Hi Tara — welcome to your first proper Biology topic. Two ideas here, and they are very different in flavour. One: cell structure. Plant cell, animal cell, bacterial cell — which parts each one has, what each part is for, and how six specialised cells have been reshaped to do one job brilliantly. This half is about recognition and precise naming: examiners will hand you an unfamiliar micrograph and ask you to identify structures you have only ever seen drawn as neat cartoons. Two: magnification. One formula, three rearrangements, and one unit conversion (mm ↔ µm) that trips up more candidates than anything else in the topic. Get the arithmetic automatic and you will pick up marks other people drop. Let's start with what is actually inside a cell.
2.1 Cell Structure & Organisation
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▶ Watch: Cell Structure, Specialised Cells and Levels of Organisation
Opens on YouTube in a new tab. Watch one, then come back and do the Check Yourself questions — watching without testing yourself feels like learning but is not.
The Big Idea: The Cell Is the Unit of Life
Every organism you will ever study in Biology — an oak tree, a blue whale, the bacterium that gives you a sore throat — is built out of cells. A cell is the smallest unit that can carry out all seven characteristics of life on its own. Some organisms are a single cell; you are roughly thirty trillion of them.
There is a second, quieter idea hiding in the syllabus, and Cambridge examines it more often than you would expect: new cells are produced by the division of existing cells. Cells do not condense out of nothing and they are not assembled from spare parts. Every cell in your body came from an earlier cell dividing into two, in an unbroken chain going back to the fertilised egg you started as — and, beyond that, back through every ancestor you have ever had. When a wound heals, when a seedling grows, when a bacterium doubles in a warm broth, the mechanism is the same: existing cells divide.
Say it the way the mark scheme says it
The marking point is “new cells are produced by division of existing cells”. Not “cells grow more cells”, not “cells multiply”, not “the body makes new cells”. In Biology the exact verb is the mark. You will meet this again and again this year — the biology is usually easier than the wording.
Plant Cells and Animal Cells Side by Side
Start with what they have in common, because that is what students forget under pressure. Both plant and animal cells have a cell membrane, cytoplasm, a nucleus, mitochondria and ribosomes. Then plant cells add three extras: a cell wall, chloroplasts and a large permanent vacuole. That is the whole comparison. Learn the shared five first and the differences look after themselves.
The five shared structures are drawn in the same colours in both cells. Notice the cell wall sits outside the cell membrane — the membrane is still there in a plant cell, pressed against the inside of the wall.
What Each Structure Actually Does
Cambridge asks for the function far more often than the name. Learn the function as a short sentence with a verb in it, not as a single word.
| Structure | Found in | Function — say it like this |
|---|---|---|
| Cell wall | Plant, bacterial (not animal) | Made of cellulose in plants. Gives the cell support and a fixed shape, and stops the cell bursting when water enters. It is fully permeable — it does not control what goes in and out. |
| Cell membrane | All cells | Controls what enters and leaves the cell — it is partially permeable. This is the structure that does the selecting, not the wall. |
| Nucleus | Plant, animal (not bacterial) | Contains the genetic material (DNA) in chromosomes and controls the activities of the cell, including cell division. |
| Cytoplasm | All cells | A jelly-like substance where most of the chemical reactions of the cell (metabolism) take place. The organelles sit in it. |
| Chloroplast | Plant only (green parts) | Contains chlorophyll, which absorbs light energy; the site of photosynthesis, which makes food (glucose) for the plant. |
| Ribosome | All cells | The site of protein synthesis — where amino acids are joined into proteins. |
| Mitochondrion (plural mitochondria) | Plant, animal (not bacterial) | The site of aerobic respiration, which releases energy from glucose for the cell to use. |
| Vacuole | Large permanent one in plants; small temporary ones in animals | In plants: a large sac filled with cell sap. It stores dissolved substances and its internal pressure pushes outwards on the cell wall, giving the cell support. |
The wall/membrane trap — the single most common error in Topic 2
“The cell wall controls what enters and leaves the cell.” This is wrong, and it is written by thousands of candidates every year. The cell wall is fully permeable — water and dissolved substances pass straight through it. The cell membrane is partially permeable and is the structure that controls movement in and out. Wall = support and shape. Membrane = control. If you only remember one sentence from this section, remember that one.
