Basque PAU Biology common mistakes
Basque PAU Biology · Understand the traps and how to avoid them.
When solving genetics problems, students often directly assign genotypes to phenotypes without considering that the same phenotype can arise from different genotypes (e.g., in incomplete dominance or codominance), leading to incorrect predictions of offspring ratios.
Students memorize Mendelian ratios (3:1, 9:3:3:1) and apply them mechanically without verifying whether the trait follows simple dominance. They overlook that in incomplete dominance, the heterozygote has a distinct phenotype, so the ratio changes (e.g., 1:2:1).
What to do instead
Before calculating, explicitly state the relationship between genotype and phenotype for each allele. For each problem, write out the cross and use a Punnett square or probability rules, but first determine the dominance pattern from the given information. Practice problems with incomplete dominance, codominance, and multiple alleles to internalize the differences.
In enzyme kinetics, students describe the effect of increasing substrate concentration on reaction rate as always increasing linearly, without recognizing that at high substrate concentrations the rate plateaus (Vmax) due to enzyme saturation.
Students confuse the initial linear portion of the Michaelis-Menten curve with the entire relationship, often because they memorize 'more substrate = faster reaction' without understanding the concept of active site availability.
What to do instead
Study the Michaelis-Menten plot and the meaning of Km and Vmax. Learn to sketch the curve and explain why saturation occurs: at high [S], all active sites are occupied, so adding more substrate does not increase rate. Practice interpreting graphs and predicting changes when enzyme concentration or inhibitors are introduced.
When describing membrane transport, students label facilitated diffusion as active transport because it uses proteins, or they claim that osmosis requires energy.
Students conflate the involvement of membrane proteins with active transport, not realizing that facilitated diffusion is passive (down the concentration gradient, no ATP). They also forget that osmosis is a special case of simple diffusion of water.
What to do instead
Create a comparison table of simple diffusion, facilitated diffusion, and active transport, listing gradient direction, energy requirement, and protein involvement. Emphasize that only active transport moves substances against their gradient and requires energy (ATP or electrochemical gradient). Use examples like glucose transporters (facilitated) vs. Na+/K+ pump (active).
In photosynthesis, students state that the light-dependent reactions produce glucose directly, or they confuse the roles of photosystems I and II in the Z-scheme.
Students oversimplify the process, remembering that 'photosynthesis makes glucose' and assuming it happens in the light reactions. They also mix up the order of electron flow and the products of each photosystem.
What to do instead
Clearly separate the light reactions (thylakoid membrane) from the Calvin cycle (stroma). Learn that light reactions produce ATP and NADPH, which are then used in the Calvin cycle to fix CO2 into G3P and eventually glucose. Memorize the Z-scheme: PSII splits water, electrons flow to PSI via an electron transport chain, and NADP+ is reduced. Practice labeling diagrams and explaining the fate of each product.
When explaining DNA replication, students describe it as conservative or dispersive, or they forget that DNA polymerase synthesizes only in the 5' to 3' direction, leading to errors about leading and lagging strand synthesis.
Students may not have internalized the semi-conservative model and the directionality of DNA polymerase. They often overlook the need for primers and the role of Okazaki fragments.
What to do instead
Review the Meselson-Stahl experiment evidence for semi-conservative replication. Emphasize that DNA polymerase adds nucleotides to the 3' end, so the leading strand is continuous while the lagging strand is synthesized discontinuously as Okazaki fragments. Draw replication forks and label 5' and 3' ends. Practice explaining why replication is bidirectional and how proofreading works.
In ecology, students calculate population growth using the exponential model (J-curve) without considering carrying capacity, even when the scenario implies limited resources.
Students memorize the exponential growth equation and apply it blindly, ignoring the logistic model. They may not recognize cues like 'limited food' or 'space constraints' that indicate logistic growth.
What to do instead
Learn the logistic growth equation and the concept of carrying capacity (K). Practice distinguishing between exponential and logistic growth from word problems. When a problem mentions resource limitation, use the logistic model. Sketch both curves and explain how growth rate changes with population size.
When describing the immune response, students claim that antibodies directly kill pathogens or that cytotoxic T cells produce antibodies.
Students confuse the roles of humoral and cell-mediated immunity. They may think antibodies are toxic to pathogens, when they actually neutralize or mark them for destruction by phagocytes or complement.
What to do instead
Create a clear table of immune components: B cells (produce antibodies, humoral), cytotoxic T cells (kill infected cells, cell-mediated), helper T cells (activate other immune cells). Emphasize that antibodies do not kill directly; they bind to antigens and facilitate phagocytosis or complement lysis. Practice matching cell types to their functions.
In protein structure, students state that the primary structure is determined by hydrogen bonds, or they confuse the levels of structure (e.g., saying tertiary structure is the sequence of amino acids).
Students may memorize the names of the four levels but not their definitions or the bonds involved. They often mix up which bonds stabilize which level.
What to do instead
Learn the four levels with their defining bonds: primary (peptide bonds), secondary (hydrogen bonds in alpha helices and beta sheets), tertiary (hydrophobic interactions, disulfide bridges, ionic bonds, hydrogen bonds), quaternary (same as tertiary but between subunits). Practice identifying the level from a description and explaining how denaturation affects each level.
When explaining evolution, students say that individuals evolve or that natural selection acts directly on genes, rather than on phenotypes.
Students often use Lamarckian thinking or conflate the process of selection with genetic change. They may not distinguish between the individual and the population.
What to do instead
Emphasize that evolution is a change in allele frequencies in a population over generations. Natural selection acts on phenotypes, which are determined by genotypes and environment. Individuals do not evolve; populations do. Use examples like antibiotic resistance in bacteria to illustrate selection on existing variation.
In cell division, students describe mitosis as producing two genetically different cells, or they confuse the events of mitosis with those of meiosis (e.g., crossing over in mitosis).
Students may not have a clear mental model of the phases and the purpose of each division type. They often memorize phase names without understanding the outcomes.
What to do instead
Compare mitosis and meiosis side by side: mitosis produces two identical diploid cells for growth and repair; meiosis produces four genetically diverse haploid gametes. Learn the key events of each phase (e.g., crossing over in prophase I of meiosis, separation of sister chromatids in anaphase of mitosis). Draw diagrams and practice identifying phases from descriptions.