Common mistakes · Biology · 11 min read
IB Biology common mistakes: 18 answers that lose marks, and how to fix them
Eighteen slips that cost marks on the current syllabus, each shown as the answer that loses the mark next to the answer that earns it.
On this page
- Which mistakes about water and enzymes lose marks?
- Where do students mix up DNA processes and cell division?
- How should you write about evolution, antibiotics and vaccines?
- What goes wrong with respiration and photosynthesis?
- Which ecology terms get used loosely?
- Which calculation and graph mistakes are most common?
- Which exam-technique mistakes cost marks regardless of content?
- How do you stop making the same mistake twice?
Many of the marks lost in IB Biology are not gaps in knowledge. They are imprecise words (an enzyme "killed" instead of denatured), two processes blurred into one (chromatids separating when it is homologous chromosomes), and data slips (reading an amount as a rate, mixing up millimetres and micrometres). Below are 18 of them, each tagged with its syllabus topic and shown as the answer that loses the mark next to the one that earns it.
Read them with your own notes open. If you recognise a mistake as one of yours, rewrite that part of your notes in the "earns marks" wording, because the wording you revise is the wording you will write in the exam.
Which mistakes about water and enzymes lose marks?
1. Calling the bonds inside water "hydrogen bonds" (A1.1)
Inside one water molecule, oxygen and hydrogen are joined by covalent bonds. Hydrogen bonds form between molecules: the slightly positive hydrogen of one molecule is attracted to the slightly negative oxygen of a neighbour. Every property you are asked to explain (cohesion, high specific heat capacity, solvent behaviour) depends on that distinction.
Loses marks Water has a high specific heat capacity because of the strong hydrogen bonds between its oxygen and hydrogen atoms.
Earns marks Water molecules are polar, so hydrogen bonds form between the δ+ hydrogen of one molecule and the δ− oxygen of another. Energy is needed to break these many hydrogen bonds before molecules can move faster, so water's temperature changes slowly.
2. Getting osmosis the wrong way round (D2.3)
At SL, describe osmosis in terms of solute concentration: water moves from the less concentrated solution (lower solute concentration) to the more concentrated one. At HL you also use water potential: water moves from a higher water potential to a lower one. Pure water at atmospheric pressure has a water potential of zero and adding solutes makes it more negative, so "lower" means "more negative". Writing "water concentration" or "concentration" without saying of what leaves the examiner unable to credit the answer.
Loses marks A cell at −300 kPa placed in a solution at −500 kPa gains water because water moves from low to high concentration.
Earns marks (HL) The solution has the lower (more negative) water potential, so there is a net movement of water out of the cell by osmosis, from −300 kPa to −500 kPa. (SL wording: the solution has a higher solute concentration than the cytoplasm, so water leaves the cell by osmosis.)
3. Saying enzymes are "killed" by heat (C1.1)
Enzymes are proteins, not organisms, so they cannot die. High temperature or extreme pH breaks the bonds that hold the tertiary structure together, the active site changes shape, and the substrate no longer binds. Equally important: cold does not denature enzymes. At low temperature they work slowly because molecules have less kinetic energy and collide less often.
Loses marks Above 40 °C the enzyme is killed, so the rate drops.
Earns marks Above 40 °C, increased vibration breaks hydrogen and ionic bonds in the enzyme, changing the shape of the active site. Fewer substrate molecules can bind, so fewer enzyme–substrate complexes form and the rate falls. This denaturation is usually permanent.
4. Saying the substrate is the "same shape" as the active site (C1.1)
A key is not the same shape as the lock; it fits it. The active site and substrate are complementary in shape and in chemical properties (charge, polarity), and under the induced-fit model the active site changes shape slightly as the substrate binds.
Loses marks The substrate has the same shape as the active site, so only that substrate can fit.
Earns marks The active site's shape and chemical properties are complementary to the substrate, so only that substrate can bind; binding induces a small change in the active site's shape that strains bonds in the substrate.
Where do students mix up DNA processes and cell division?
5. Blurring DNA replication and transcription (D1.1, D1.2)
Both start with the double helix opening, which is exactly why they get mixed up. Keep a two-column comparison in your notes and check every answer against it.
| Item | Replication | Transcription |
|---|---|---|
| Product | Two DNA molecules | One mRNA molecule |
| Strands used as template | Both | One (the template strand), for one gene |
| Enzymes to name | Helicase unwinds and separates strands; DNA polymerase links nucleotides | RNA polymerase separates strands and links RNA nucleotides |
| Base paired with adenine | Thymine | Uracil |
| "Semi-conservative"? | Yes | No; the term does not apply |
Loses marks In transcription, helicase unzips the DNA and DNA polymerase makes mRNA by semi-conservative copying.
Earns marks In transcription, RNA polymerase separates the two DNA strands and joins free RNA nucleotides that pair with the template strand, with uracil pairing with adenine.