The Bacterial Cell — Same Job, Different Toolkit
A bacterium is a complete living organism made of one cell, and it manages this with a much shorter parts list. The syllabus limits you to six structures: cell wall, cell membrane, cytoplasm, ribosomes, circular DNA and plasmids.
The headline is that a bacterial cell has no nucleus — but it certainly has DNA. Instead of being packaged into chromosomes inside a nuclear envelope, the DNA lies free in the cytoplasm as a single circular loop (often called the bacterial chromosome). Alongside it float plasmids: much smaller separate rings of DNA carrying a few extra genes, frequently the genes for antibiotic resistance. A bacterium can gain or lose plasmids without losing its main chromosome.
Bacterial cells also have no mitochondria and no chloroplasts. They still respire — the reactions simply happen in the cytoplasm and at the cell membrane instead of inside a separate organelle.
Circular DNA (orange, large) is the bacterium's main chromosome. Plasmids (small orange rings) are extra, separate loops. Both are DNA; only their size and role differ.
The Three-Way Comparison Table
Cambridge loves a tick-table like this. Read down the columns until you can rebuild it from memory on a blank page.
| Structure | Animal cell | Plant cell | Bacterial cell |
|---|---|---|---|
| Cell membrane | ✓ | ✓ | ✓ |
| Cytoplasm | ✓ | ✓ | ✓ |
| Ribosomes | ✓ | ✓ | ✓ |
| Cell wall | ✗ | ✓ (cellulose) | ✓ (not cellulose) |
| Nucleus | ✓ | ✓ | ✗ — circular DNA free in cytoplasm |
| Mitochondria | ✓ | ✓ | ✗ |
| Chloroplasts | ✗ | ✓ (in green parts) | ✗ |
| Large permanent vacuole | ✗ (small, temporary ones only) | ✓ | ✗ |
| Plasmids | ✗ | ✗ | ✓ |
Three questions that identify any cell in a diagram
1. Is there a nucleus? No → bacterial. Yes → plant or animal. 2. Is there a straight, thick outer boundary making an angular box shape? Yes → plant (cell wall). No, it is a soft rounded outline → animal. 3. Are there green oval bodies and one huge central space? Yes → definitely plant. Work through those three in order and you will never mislabel a cell diagram.
Specialised Cells: Structure Follows Function
Most cells in a multicellular organism are specialised — their structure has been modified so they do one job extremely well. The syllabus names six, and for each one Cambridge wants you to link a visible feature to a function. That link is the mark. “A root hair cell has a root hair” earns nothing; “the long narrow extension gives a large surface area for faster absorption of water and mineral ions” earns everything.
Every one of these six is a normal cell that has been reshaped. In an exam, look for the exaggerated feature first — the cilia, the hair, the tail, the missing nucleus — then say what it is for.
| Specialised cell | Key adaptation | Function |
|---|---|---|
| Ciliated cell | Cilia — tiny hair-like projections that beat | Moves mucus (carrying trapped dust and bacteria) along the trachea and bronchi, away from the lungs |
| Root hair cell | Long, narrow extension giving a large surface area; no chloroplasts; many mitochondria | Absorption of water and mineral ions from the soil |
| Palisade mesophyll cell | Many chloroplasts; tall column shape; near the upper leaf surface | Photosynthesis |
| Neurone | Very long axon; branched ends | Conduction of electrical impulses over long distances |
| Red blood cell | Biconcave disc (large surface area); haemoglobin; no nucleus | Transport of oxygen |
| Sperm and egg cells (gametes) | Sperm: tail plus many mitochondria. Egg: large with food store | Reproduction — they fuse at fertilisation |
Answering “explain how this cell is adapted” questions
Use the sentence pattern feature → so that → consequence. “The red blood cell has no nucleus, so that there is more space for haemoglobin, so it can carry more oxygen.” Two linked steps, two marks. A bare list of features (“biconcave, no nucleus, has haemoglobin”) usually scores one mark at most, because you have described the cell without explaining anything.
