UEE Biology common mistakes
UEE Biology · Understand the traps and how to avoid them.
Writing the net ATP yield of aerobic respiration as 36 or 38 without accounting for the 2 ATP spent in the investment phase of glycolysis, or forgetting that the exam expects the current figure of about 30–32 ATP per glucose when NADH shuttles and proton leak are counted.
Older textbooks still print 36–38, so candidates memorise a single number without tracking where each ATP and NADH comes from; they also forget that cytosolic NADH must be shuttled into the mitochondrion and yields less than mitochondrial NADH.
What to do instead
Build a ledger: glycolysis gives net 2 ATP and 2 NADH; pyruvate oxidation gives 2 NADH; the Krebs cycle gives 2 ATP (or GTP), 6 NADH and 2 FADH2. Then state the assumption you are using (e.g. malate–aspartate shuttle, ~2.5 ATP per NADH, ~1.5 per FADH2) and give the range the syllabus accepts. Practise writing the ledger from memory in under two minutes.
Treating enzymes as consumed reagents or claiming an enzyme shifts the equilibrium constant of a reaction, rather than lowering activation energy and being regenerated unchanged.
Candidates conflate 'catalyst' with 'reactant' and confuse the effect on rate with the effect on thermodynamics; diagrams showing the enzyme–substrate complex can look like a reaction step.
What to do instead
State three separate claims every time: the enzyme is not consumed; it lowers activation energy by stabilising the transition state; it changes the rate at which equilibrium is reached but not the position of equilibrium (ΔG and Keq are unchanged). Then link to a concrete case such as competitive inhibition raising apparent Km without changing Vmax.
Predicting offspring ratios from a dihybrid cross by multiplying probabilities but ignoring linkage, or assuming independent assortment for genes on the same chromosome.
Mendel's second law is drilled as a default, and candidates do not check whether the question states the genes are on the same chromosome or gives recombination frequencies.
What to do instead
Before calculating, ask: are the loci stated to be on different chromosomes or far apart? If linkage is given, use recombination frequency to identify parental and recombinant gamete classes, and remember that a testcross reveals the parental types as the most frequent progeny. If independent assortment is stated, only then apply the 9:3:3:1 ratio.
Confusing transcription and translation in terms of location, template and product, for example saying mRNA is made in the cytoplasm or that ribosomes read DNA directly.
The two processes are taught together and share vocabulary (template, complementary base pairing), so the details blur under exam pressure; prokaryote and eukaryote differences add further confusion.
What to do instead
Anchor a three-column table: process, template, product, location, enzyme or machine. Transcription uses the template strand of DNA, RNA polymerase, produces mRNA, occurs in the nucleus in eukaryotes; translation uses mRNA, ribosomes and tRNA, produces polypeptide, occurs in the cytoplasm. Add one line on prokaryotes lacking a nuclear envelope so the processes are coupled.
Explaining transpiration pull as 'water is pushed up by root pressure' or as if plants actively pump water, instead of describing cohesion–tension driven by evaporation at the leaf.
Root pressure is a real phenomenon and is easy to overgeneralise; the passive, tension-driven nature of the xylem is counterintuitive because it involves negative pressure.
What to do instead
Write the causal chain explicitly: evaporation from mesophyll surfaces creates tension in the leaf apoplast; cohesion between water molecules and adhesion to xylem walls transmit the tension down a continuous water column; water enters roots by osmosis. Then note that root pressure contributes only a small amount, mainly at night, and cannot explain tall-tree transport.
Describing the light-dependent and light-independent reactions as if the Calvin cycle directly uses light, or failing to name where each occurs and what each consumes and produces.
The two stages are often summarised as 'photosynthesis uses light, CO2 and water to make glucose', which hides the intermediate carriers and the fact that ATP and NADPH link the stages.
What to do instead
Memorise a two-row table: light-dependent reactions occur on the thylakoid membrane, use light and water, produce ATP, NADPH and O2; the Calvin cycle occurs in the stroma, uses ATP and NADPH plus CO2, and produces triose phosphate (G3P) that is built into sugars. State that the Calvin cycle does not use light directly but depends on the products of the light reactions.
Describing the mammalian circulatory system as a single circuit, or mixing up which side of the heart receives oxygenated blood and where the systemic and pulmonary circuits begin and end.
Diagrams are often studied visually without tracing the path verbally, so left/right and oxygenated/deoxygenated get swapped, and the pulmonary circuit is forgotten.
What to do instead
Trace the path aloud: vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body → vena cava. Note that the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood, and that the left ventricle wall is thicker because it drives the systemic circuit.
Explaining natural selection as if individuals adapt during their lifetime, or as if the environment directs mutations to produce needed traits.
Everyday language uses 'adapt' for both physiological acclimatisation and evolutionary change, and the phrase 'survival of the fittest' encourages a purposeful reading.
What to do instead
Use a strict three-step statement: variation already exists in the population (generated by mutation and recombination, not by need); some variants leave more offspring in the current environment; allele frequencies therefore change across generations. Reserve 'adaptation' for the heritable trait and use 'acclimatisation' for within-lifetime change. Test yourself by rewriting a Lamarckian sentence into a Darwinian one.
Classifying organisms by superficial features or common names, for example calling a whale a fish or treating fungi as plants, instead of using the defining characters of the group.
Common names and everyday categories conflict with phylogenetic classification, and candidates rely on habitat or appearance rather than diagnostic characters.
What to do instead
For each group, memorise two or three diagnostic characters: mammals have hair and mammary glands; fungi have chitinous cell walls and are heterotrophic by absorption; plants have cellulose walls and are photoautotrophic. When a question gives an unfamiliar organism, classify by the stated characters rather than the name, and remember that classification reflects evolutionary relatedness, not habitat.
Reading a graph of population growth or an experimental result and describing the shape ('it goes up then levels off') instead of extracting the requested quantity, such as the carrying capacity, the exponential phase or the effect of the manipulated variable.
Candidates answer the question they can see rather than the one asked, and they do not check axes, units or the control condition before writing.
What to do instead
Before writing, underline the command word (state, calculate, describe, explain) and the quantity requested. Read the axes and units, identify the independent and dependent variables and the control, then give a value with units where a number is asked. For growth curves, name the phases (lag, exponential, stationary) and quote the plateau value as carrying capacity rather than describing the line.