6. Separating chromatids in meiosis I (D2.1)
In meiosis I, homologous chromosomes separate; that is the reduction division that halves the chromosome number. In meiosis II, and in mitosis, sister chromatids separate. Count carefully too: after DNA replication a chromosome consists of two chromatids, but it is still one chromosome. A human cell entering mitosis has 46 chromosomes and 92 chromatids.
Loses marks In anaphase I the chromatids are pulled to opposite poles, halving the number of chromosomes.
Earns marks In anaphase I, homologous chromosomes are pulled to opposite poles, each still made of two chromatids. The chromosome number halves. Sister chromatids separate later, in anaphase II.
How should you write about evolution, antibiotics and vaccines?
7. Making natural selection sound intentional (D4.1)
"Evolved to", "needed to", "adapted in order to" all suggest that organisms change on purpose. Examiners look for the mechanism: variation that already exists, a selection pressure, differential survival and reproduction, inheritance, and a change in allele frequency over generations.
Loses marks The finches developed bigger beaks so they could crack the harder seeds during the drought.
Earns marks Beak size already varied in the population. During the drought, birds with larger beaks could crack the remaining hard seeds, so more of them survived and reproduced, passing on alleles for larger beaks. Over generations the frequency of those alleles increased.
8. Saying bacteria become "immune" to antibiotics (C3.2)
Immunity belongs to the host. Bacteria become resistant, and individual bacteria do not change in response to the drug. Write the selection argument in full, and add the bacteria-specific detail that resistance genes can spread between bacteria on plasmids.
Loses marks Bacteria get used to the antibiotic and become immune to it, especially if people don't finish the course.
Earns marks A random mutation gives some bacteria a resistance allele. The antibiotic kills susceptible bacteria, so resistant ones survive, reproduce and pass the allele on; it can also be transferred to other bacteria on plasmids. Widespread or unnecessary antibiotic use increases this selection pressure.
9. Saying vaccines contain antibodies (C3.2)
A vaccine contains antigens, or nucleic acid (such as mRNA) coding for an antigen. The person's own lymphocytes then make antibodies, and memory cells are produced. Giving ready-made antibodies is passive immunity, a different idea.
Loses marks The vaccine gives you antibodies against the virus so you are protected.
Earns marks The vaccine introduces an antigen (or mRNA coding for it), which triggers a primary immune response: specific lymphocytes are activated, produce antibodies and form memory cells. On later infection, memory cells allow a faster, larger secondary response.
What goes wrong with respiration and photosynthesis?
10. "Respiration produces energy" (C1.2)
Energy cannot be created. Respiration releases energy from organic molecules such as glucose and uses it to make ATP. Two related slips: respiration is not breathing (that is ventilation, part of gas exchange), and plants respire all the time, in light and dark, not only when photosynthesis stops.
Loses marks Plants photosynthesise during the day and respire at night to produce energy.
Earns marks Plant cells respire continuously, releasing energy from glucose to produce ATP. In bright light, the rate of photosynthesis exceeds the rate of respiration, so there is net uptake of CO2 and net release of O2.
11. Getting the source of oxygen, or the location of each stage, wrong (C1.3, C1.2)
The oxygen released in photosynthesis comes from splitting water (photolysis), not from carbon dioxide; the carbon in glucose comes from CO2. If you are HL and need the stages, pin each to a place: light-dependent reactions on the thylakoid membranes, the Calvin cycle in the stroma; glycolysis in the cytoplasm, the link reaction and Krebs cycle in the mitochondrial matrix, the electron transport chain on the inner membrane (cristae).
Loses marks The Krebs cycle takes place on the cristae, and the O2 released by plants comes from CO2.
Earns marks The Krebs cycle takes place in the matrix; the electron transport chain is on the cristae. The O2 released by plants comes from photolysis of water.
Which ecology terms get used loosely?
12. Population, community, ecosystem (C4.1)
A population is the individuals of one species in the same area at the same time. A community is all the populations of different species living and interacting in an area. An ecosystem is the community together with its abiotic environment. Using "population" when you mean community is one of the quickest ways to lose a definition mark.
Loses marks The population of the pond includes frogs, pondweed and water beetles.
Earns marks The community of the pond includes populations of frogs, pondweed and water beetles; with the water, sediment and light, it forms an ecosystem.
13. Saying energy is recycled (C4.2)
Chemical elements such as carbon and nitrogen are recycled. Energy flows through a food chain and leaves it, mostly as heat released by respiration, which is why chains are short. Energy is not "used up" either; it is transferred and eventually lost from the system as heat.
Loses marks Energy is recycled by decomposers and returned to the producers.
Earns marks Decomposers recycle nutrients such as carbon and nitrogen, but energy is not recycled: at each trophic level much of it is lost as heat from respiration, so ecosystems need a continuous input of light energy.
Which calculation and graph mistakes are most common?
14. Unit slips in magnification (A2.2)
Magnification = image size ÷ actual size, with both in the same unit. Remember 1 mm = 1000 µm. Magnification has no unit; write it as ×4000.