From One Cell to a Whole Organism
Multicellular organisms are organised in a hierarchy, and Cambridge wants the definitions, not just the order.
cell → tissue → organ → organ system → organism
Cell — the basic unit from which all living organisms are built up.
Tissue — a group of cells with similar structures, working together to perform a shared function.
Organ — a structure made up of a group of different tissues, working together to perform a specific function.
Organ system — a group of organs with related functions, working together to perform a body function.
Organism — a complete living thing that shows all seven characteristics of life. It may be a single cell (a bacterium, Amoeba) or many cells organised into tissues, organs and organ systems.
The word that carries the mark is similar for a tissue and different for an organ. Swap them and both definitions score zero.
Is the leaf a tissue or an organ?
An organ. A leaf contains palisade mesophyll, spongy mesophyll, epidermis and xylem — several different tissues working together for one job (photosynthesis). Students often call a leaf a tissue because it looks simple. Apply the test: more than one kind of tissue? Then it is an organ. Same reasoning makes the stomach, the heart, the root and the flower all organs.
Worked Example 1
A student is shown an electron micrograph of an unfamiliar cell. It has a firm angular outer boundary, a dense round structure about one fifth of the cell's width, several sausage-shaped bodies with folded inner membranes, and one very large clear space occupying most of the middle. No green oval bodies are visible. Identify the type of cell, giving reasons, and explain why the absence of green bodies does not contradict your answer. [4]
Step 1: Run the three identification questions
Is there a nucleus? Yes — the dense round structure. So this is not a bacterial cell. Is the outer boundary firm and angular? Yes — that is a cell wall, which animal cells never have. Is there one huge central space? Yes — a large permanent vacuole. Two features that only plants have, so it is a plant cell.
Step 2: Name the sausage-shaped bodies
Folded inner membranes inside an oval body identify mitochondria — the site of aerobic respiration. Plant cells respire just as animal cells do, so finding mitochondria is entirely expected and does not push you towards “animal”.
Step 3: Deal with the missing chloroplasts
Chloroplasts are only found in the green parts of a plant. A cell from a root, a potato tuber or the inside of an onion bulb is a genuine plant cell with a wall and a vacuole but no chloroplasts, because it never receives light. So their absence is consistent with a plant cell — it simply tells you which part of the plant the cell came from.
The answer
A plant cell, shown by the cell wall [1] and the large permanent vacuole [1]; the sausage-shaped bodies are mitochondria [1]; chloroplasts are absent because this cell is not from a green, light-exposed part of the plant, e.g. a root or an onion bulb [1].
Worked Example 2
A bacterium is grown in a medium containing an antibiotic. Most of the population dies, but a few cells survive and divide. The survivors are found to contain a small ring of DNA that the dead cells lacked. (a) Name this structure and state where it is found in the cell. (b) Explain why a bacterium can lose this structure and still be alive, but cannot lose its circular DNA. [4]
Step 1: Name it precisely
A small, separate ring of DNA in a bacterial cell is a plasmid. It lies free in the cytoplasm — a bacterial cell has no nucleus, so there is nowhere else for it to be.
Step 2: Compare what the two kinds of DNA carry
The circular DNA (the bacterial chromosome) carries the genes the cell needs for its essential activities — without it, no proteins and no control of the cell, so the cell cannot survive or divide. A plasmid carries only a few extra genes, useful in particular conditions (here, antibiotic resistance) but not essential for staying alive.
The answer
(a) A plasmid [1], found free in the cytoplasm [1]. (b) The plasmid carries only extra, non-essential genes such as antibiotic resistance, so a cell without it still functions [1]; the circular DNA carries the genes that control the cell's essential activities and is copied when the cell divides, so a cell cannot survive without it [1].
