Extremadura PAU Biology common mistakes
Extremadura PAU Biology · Understand the traps and how to avoid them.
When asked to explain a transport process, the candidate describes what happens ("water moves across the membrane") without naming the mechanism (osmosis, facilitated diffusion via aquaporins, active transport by Na+/K+ ATPase) or stating whether ATP is consumed and whether the movement is down or against the gradient.
The syllabus presents membranes and transport as a list of named processes, so candidates memorise examples and skip the classification step; under exam pressure they answer with the phenomenon instead of the category, which is what the marking criteria reward.
What to do instead
Before writing, force a two-line plan: (1) direction relative to the concentration gradient, (2) energy source (none, ATP, or an ion gradient). Then write the answer starting with the mechanism name. Practise with a table of six processes (simple diffusion, osmosis, facilitated diffusion by channels, facilitated diffusion by carriers, primary active transport, secondary active transport) and for each write one sentence with gradient direction and energy source.
The candidate writes the overall equation for aerobic respiration or photosynthesis from memory but cannot place the ATP yield, the location, or the hydrogen acceptors correctly - for example claiming the Krebs cycle occurs in the cytoplasm, or that all 38 ATP come from the electron transport chain.
The pathway is learned as a narrative sequence of names rather than as a compartment-by-compartment map with inputs and outputs, so location and stoichiometry are the first details to blur.
What to do instead
Draw a single page per pathway: three boxes for glycolysis (cytosol), Krebs (mitochondrial matrix), and oxidative phosphorylation (inner membrane), and on each arrow write substrate, product, and coenzyme. Rebuild the page from blank paper daily until the locations and approximate ATP/NADH/FADH2 counts are automatic.
In a genetics problem the candidate computes gamete frequencies or a cross correctly but never states the parental genotypes, the linkage phase, or the null hypothesis, so a correct number sits next to an unjustified answer and loses marks; alternatively they apply a dihybrid 9:3:3:1 ratio to a case with linkage.
Candidates treat genetics problems as arithmetic and jump to the Punnett square or chi-square formula, skipping the formal setup that the exam expects to see written down.
What to do instead
Adopt a fixed written template for every problem: (1) define allele symbols, (2) write parental genotypes with linkage phase (cis/trans), (3) state expected ratios, (4) compute, (5) interpret in a sentence. Use the template even when the answer seems obvious; the setup itself carries marks.
When interpreting a pedigree, the candidate decides on dominant versus recessive inheritance from a single affected individual instead of testing the decisive patterns (affected child from two unaffected parents proves recessive; every affected person having an affected parent suggests dominant; male-to-male transmission rules out X-linked recessive).
Pedigrees are read visually, left to right, and the first plausible hypothesis is accepted without being tested against the whole diagram.
What to do instead
Apply a checklist in a fixed order: does an affected child have two unaffected parents? Are all affected males sons of affected mothers? Is there male-to-male transmission? Is the trait far more common in males? Write the checklist at the top of the answer and tick or cross each item before committing to a mode of inheritance.
The candidate confuses replication, transcription, and translation details - writing that DNA polymerase makes RNA, that the template strand and coding strand are the same thing, that tRNA carries amino acids to the ribosome while also claiming the ribosome reads the DNA directly, or placing 5' and 3' ends the wrong way round.
The three processes share vocabulary (template, polymerase, directionality) and are revised together, so they blur into one generic 'central dogma' script.
What to do instead
Build a comparison table with rows for template, enzyme, product, direction of synthesis, start and stop signals, and location in eukaryotes versus prokaryotes. For each process, write out one short sequence by hand, correctly labelling 5' and 3', and mark the coding versus template strand explicitly.
The candidate answers an immunology question by listing cell types (macrophages, T cells, B cells) without assigning them to innate versus acquired immunity, or without distinguishing humoral from cell-mediated response and the role of antibodies versus cytotoxic T cells.
Immunology is taught as a cast of characters, and candidates memorise names without the branching structure that determines which cell acts when.
What to do instead
Draw a flowchart from pathogen entry: barrier, innate (phagocytes, complement, inflammation), then antigen presentation, then the fork into humoral (B cells, plasma cells, antibodies) and cell-mediated (cytotoxic T cells). Practise describing a viral infection and a bacterial toxin through the flowchart, naming which branch dominates and why.
On a virus question the candidate describes a virus as a cell, claims it has its own metabolism, or states that antibiotics act on viruses; on a microbiology question they describe binary fission and bacterial conjugation as if they were the same process.
Everyday language ('the virus replicates', 'the bacteria reproduce') encourages loose phrasing, and the distinction between acellular and cellular organisation is not rehearsed as a formal definition.
What to do instead
Write and memorise two one-sentence definitions: a virus as an acellular obligate intracellular parasite with nucleic acid plus capsid and no own metabolism, and a bacterium as a prokaryotic cell with binary fission and no membrane-bound nucleus. Then list three differences between binary fission and conjugation (division versus DNA transfer, vertical versus horizontal, no pilus versus pilus).
In population genetics the candidate uses the Hardy-Weinberg equations but assigns the wrong quantity to p², 2pq, and q², or applies the equilibrium to a locus that is clearly under selection without saying so; they also fail to state what the chi-square result means.
The algebra is easy to perform mechanically, so candidates plug numbers in without first writing what each term represents and what assumptions the model requires.
What to do instead
Before calculating, write the legend: p = frequency of dominant allele, q = recessive allele, p² = homozygous dominant, 2pq = heterozygous, q² = homozygous recessive. State the five assumptions (no selection, no mutation, no migration, infinite population, random mating). After computing chi-square, always write one sentence comparing the value with the critical value and stating whether the null hypothesis is rejected.
In an experimental-data question the candidate describes the graph ('the line goes up') instead of identifying the variables, the trend with values, the control, and a limitation; they also propose a conclusion that the data cannot support, such as inferring causation from a correlation.
Candidates treat data questions as comprehension rather than analysis, and the pressure to fill space favours description over interpretation.
What to do instead
Use a four-sentence frame for every graph: (1) identify independent and dependent variables with units, (2) state the trend with at least two numerical values read from the axes, (3) name the control and what it rules out, (4) state one limitation and one conclusion the data do support. Never write a conclusion without citing the numbers that justify it.
The candidate answers biotechnology questions by naming a technique (PCR, electrophoresis, CRISPR, cloning) without stating its purpose, the key reagent or enzyme, and one application or limitation - for example describing PCR without mentioning primers, thermostable polymerase, and thermal cycling.
Technique names are highly memorable and appear frequently in the media, so candidates learn labels without the operational detail the exam asks for.
What to do instead
For each technique keep a three-line card: purpose, key components or steps, and one real application plus one limitation. For PCR specifically, memorise denaturation, annealing of primers, extension by Taq polymerase, and the exponential amplification. Recite each card aloud from memory before moving on.