibiotic resistance, since that example forces you to say the mutation existed before the antibiotic was applied.
Describing the immune response as a single event, without separating innate from adaptive immunity and without ordering the steps of clonal selection, helper T-cell activation, B-cell differentiation into plasma cells and memory cells, and the secondary response.
The vocabulary (antígeno, anticuerpo, linfocito T, macrófago) is learned as a glossary rather than as a sequence, so answers list cells without showing who presents antigen to whom or what happens on second exposure.
What to do instead
Draw the response as a timeline with two lanes (innata: barreras, fagocitos, inflamación, complemento; adaptativa: presentación antigénica, selección clonal, expansión, diferenciación, memoria). Annotate where antibodies act (neutralización, aglutinación, opsonización) and why the secondary response is faster and stronger. Then answer '¿por qué una vacuna funciona?' using only that timeline.
Treating biotechnology techniques as names without mechanism, e.g. saying PCR 'amplifies DNA' without mentioning primers, thermostable polymerase and thermal cycles, or describing a plasmid vector without restriction sites, origin of replication and selectable marker.
Technique names are easy to memorise and the exam often asks for a description of a procedure or an interpretation of a gel, where the mechanism is exactly what earns marks.
What to do instead
For each technique write a numbered protocol in five to seven steps with the purpose of each reagent: PCR (desnaturalización, hibridación de cebadores, extensión, Taq polimerasa, ciclos); clonaje (corte con enzimas de restricción, ligasa, transformación, selección por antibiótico); electroforesis (carga del DNA, migración hacia el ánodo, tamaño frente a movilidad). Then practise reading a hypothetical gel result and explaining why a band appears where it does.