Worked Example 3
A pathologist examines a sample taken from the lining of a person's trachea. She notes that the surface cells carry many short projections that beat together, and that in this patient many of these projections have been destroyed by long-term smoking. Explain the normal function of these cells and predict two consequences of losing them. [4]
Step 1: Identify the cell from the feature
Short beating projections on the surface of a cell lining the trachea are cilia, so these are ciliated cells. Their job is to sweep mucus — which has trapped dust particles and bacteria — upwards along the trachea and bronchi, away from the lungs, so it can be swallowed or coughed out.
Step 2: Reason forwards from “the sweeping stops”
If the cilia are destroyed, the mucus is no longer moved. Consequence one: mucus accumulates in the airways, so the person coughs to shift it (the “smoker’s cough”). Consequence two: the trapped dust and bacteria stay in the lungs instead of being removed, so the risk of chest infections rises.
The answer
Ciliated cells [1] use beating cilia to move mucus containing trapped dust and bacteria up and out of the trachea and bronchi [1]. Without them, mucus builds up in the airways causing persistent coughing [1] and bacteria are not removed, so infections of the lungs become more likely [1].
Apply It: Cells in the Real World
Four situations where knowing what is inside a cell — and what has been left out — explains something you can actually see.
1
A student in Bangalore leaves a stick of celery in a glass of water overnight. It becomes noticeably stiffer and snaps crisply. A second stick, left on the kitchen counter in the dry air, goes limp and bends without breaking.
Which two cell structures explain both observations, and why can an animal cell never do this?
The vacuole fills, the wall resists
Water enters the celery cells and collects in the large permanent vacuole, which swells and pushes the cytoplasm outwards against the cell wall. The wall is strong and does not stretch much, so it pushes back — and that outward pressure is what makes the tissue rigid. The limp stick has lost water, its vacuoles have shrunk, and there is nothing pressing on the walls.
Why an animal cell cannot copy the trick
An animal cell has no cell wall and no large permanent vacuole. Extra water simply stretches the flexible cell membrane, and if too much enters the cell bursts. Plants get free structural support from a design that would be fatal in an animal.
Biology Connection
This is the everyday version of Topic 3's osmosis. The point to carry forward now: the wall gives support, the vacuole provides the push, and the two only work as a pair.
2
A tissue sample is stained and examined. Cell type P contains 25 times more mitochondria per unit volume than cell type Q. Both cells come from the same mammal and are the same size.
What can you conclude about the two cells, and which of the six specialised cells would you expect to be at each extreme?
Mitochondria count as an energy meter
Mitochondria are the site of aerobic respiration, which releases energy. Many mitochondria therefore mean the cell releases a lot of energy, which means it does something energy-expensive. Cell P is highly active; cell Q is not.
Matching the extremes
Highest: the sperm cell, which must swim, and (in plants) the root hair cell, which absorbs mineral ions by active transport. Lowest: the red blood cell, which has no nucleus and very few organelles — it is essentially a bag of haemoglobin being carried along by the blood, doing no work of its own.
Biology Connection
Examiners often give you an organelle count instead of naming the cell. The reasoning chain is always the same: organelle → its function → what the cell must be doing a lot of.
3
Some antibiotics kill bacteria by preventing them from building a proper cell wall. Human cells are unharmed. Other drugs that attack ribosomes have to be designed extremely carefully, because bacterial and human cells both contain ribosomes.
Use the structures in each cell type to explain both facts.
A target that only one side has
Bacterial cells have a cell wall; human (animal) cells do not. A drug aimed at wall-building therefore has no target at all in a human cell, so it damages the bacterium and leaves the patient alone. Without a working wall the bacterium cannot resist the pressure of water entering it and bursts.
A target both sides share
Ribosomes are in the syllabus list for animal, plant and bacterial cells, because every cell must make proteins. A drug that stops ribosomes working would stop protein synthesis in human cells too, so it must exploit small differences between bacterial and human ribosomes — a far harder design problem.
