ORT Biology common mistakes
ORT Biology · Understand the traps and how to avoid them.
When asked to compare aerobic respiration and anaerobic respiration, the candidate writes 'anaerobic produces less ATP' and stops, without stating where each stage occurs (cytoplasm vs. mitochondrial matrix/inner membrane) or giving the ATP yield per glucose.
Candidates memorize the headline contrast from a textbook summary and never build a stage-by-stage map, so under time pressure they retrieve only the single most rehearsed sentence and cannot expand it into the marks the question allocates.
What to do instead
Build a one-page comparison table for glycolysis, link reaction, Krebs cycle, oxidative phosphorylation, and fermentation with columns for location, inputs, outputs, and net ATP/NADH/FADH2. Before the exam, practise writing it from memory in under five minutes, then answer three past-style comparison questions using the table's structure.
The candidate describes an enzyme as 'denatured' whenever activity falls, including cases where a competitive inhibitor or a pH away from the optimum is the actual cause, and never distinguishes reversible inhibition from irreversible denaturation.
Denaturation is the most frequently repeated explanation in lessons, so it becomes a default answer. Candidates also conflate 'shape changed' with 'shape permanently destroyed' and do not check whether the stem specifies temperature, pH, or an inhibitor.
What to do instead
Learn a decision sequence: check for an inhibitor first (does adding substrate restore rate? if yes, competitive), then check pH/temperature against the stated optimum, and only then invoke denaturation. Write three short explanations, one for each cause, and check each against the stem's specific data before committing.
In a dihybrid or sex-linked cross, the candidate writes the phenotype ratio without showing the gametes or a Punnett grid, and mislabels which parent is heterozygous when the question gives a test-cross result.
Candidates learn the 9:3:3:1 and 3:1 ratios as facts to recall rather than as outcomes to derive, so they apply the ratio even when linkage, epistasis, or sex linkage makes it wrong. Drawing grids feels slow, so it is skipped under time pressure.
What to do instead
Always write parental genotypes, then list gametes, then draw the grid, even when the ratio seems obvious. Practise identifying from the wording whether the cross is a test cross (crossed with homozygous recessive) and whether genes are on sex chromosomes. Do five crosses where the expected ratio is deliberately not 9:3:3:1.
The candidate describes transcription and translation but omits the directionality and template details: which strand is the template, that RNA polymerase reads 3' to 5' and synthesises 5' to 3', and that the ribosome reads mRNA codons 5' to 3' while tRNA anticodons pair antiparallel.
The overall flow of information is easy to state, so candidates stop at the summary level and never drill the polarity conventions. These are abstract and easy to mix up, and exam diagrams often show strands without labels, encouraging guessing.
What to do instead
Draw a labelled gene with both strands marked 5' and 3', then annotate the template strand, the mRNA, and the tRNA anticodon with their polarities. Recite the rule 'polymerase moves 3' to 5' on the template, new strand grows 5' to 3'' until it is automatic, then apply it to three questions involving a given DNA sequence.
When explaining gas exchange or absorption, the candidate lists structures (alveoli, villi, nephrons) without linking each structural feature to the specific property it provides, such as surface area, thin diffusion distance, or steep concentration gradient.
Candidates revise structure and function as two separate lists and never build the explicit 'feature therefore property therefore effect on rate' chain. Mark schemes reward the chain, so a list of nouns earns little even though the candidate knows the anatomy.
What to do instead
For each exchange surface, write sentences in the form 'The wall is one cell thick, which shortens the diffusion distance, which increases the rate of diffusion.' Do this for alveoli, villi, gill lamellae, and the loop of Henle. Then answer two questions where the examiner asks specifically how a named feature increases rate.
The candidate calculates a cardiac output, ventilation rate, or surface-area-to-volume ratio but keeps units inconsistent, for example multiplying stroke volume in cm3 by heart rate in beats per minute and reporting the answer in litres without converting, or squaring a radius given in millimetres and comparing it to an area in cm2.