Example. A mitochondrion measures 45 mm on a micrograph at ×15 000. Actual length = 45 mm ÷ 15 000 = 0.003 mm = 3 µm, a sensible size for a mitochondrion. A scale bar 20 mm long labelled 5 µm gives magnification = 20 000 µm ÷ 5 µm = ×4000.
Loses marks 20 ÷ 5 = ×4 magnification.
Earns marks 20 mm = 20 000 µm; 20 000 ÷ 5 = ×4000.
15. Reading an amount graph as a rate graph
On a graph of product formed against time, the rate is the gradient. A flat line at the top means the rate has fallen to zero, usually because the substrate has run out, not that the enzyme has stopped working or that the rate is "at its maximum". The worked example in our Paper 1B guide shows this with two enzyme concentrations.
Loses marks After 90 s the line is flat, so the reaction is at its fastest rate.
Earns marks After 90 s no more product forms, so the rate is zero; all the substrate has been converted. The enzyme is unchanged and could act on more substrate.
16. Saying data "prove" a cause
Data from a sample can support or suggest a conclusion; they do not prove it. A correlation shows two variables change together, not that one causes the other. Mention sample size, repeats, error bars or a statistical test when the question invites it; the statistical tests guide explains which test fits which data.
Loses marks The graph proves that higher nitrate levels cause more algae.
Earns marks There is a positive correlation between nitrate concentration and algal density, which suggests nitrate may limit algal growth, but other factors such as temperature or light could also differ between the sites.
Which exam-technique mistakes cost marks regardless of content?
17. Explaining when asked to describe, or describing when asked to explain
Describe means what the data show, with values. Explain means why, using biology. "Outline" is a short account. "Compare" asks for similarities, and "compare and contrast" for similarities and differences; either way, mention both items in each point. The full list, with the trap in each, is in IB command terms.
Loses marks (Describe the trend.) The rate increases because more collisions happen.
Earns marks (Describe the trend.) The rate rises steadily from 2 to 8 µmol min−1 between 10 and 35 °C, then falls sharply to 1 µmol min−1 at 50 °C.
18. Labels that point at nothing in particular
For "draw" and "label": ruled label lines that touch exactly the structure named, no arrowheads, no shading, labels outside the drawing, and only the structures you can see. "Annotate" means label plus a short note on function. Typical losses: a label line ending between the cell wall and the membrane, and ribosomes drawn inside the nucleus.
Loses marks A line from "cell membrane" that ends in the gap between membrane and wall; mitochondria drawn without the inner membrane folds.
Earns marks A ruled line touching the membrane itself; mitochondria with a double membrane and cristae visible, labelled "cristae: folded inner membrane, large area for the electron transport chain" when annotation is asked for.
How do you stop making the same mistake twice?
Recognising a mistake while reading this page is not the same as not making it under time pressure. Three habits that work:
- Keep an error log. Every time a mark scheme shows you a point you missed, write the losing wording and the winning wording side by side, as above. Read it the night before each exam. Our guide on why you're not improving explains why this beats doing more questions.
- Turn each fix into a question. "Which structures separate in anaphase I?" on a flashcard drills the distinction far better than rereading the paragraph.
- Practise data, not just recall. Mistakes 14–16 only show up when you handle numbers. Paper 1B and the data question in Paper 2 both demand it; see what the Biology exams look like now.
Questions students ask
How do I get a 7 in IB Biology?
Know the content precisely enough to use the exact terms (denatured, complementary, homologous), practise data questions as often as recall, and check every practice answer against a mark scheme to see which point you missed. Marks between a 5 and a 7 are often lost on wording and data handling rather than on unfamiliar topics.
Do spelling mistakes lose marks in IB Biology?
Examiners are there to credit biology, not spelling, but a misspelling that turns one term into another cannot be credited, because the examiner cannot tell which you meant. Mitosis and meiosis, glucagon and glycogen, ileum and ilium are the classic pairs. Spell those carefully.
Is "increases" enough to describe a trend on a graph?
Rarely. A description that earns marks gives the direction, the shape (steady rise, plateau, peak) and at least one pair of values with units read off the graph. If the question says explain, add the biological reason as well.
Try one: Biology
A species has a diploid number of 8 (2n = 8). Which describes each cell produced at the end of meiosis I?
- 8 chromosomes, each made of one chromatid
- 4 chromosomes, each made of one chromatid
- 4 chromosomes, each made of two chromatids
- 8 chromosomes, each made of two chromatids
Show the answer and why
Answer: C
In meiosis I the homologous chromosomes separate, so each cell receives one chromosome from each pair: 4 chromosomes. The sister chromatids stay joined at the centromere until anaphase II, so each of those 4 chromosomes still has two chromatids. B describes the cells at the end of meiosis II. The tempting error is to think chromatids separate in meiosis I, which is what happens in mitosis and in meiosis II.