Biology Connection
This is why the shared-versus-unique columns of the comparison table matter medically, not just for an exam tick-box. Unique structures make safe drug targets; shared structures do not.
4
A student writes: “The stomach is a tissue because it is made of stomach cells. Blood is an organ because it does a lot of jobs. The leaf is a tissue because it is thin and flat.”
Correct all three statements, and give the test you used each time.
Apply one test to all three
The test is: does this structure contain more than one kind of tissue? The stomach contains muscle tissue, glandular tissue and epithelial tissue — several different tissues, so it is an organ. The leaf contains palisade mesophyll, spongy mesophyll, epidermis and xylem — also an organ. Doing many jobs is irrelevant; what matters is the number of tissue types.
The awkward one: blood
Blood is a tissue. It is a group of cells (red blood cells, white blood cells, platelets) suspended in plasma, working together on one broad function — transport. It is not made of several different tissues, so it cannot be an organ. It is a useful reminder that a tissue does not have to be a solid sheet.
Biology Connection
Never classify by appearance or by how important something seems. Count the tissue types. That single habit answers every hierarchy question in the syllabus.
Check Yourself: 2.1 Cell Structure and Organisation
20 multiple choice questions. Click an option to check your answer.
Your Score
0 / 20
Question 1
Which structure controls what enters and leaves a plant cell?
The cell membrane is partially permeable and controls movement in and out. The cell wall is fully permeable — substances pass straight through it — so it controls nothing. This wall/membrane swap is the most common single error in Topic 2.
Question 2
A cell is seen to have a cell wall, cytoplasm, ribosomes and a circular loop of DNA lying free in the cytoplasm. The cell is
Free circular DNA with no nucleus identifies a bacterial cell. Students often pick “plant” because of the cell wall, but bacteria have walls too — the decisive feature is that the DNA is not enclosed in a nucleus.
Question 3
The function of the mitochondrion is
Mitochondria are the site of aerobic respiration. The classic wrong answer is “produces energy” — energy is released from glucose, never created. Protein synthesis happens at ribosomes and photosynthesis in chloroplasts.
Question 4
Which structures are found in a plant cell but never in an animal cell?
Plants add exactly three things to the shared list: cell wall, chloroplasts, large permanent vacuole. Option C fails because the membrane and cytoplasm are in every cell; option D fails because plasmids are bacterial.
Question 5
New cells are produced by
The syllabus wording is “new cells are produced by division of existing cells”. Cells are never built from spare parts; every cell comes from a pre-existing cell dividing.
Question 6
A leaf is classified as an organ because it
An organ = a structure made of a group of different tissues. A leaf contains palisade mesophyll, spongy mesophyll, epidermis and xylem. Option C is the definition of a tissue, and doing several jobs (option A) is not the test.
Question 7
Which cell has no nucleus in its mature form?
Mature red blood cells lose their nucleus, which leaves more room for haemoglobin and so more oxygen carried. Candidates sometimes confuse this with bacteria, which also lack a nucleus — but a red blood cell is an animal cell that has lost one.
Question 8
Chloroplasts are absent from root hair cells because
Chloroplasts contain chlorophyll to absorb light; underground there is none, so they would be useless. Roots certainly need energy — root hair cells are packed with mitochondria for active transport.
Question 9
A plasmid is best described as
A plasmid is a small, separate loop of DNA carrying a few extra genes, such as antibiotic resistance. It is not a nucleus — bacteria have no nucleus at all — and it is DNA, not protein.
Question 10
Which row correctly matches a specialised cell to its function?
Palisade mesophyll cells are packed with chloroplasts for photosynthesis. The other three have been deliberately shuffled: cilia move mucus, neurones conduct impulses, root hairs absorb water and mineral ions.
Question 11
The cytoplasm of a cell is where
Cytoplasm is the jelly-like site of most of the cell's chemical reactions (its metabolism). Genetic information is in the nucleus, light absorption happens in chloroplasts and cell sap fills the vacuole.