The formula is recalled correctly, but unit handling is treated as an afterthought and never rehearsed. Candidates also forget that surface area scales with the square while volume scales with the cube, so they compare ratios computed from mismatched dimensions.
What to do instead
Adopt a fixed routine: write the formula, convert every quantity to SI or to a single stated unit before substituting, carry units through the calculation, and check that the final unit matches what was asked. Practise ten calculations that deliberately mix cm, mm, and litres, and always state the SA:V ratio with its units.
The candidate describes a food chain or energy transfer but omits the quantitative losses: no mention of respiration, egestion, excretion, or the approximately 10% transfer between trophic levels, and no distinction between gross and net primary productivity.
Energy flow is taught as a diagram with arrows, and candidates reproduce the arrows without attaching numbers or named loss routes. Gross versus net productivity is often confused because both appear in the same topic and differ by one subtraction.
What to do instead
Annotate one energy-flow diagram with the four loss routes and typical percentages at each arrow. Learn the definitions as equations (NPP = GPP minus respiratory loss) and practise explaining why a pyramid of energy is always upright while a pyramid of numbers need not be, citing a specific counterexample such as a single oak tree supporting many insects.
The candidate explains antibiotic resistance or pesticide resistance by saying individual bacteria or insects 'adapt' or 'develop resistance' in response to the chemical, rather than stating that resistant variants already exist and are selected for.
Everyday language uses 'adapt' for both physiological acclimatisation and evolutionary change, and this ambiguity survives into exam answers. Candidates also skip the requirement that the variation must be heritable and that selection changes allele frequencies over generations.
What to do instead
Practise rewriting any sentence containing 'adapt' into the selection sequence: pre-existing variation from mutation, differential survival, reproduction, increase in allele frequency. Write the full sequence for one bacterial and one insect example, and check that the words 'already present' and 'frequency' appear.
The candidate describes mitosis and meiosis as lists of phases without stating the ploidy at each stage, the behaviour of homologous chromosomes versus sister chromatids, and the two sources of genetic variation unique to meiosis (crossing over and independent assortment).
Phase names are memorable and easy to sequence, so revision stops there. Ploidy and chromosome behaviour are harder to hold in working memory, and candidates conflate 'homologous chromosomes separate' with 'sister chromatids separate', which happens at different divisions.
What to do instead
Draw one cell with two pairs of chromosomes and carry it through meiosis I and II, labelling ploidy and chromatid number at every stage. State explicitly that homologues separate in meiosis I and sister chromatids in meiosis II. Then write two sentences explaining how crossing over and independent assortment each generate variation, naming the stage.
In a plant transport or transpiration question, the candidate describes water movement as 'sucked up' by the leaves or attributes it solely to transpiration pull, omitting cohesion-tension, root pressure, and the role of the Casparian strip, and confuses xylem with phloem direction of flow.
Transpiration is taught with a vivid demonstration, so the pull dominates recall and the supporting mechanisms are forgotten. Xylem and phloem are revised together, which encourages the error of treating both as bidirectional.
What to do instead
Write a single paragraph tracing water from soil to air that must include root hair absorption, the Casparian strip forcing the symplast route, root pressure, cohesion and adhesion in the xylem, and evaporation from the leaf. Then write a parallel paragraph for phloem translocation stating that flow is bidirectional and requires active loading at the source.
When explaining the use of a biotechnological technique such as PCR, gel electrophoresis, or genetic modification, the candidate describes the purpose but omits the mechanism and the specific conditions, for example not stating that PCR needs a thermostable DNA polymerase, primers, and temperature cycling, or describing electrophoresis without mentioning charge and fragment size.
These techniques are often presented as applications with a headline use, so candidates learn what they are for but not how they work. The procedural detail feels peripheral until a question asks for it directly.
What to do instead
For each technique, write a numbered protocol of no more than six steps that includes the key reagent and the physical principle at each step. For PCR, state the three temperatures and what happens at each. For electrophoresis, state why DNA moves toward the anode and how fragment size affects migration distance. Test yourself by explaining each technique to someone with no biology background.