LC Biology common mistakes
LC Biology · Understand the traps and how to avoid them.
Defining a hypothesis as a guess or prediction rather than a testable, falsifiable statement, and then describing the experiment as proving the hypothesis correct.
Everyday English uses 'guess' and 'prove' loosely, and textbook summaries of the scientific method compress it into a slogan. Candidates also confuse the hypothesis with the prediction derived from it.
What to do instead
Practise writing hypotheses in the form 'X causes Y because Z', then state the prediction that would follow and one result that would falsify it. When reporting, use 'supported' or 'not supported by the data' instead of 'proved', and note that a single experiment cannot establish a general law.
Listing the variables of an experiment without identifying which is independent, which is dependent, and which must be held constant, or naming a control that is not actually a control.
Candidates memorise 'fair test' language without applying it to the specific design, and they often call any comparison group a control even when it lacks the defining feature (for example, a control for an enzyme experiment must contain everything except the enzyme).
What to do instead
For each practical, draw a three-column table (independent, dependent, controlled) before writing the report, and justify each controlled variable in one clause. State explicitly what the control is and what it rules out, e.g. boiled enzyme rules out the substrate reacting on its own.
Recording raw counts or colour changes as results and drawing a conclusion without any replicate, mean, or indication of variation.
School practicals often produce a single dramatic result (a colour change, a bubble count), so candidates treat one observation as data. They have rarely been required to calculate a mean or range, so they omit them.
What to do instead
Adopt a fixed reporting template: repeat each treatment at least three times, tabulate raw values, calculate the mean, and quote the range or a simple measure of spread. In the conclusion, refer to the mean and acknowledge overlap between replicates before claiming a trend.
Confusing chemical digestion with mechanical digestion and stating that enzymes 'digest' food in the mouth, stomach, and small intestine without naming the substrate, product, and optimum pH at each site.
The digestive system is taught as an anatomy diagram with enzymes listed separately, so the two are memorised in isolation. Candidates also carry the misconception that all enzymes work best at pH 7.
What to do instead
Build a table with one row per enzyme: substrate, product, site of action, optimum pH, and the source gland. Include salivary amylase (starch to maltose, pH near neutral), pepsin (protein to peptides, pH 2), and pancreatic lipase (fat to fatty acids and glycerol, alkaline). Check each entry against the pH of the compartment, not the body.
Describing the cell membrane as a static wall with holes and explaining osmosis as water moving from a low to a high water concentration.
The 'fluid mosaic' phrase is memorised as a label rather than a model, and the direction of osmosis is easily inverted because candidates reason about solute rather than water concentration.
What to do instead
Redraw the membrane from memory including phospholipid bilayer, embedded and peripheral proteins, cholesterol, and glycoproteins, and state one function for each. For osmosis, always write the comparison as water potential or water concentration: water moves from the higher water concentration (lower solute concentration) to the lower. Test yourself with a turgid plant cell and a red blood cell in distilled water.
Predicting offspring ratios from a genetic cross without drawing a full Punnett square, or writing genotypes without distinguishing homozygous, heterozygous, and the phenotype each produces.
Candidates shortcut to remembered ratios such as 3:1 and apply them to crosses that do not fit, for example when the parents are not both heterozygous. They also mix up genotype symbols with phenotype descriptions.
What to do instead
Always define alleles first (e.g. T = tall, t = dwarf), write parental genotypes, draw the gametes on the axes, fill the square, and only then count phenotypes. State the ratio with its basis ('out of four offspring'). Practise a test cross and a cross involving a sex-linked trait to break the 3:1 habit.
Explaining aerobic and anaerobic respiration as if anaerobic respiration produces the same ATP yield, or omitting the products and locations of each stage of aerobic respiration and photosynthesis.
The two processes are taught under one heading and summarised as 'respiration releases energy', so the stage-by-stage detail collapses. Candidates also confuse the light-dependent and light-independent stages of photosynthesis.
What to do instead
Write two flow diagrams from memory: glycolysis (cytoplasm, glucose to pyruvate, small ATP yield), Krebs cycle and electron transport (mitochondrion, large ATP yield, water formed), and fermentation (cytoplasm, lactate or ethanol plus carbon dioxide, no further ATP). For photosynthesis, place the light stage in the thylakoid membrane (water split, oxygen released, ATP and NADPH made) and the dark stage in the stroma (carbon dioxide fixed, sugar formed).
Describing a food chain or energy flow without units, or stating that energy is 'used up' or 'lost as heat' without quantifying the transfer between trophic levels.
Ecology answers are often written as prose narratives, so candidates never attach numbers. The phrase 'energy is lost' is memorised without the idea that energy is conserved and only its form and availability change.
What to do instead
Annotate every energy flow diagram with values in kJ m^-2 yr^-1 and calculate the percentage transferred between each level. Explain the losses explicitly: respiration as heat, excretion, and material not consumed. State that energy is not destroyed but becomes unavailable, and link this to why food chains are short.
Confusing the roles of the nervous and hormonal systems in coordination, or describing a reflex arc with the receptor and effector reversed and no synapse.
Both systems are summarised as 'control', and the reflex arc is learned as a list of words rather than a pathway with direction. Candidates also forget that hormones travel in blood and act more slowly and for longer.
What to do instead
Draw the reflex arc in order (stimulus, receptor, sensory neurone, synapse, interneuron in the spinal cord, motor neurone, effector, response) and mark the direction of the impulse. Build a comparison table for nervous versus hormonal control covering transmission route, speed, duration, and target specificity.
Describing plant transport and responses by naming structures without linking them to mechanism, for example saying 'xylem carries water up' without transpiration pull, cohesion and adhesion, or describing phototropism without auxin distribution.
Plant biology is often taught as labelling exercises, so the causal chain from evaporation at the leaf to water entry at the root is never practised. Tropisms are reduced to 'grows towards light'.
What to do instead
Write the transpiration stream as a causal chain: evaporation from leaf mesophyll, tension in xylem, cohesion between water molecules, adhesion to xylem walls, water drawn from root, uptake by root hair cells by osmosis. For phototropism, state that auxin moves to the shaded side, elongates cells there, and causes bending towards light, and contrast with geotropism in roots.