Question 12
Which structure is present in animal, plant AND bacterial cells?
Ribosomes appear in all three lists because every cell must make proteins. Bacteria have no mitochondria and no nucleus, and animal cells have no cell wall.
Question 13
A cell in the trachea is covered with tiny beating projections. Their function is to
Cilia sweep mucus up the trachea and bronchi. The tempting wrong answer is “increase surface area” — that is what root hairs and villi do; cilia work by movement, not by area.
Question 14
Which statement about the cell wall is correct?
Plant cell walls are cellulose and provide support and a fixed shape. They are fully permeable, absent from animal cells, and contain no chlorophyll — chlorophyll is inside chloroplasts.
Question 15
Muscle tissue is described as a tissue rather than an organ because it is
The definition of a tissue is a group of cells with similar structures working together. The word that earns the mark is similar; swapping it for different turns your answer into the definition of an organ.
Question 16
A sperm cell contains an unusually large number of mitochondria. This is because it needs energy to
The energy released by respiration in the mitochondria powers the tail movement that carries the sperm to the egg. Sperm cells do not photosynthesise, do not carry oxygen and never build a cell wall.
Question 17
Which of these is NOT found in a bacterial cell?
Bacteria have no mitochondria (and no nucleus and no chloroplasts). They still respire — the reactions take place in the cytoplasm and at the cell membrane instead of inside an organelle.
Question 18
A student examines a cell and finds a large permanent vacuole, a nucleus and a cell wall, but no chloroplasts. The most likely origin of the cell is
A wall plus a big vacuole plus a nucleus makes it a plant cell, and the missing chloroplasts point to a part of the plant that gets no light — an onion bulb grows underground. A leaf mesophyll cell would be full of chloroplasts.
Question 19
The correct order of the levels of organisation is
cell → tissue → organ → organ system → organism. The usual slip is swapping tissue and organ; remember that tissues are the building blocks of organs, so tissue must come first.
Question 20
Which pair of features would let you distinguish a plant cell from a bacterial cell in an electron micrograph?
Both cells have a wall, ribosomes, cytoplasm, a membrane and DNA, so options A, B and D cannot separate them. Only the nucleus and the large permanent vacuole belong to the plant cell alone.
2.2 Size of Specimens — Magnification
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▶ Watch: Magnification and Size of Specimens
Opens on YouTube in a new tab. Watch one, then come back and do the Check Yourself questions — watching without testing yourself feels like learning but is not.
The Big Idea: A Picture Is a Lie About Size
A drawing of a cell in your textbook is about the size of your thumbnail. The real cell is invisible. Everything in this section exists to convert between those two numbers — the size on the page and the size in real life — and the bridge between them is a single number called the magnification.
magnification = image size ÷ actual size
image size — how big the specimen looks in the drawing or photograph, measured with a ruler. In IGCSE this is nearly always in millimetres.
actual size — how big the specimen really is.
magnification — a pure number with no units, because it is one length divided by another. Write it as ×400, not “400 mm” and not “400 times bigger mm”.
The golden rule: image size and actual size must be in the same unit before you divide. Nearly every lost mark in 2.2 comes from breaking this one rule.
Only I sits on the top row, so I is the only one you ever multiply for. If your answer for an actual size is bigger than the image size, you have multiplied when you should have divided.
Sanity Checks That Catch Almost Every Error
Before you write an answer down, run these three checks. They take four seconds and they save whole questions.
| Check | What it means | What it catches |
|---|---|---|
| Is the actual size smaller than the image? | A microscope makes things look bigger, so the real specimen is almost always smaller than its picture. | Dividing the wrong way round — the classic upside-down fraction. |
| Are both lengths in the same unit? | You cannot divide 40 mm by 20 µm and get a meaningful number until one of them is converted. | Answers wrong by a factor of exactly 1000. |
| Is the answer a believable size? | Plant cell ≈ 100 µm, animal cell ≈ 20 µm, red blood cell ≈ 7 µm, bacterium ≈ 1–5 µm. | Answers like “a bacterium is 3 mm long”, which would make it visible on your desk. |
Supplement
Converting between millimetres and micrometres
A micrometre (µm, sometimes called a micron) is one thousandth of a millimetre.
