Most wrong answers on a Biology Challenge paper aren't guesses — they're a well-known misconception wearing the shape of a plausible option. Multiple-choice questions are often built exactly this way: the right answer sits next to two or three wrong answers that feel right if you half-understand the topic. Below are the misconceptions behind our own Biology Challenge flashcard deck — worth correcting whether or not your child ever sits the paper, because each one is a genuine KS3 biology stumbling block. For the facts themselves, see Biology Challenge facts every KS3 child should know.

"Plants photosynthesise instead of respiring"

A green plant does not switch off respiration when the sun comes out. Every living cell — plant or animal — respires constantly, day and night, releasing energy from glucose. Photosynthesis is a separate reaction that only runs in daylight, in cells that contain chlorophyll. So during the day a leaf is doing both at once, and because photosynthesis usually happens faster, the plant is a net taker of carbon dioxide; at night, with photosynthesis switched off, it's a net giver. Treating the two as alternatives — "is the plant respiring or photosynthesising?" — is the wrong question; the right one is "which is happening faster right now?"

"Muscles make ethanol and carbon dioxide when they run out of oxygen"

That's what yeast does, not muscle. When a sprinting muscle can't get enough oxygen, human cells respire anaerobically by turning glucose into lactic acid alone — no ethanol, no carbon dioxide. Yeast, and some other microbes, run a different anaerobic pathway called fermentation, glucose to ethanol and carbon dioxide, which is why bread rises and why fermentation makes alcohol. The two processes get taught close together, which is exactly why they get muddled; the fix is to fix one keyword to each: muscle → lactic acid, yeast → ethanol.

Reading a heart diagram as a mirror image

This one has nothing to do with understanding the heart and everything to do with reading a diagram. "Left" and "right" on a heart diagram refer to the patient's own left and right — so on a diagram facing you, the left ventricle (the thick-walled chamber that pumps oxygenated blood out to the whole body) sits on the right-hand side of the page. A student who knows the biology perfectly can still get this wrong by reading the diagram as if it were a mirror. The fix is procedural, not conceptual: before answering, mentally put yourself in the diagram's position, facing the same way as the heart's owner.

"Oxygen travels dissolved in the blood plasma"

Only a trace does. Almost all the oxygen your blood carries rides on haemoglobin, the iron-rich protein packed into red blood cells — which is the entire reason red blood cells exist in the huge numbers they do, and why they're shaped the way they are (biconcave, with no nucleus, to maximise the space for haemoglobin and the surface area for gas exchange). Plasma does carry things dissolved — food, carbon dioxide, hormones — but bulk oxygen transport is not one of them.

"A gamete carries the full 46 chromosomes"

Follow this one to its logical end and it breaks immediately: if an egg and a sperm each carried 46 chromosomes like an ordinary body cell, a fertilised egg would have 92, and the chromosome number would double every single generation. It doesn't, because a gamete carries only 23 — half the full set — precisely so that fertilisation restores 46 and the number stays stable. The pairing to remember is body cell 46, gamete 23, not "gametes are just smaller cells."

"Bacteria learn to resist antibiotics"

No individual bacterium changes its mind or adapts on purpose. In any large bacterial population, a few cells already happen to carry a mutation that lets them survive a particular antibiotic — pure chance, present before the drug ever arrives. The antibiotic kills everything that can't cope; the resistant survivors multiply; the next generation is mostly resistant. That's natural selection acting on a population, not learning happening inside an individual — the same logic behind the peppered moth and the Galápagos finches, just running on a twenty-minute generation time instead of a twenty-year one.

Drawing a food-chain arrow the wrong way round

It's tempting to draw the arrow towards the food, since that's the direction the predator moves — fox ← rabbit, because the fox eats the rabbit. But a food-chain arrow means "is eaten by," and it shows the direction energy is transferred, not the direction of movement or appetite: grass → rabbit → fox. Getting this backwards doesn't just cost one mark; it also flips how a pyramid of biomass gets read, since the arrow direction and the "which level has more energy" question are the same idea viewed two ways.

"Antibiotics treat any infection"

Antibiotics kill bacteria, full stop — they do nothing against viruses, which is why a doctor won't prescribe them for flu, a cold, or COVID-19. The reason isn't that antibiotics are "too weak" for viruses; it's that a virus hides and replicates inside your own cells, somewhere an antibiotic's mechanism can't reach. Taking antibiotics for a viral infection doesn't just fail to help — it also breeds resistant bacteria elsewhere in the body, for no benefit at all.

Why correcting the misconception beats memorising the answer

Every one of these traps has a right answer that's easy to memorise in isolation — but a multiple-choice paper is specifically designed to catch a student who knows the answer without knowing why. A child who understands that a gamete's 23 chromosomes exist to stop the count doubling will get any rephrased version of that question right; a child who's only memorised "the answer is 23" might not recognise the same idea asked a different way. That's the case for spaced, active review over a single read-through — see preparing for the Biology Challenge with spaced repetition.

FAQ

Are these mistakes specific to the Biology Challenge?

No — they're general KS3 biology misconceptions that happen to be exactly the kind of thing a well-written multiple-choice question is designed to probe. They're worth correcting regardless of whether your child ever sits the paper.

Why do multiple-choice questions catch these particular mistakes?

Because the wrong options are usually built from the misconception itself, not chosen at random. A student who has genuinely understood the concept sails past the trap; a student who has only memorised the right answer for a differently-worded question can still fall into it.

How do I help my child unlearn a misconception like these?

Naming it explicitly helps more than repeating the correct fact alone — "muscles don't make ethanol, only yeast does" sticks better than "muscles make lactic acid" on its own. Regular, spaced review of the correction is what makes it stick long-term; see preparing for the Biology Challenge with spaced repetition.

Duke Harewood runs aitutors.me and built its KS3 science tutors, including Professor Darwin for biology.