Biology is brand new to you, so read this page before you sit any of the challenge papers — it is the on-ramp. Topic 2 loses marks in two places and almost nowhere else. One: wording. The wall does not control what enters a cell, mitochondria do not make energy, a bacterium without a nucleus still has DNA, and a tissue is cells that are similar while an organ is tissues that are different. Two: arithmetic. Magnification is image ÷ actual, both lengths in the same unit, and 1 mm = 1000 µm — miss that conversion and your answer is wrong by exactly a factor of a thousand, every time. Twelve traps below, then reasoning walkthroughs, worked magnification examples that get harder as you go, and a practice bank with full mark schemes.
Twelve traps that cost marks on Topic 2 questions. Every one of them turns up on challenge papers regularly.
Six challenge questions broken down step by step. Walkthroughs 2, 3 and 4 are magnification calculations of rising difficulty — do them in order. Try each step yourself before revealing the next.
A soft rounded outline means an animal cell; a straight-edged, angular boundary with a distinct thick layer means a cell wall. Animal cells never have one. So this cell is either plant or bacterial — and the presence of a large nucleus rules out bacterial immediately. Do not guess from the organelles first; the outline is the fastest and most reliable clue.
Typical sizes worth carrying in your head: plant cell ≈ 100 µm, animal cell ≈ 20 µm, red blood cell ≈ 7 µm, bacterium ≈ 1–5 µm, mitochondrion ≈ 2 µm. A 90 µm cell is far too big to be bacterial and comfortably in the plant range. The scale bar has just given you a second, independent piece of evidence.
The dense spherical body with a darker region inside is the nucleus (the darker region is the nucleolus) — it contains the genetic material and controls the activities of the cell. The oval bodies 2 µm long with folded internal membranes are mitochondria — the site of aerobic respiration, releasing energy. The large pale region is the large permanent vacuole, filled with cell sap; it stores dissolved substances and its pressure on the cell wall supports the cell.
Chloroplasts are only present in the parts of a plant that receive light, because they contain chlorophyll for photosynthesis. A cell with a wall and a vacuole but no chloroplasts therefore comes from a part of the plant kept in the dark — a root, an onion bulb or a potato tuber. This does not weaken the identification; it refines it.
Never start with the calculator. Writing M = I ÷ A costs three seconds and is very often worth a mark on its own. Identify which letter you are being asked for: here it is M, so no rearrangement is needed.
Image = 96 mm, actual = 0.12 mm. They are in the same unit, so no conversion is needed — but you should notice that, because in the next two walkthroughs they will not match, and the checking habit is what saves you.
The drawing is much bigger than the cell, so a magnification well above 1 is exactly right. Write it as ×800, with no units — a magnification is a ratio of two lengths, so the units cancel.
Image size is the quantity on the top of the formula triangle, so it is the only one you ever multiply for. I = 0.09 × 800 = 72 mm. Check: this cell is smaller than the first one, so its drawing should be shorter than 96 mm. It is.
Cover A on the formula triangle and you are left with I over M. A = 34 ÷ 400 = 0.085 mm. Do the division in millimetres first — converting before you divide is possible but adds an extra chance to slip.
1 mm = 1000 µm, so mm → µm means multiply by 1000: 0.085 × 1000 = 85 µm. Direction check: a micrometre is a smaller unit, so you need more of them and the number must get bigger. It did. Plausibility check: 85 µm is a very believable plant cell.
Now the image size is 85 mm and the actual size is 85 µm = 0.085 mm. M = 85 ÷ 0.085 = ×1000. Notice the trap built into the numbers: 85 and 85 look like they should cancel to 1, and a candidate who forgets the conversion writes “×1”, which would mean the printed picture is life-size.
34 mm is an image measurement. Without the magnification it says nothing at all about the real cell — the same cell photographed at ×100 would measure 8.5 mm and at ×1600 would measure 136 mm. The actual size, 85 µm, is in fact typical of a plant cell, so the conclusion is wrong as well as the reasoning.
The bar's image length is 25 mm; its actual length is 10 µm = 0.01 mm. So M = 25 ÷ 0.01 = ×2500. Forget the conversion and you get 25 ÷ 10 = 2.5, which would mean the electron microscope barely enlarges anything — an alarm bell you should hear immediately.
A = 150 ÷ 2500 = 0.06 mm, and 0.06 × 1000 = 60 µm. Plausibility: 60 µm is a reasonable cell. Notice you are reusing exactly the same two operations as in Walkthrough 3 — the question is longer, not harder.
A = 5 ÷ 2500 = 0.002 mm, and 0.002 × 1000 = 2 µm. Plausibility: mitochondria really are about 2 µm long, so both answers pass the reality test. If one of your two answers had come out at 2 mm you would know instantly that a conversion had gone missing.
Both structures were photographed on the same image, so both were enlarged by the same factor. When you divide one by the other, that factor cancels. You can therefore compare the image measurements directly: 150 ÷ 5 = 30. Saying so explicitly is worth the second mark, because it proves you understand what magnification actually is.
A bacterial cell has no nucleus, but its genetic material is present as a single circular DNA molecule lying free in the cytoplasm. The absence of a nucleus describes only how the DNA is packaged. Without DNA a cell could not make proteins or divide, so the student's conclusion contradicts the fact that the bacteria are visibly growing.
A small ring of DNA separate from the main circular DNA is a plasmid. Plasmids carry a few extra genes — frequently genes for antibiotic resistance. Here the plasmid must carry a gene that lets the cell survive the antibiotic, for example by producing a protein that inactivates the drug. That is why only the plasmid-carrying cells formed colonies.
Bacterial cells have a cell wall; human (animal) cells do not. A drug aimed at wall construction therefore has no target at all in a human cell. In the bacterium, a defective wall cannot resist the pressure of water entering the cell, so the cell bursts. Unique structures make safe drug targets — this is the practical value of the comparison table.
Ribosomes are present in animal, plant and bacterial cells, because every cell must carry out protein synthesis. A drug that stopped ribosomes working would stop protein synthesis in the patient's cells too. Such antibiotics have to exploit small differences between bacterial and human ribosomes, which is a far more delicate design problem.
Hundreds of short beating projections are cilia, so those are ciliated cells; in humans they line the trachea and bronchi and move mucus, and here they will move food particles or fluid along the tube. Long thin cells carrying electrical signals are neurones, whose function is the conduction of electrical impulses. The organism is unfamiliar; the cells are not.
The feeding tube contains a lining of ciliated cells (one tissue) and a surrounding layer of nerve cells (a second tissue). A structure made of a group of different tissues working together for a specific function is an organ. It is not a tissue, because tissue means cells of similar structure only.
The digestive sac, the feeding tube and the muscular pump are three organs with related functions, working together to perform a body function (feeding and digestion). A group of organs like that is an organ system — the next level in the hierarchy, one below the whole organism.
Moving cilia continuously requires energy, which is released by aerobic respiration in the mitochondria. A cell that does constant mechanical work therefore contains many mitochondria — the same reasoning that explains why sperm cells and root hair cells are packed with them. Say releases energy, never produces it.
Pairs of questions that look nearly identical but have different answers. Find the key distinction before reading it.
Click each node to see how the ideas connect.
Spot the error in each student's answer. Think before revealing.
Eight Cambridge-style challenge questions. Write your answer, then reveal the model answer with the mark scheme and examiner's notes.