1 mm = 1000 µm
Going from mm to µm → multiply by 1000 (the number gets bigger, because micrometres are smaller units so you need more of them). Going from µm to mm → divide by 1000.
Worked both ways: 0.08 mm × 1000 = 80 µm. 250 µm ÷ 1000 = 0.25 mm. 0.0015 mm × 1000 = 1.5 µm. 7 µm ÷ 1000 = 0.007 mm.
The direction test that never fails: if you are changing to a smaller unit, the number must get bigger. A micrometre is smaller than a millimetre, so mm → µm must make the number larger. If your number shrank, you divided when you should have multiplied.
Reading a Scale Bar
Photographs of cells rarely tell you the magnification directly. Instead they carry a scale bar: a short line with a length written next to it, meaning “this much of the picture represents this much in real life”. The scale bar is a magnification calculation waiting to happen, and you handle it with the ordinary formula.
Once you have the magnification from the scale bar, the picture becomes a measuring instrument: measure any structure in millimetres, divide by 400, multiply by 1000, and you have its real size in micrometres.
Where the marks sit in a magnification question
A two-mark calculation almost always pays one mark for correct working and one for the correct answer with the correct unit. That means a wrong final number can still earn a mark if the examiner can see “M = image ÷ actual = 20 ÷ 0.05” written out. Never write a bare answer. And never leave the answer in the wrong unit when the question says “give your answer in µm” — that is a specific instruction and an answer in mm scores nothing.
Worked Example 1
A student draws a plant cell. The drawing is 75 mm long. The actual cell is 0.15 mm long. Calculate the magnification of the drawing. [2]
Step 1: Check the units match
Image = 75 mm, actual = 0.15 mm. Both are already in millimetres, so no conversion is needed. (This is the step people skip when it is needed.)
Step 2: Divide image by actual
M = 75 ÷ 0.15 = 500.
Step 3: Write it as a magnification
Answer: ×500. No units — a magnification is a ratio of two lengths, so the units cancel. Sanity check: the drawing is much bigger than the cell, so a magnification well above 1 is exactly what we expect.
Worked Example 2
A photomicrograph is taken at a magnification of ×500. A cell in the photograph measures 45 mm across. Calculate the actual width of the cell. Give your answer in µm. [3]
Step 1: Choose the rearrangement
We want the actual size, so cover A on the triangle: A = I ÷ M. The image is bigger than the real cell, so dividing is right.
Step 2: Do the division in mm
A = 45 ÷ 500 = 0.09 mm.
Step 3: Convert to micrometres, because the question said so
0.09 mm × 1000 = 90 µm. Changing to a smaller unit, so the number gets bigger — the direction is right. And 90 µm is a believable size for a plant cell, which is the third sanity check passed.
Worked Example 3
An electron micrograph of a bacterium carries a scale bar labelled 2 µm. The bar measures 40 mm on the page. (a) Calculate the magnification. (b) The bacterium itself measures 62 mm long on the page. Calculate its actual length in µm. [4]
Step 1 (a): Convert the scale bar label to mm
2 µm ÷ 1000 = 0.002 mm. Now both the bar's image length (40 mm) and its actual length are in millimetres.
Step 2 (a): Divide
M = 40 ÷ 0.002 = ×20 000. A five-figure magnification is normal for an electron micrograph, so this is believable.
Step 3 (b): Use that magnification on the bacterium
A = I ÷ M = 62 ÷ 20 000 = 0.0031 mm.
Step 4 (b): Convert and check
0.0031 mm × 1000 = 3.1 µm. Bacteria are typically 1–5 µm long, so the answer is sensible. Shortcut worth knowing: because both lengths were measured on the same picture, you can also do 62 ÷ 40 × 2 µm = 3.1 µm directly — the magnification cancels out.
Worked Example 4
A red blood cell has an actual diameter of 7 µm. A textbook wants to print it 21 mm wide. What magnification must the printed image use? [3]
Step 1: Spot which quantity is missing
You are given the image size (21 mm) and the actual size (7 µm) and asked for the magnification, so it is the plain formula M = I ÷ A. No rearrangement needed.
Step 2: Force the units to match
7 µm ÷ 1000 = 0.007 mm. (You could instead convert 21 mm to 21 000 µm — either works, as long as you do one of them.)
Step 3: Divide
M = 21 ÷ 0.007 = ×3000. Check with the other route: 21 000 µm ÷ 7 µm = 3000. Same answer, which is a useful way to confirm you converted correctly.
Worked Example 5 — challenge
An electron micrograph of a palisade mesophyll cell is printed at ×4000. On the print, the whole cell measures 320 mm long and one mitochondrion measures 6 mm long. (a) Calculate the actual length of the cell in µm. (b) Calculate the actual length of the mitochondrion in µm. (c) How many times longer is the cell than the mitochondrion? Explain why you did not need the magnification for part (c). [5]
Step 1 (a): Cell
A = 320 ÷ 4000 = 0.08 mm, and 0.08 × 1000 = 80 µm. A palisade cell of 80 µm is entirely realistic.
Step 2 (b): Mitochondrion
A = 6 ÷ 4000 = 0.0015 mm, and 0.0015 × 1000 = 1.5 µm. Mitochondria are a couple of micrometres long, so again believable.
Step 3 (c): The ratio
80 ÷ 1.5 = 53 times longer (to two significant figures).
Step 4 (c): Why the magnification cancels
Both structures were measured on the same image, so both were enlarged by the same factor. Dividing one by the other cancels that factor: 320 ÷ 6 = 53 as well. Whenever a question asks “how many times bigger is X than Y” and both are in the same picture, you can compare the image measurements directly — and it is worth saying so, because it shows you understand what magnification is.
Making a Biological Line Drawing
When a question says “make a large, clear drawing”, the examiner marks the drawing itself. Six rules:
- Draw large: use at least half of the space given.
- Use a sharp pencil and single, clear, continuous lines. No sketchy or overlapping lines.
- No shading and no colouring.
- Keep the proportions right: if one part is twice as long as another on the specimen, it must be twice as long on your drawing.
- Draw only what you can see, not what the textbook shows.
- Labels go outside the drawing. Label lines are drawn with a ruler, do not cross each other, have no arrowheads, and touch the structure they name.
Then calculate the magnification of your drawing: measure a length on the drawing with a ruler (the image size, in mm), measure the same length on the real specimen (the actual size, in mm), and divide image size by actual size. Magnification has no units: write it as ×6, not ×6 mm.
Worked Example 6
A student draws a bean seed. The seed is 16 mm long; her drawing of it is 96 mm long. Calculate the magnification of her drawing. [2]
Step 1: Same units
Both lengths are already in mm, so they can be divided directly.
Step 2: Image ÷ actual
magnification = 96 ÷ 16 = 6
Magnification = ×6 [2] (no units).
Worked Example 7
A pollen grain is drawn 60 mm across at a magnification of ×1500. Calculate its actual diameter in µm. [2]
Step 1: Actual size = image ÷ magnification
actual size = 60 ÷ 1500 = 0.04 mm
Step 2: Convert mm to µm
There are 1000 µm in 1 mm, so 0.04 × 1000 = 40 µm.
Actual diameter = 40 µm [2].
Apply It: Measuring the Invisible
Three situations where getting the magnification arithmetic right changes the conclusion completely.
1
Two students measure the same cell on the same photomicrograph, which has a scale bar labelled 100 µm that is 25 mm long. The cell is 60 mm wide on the page. Student A writes: “M = 25 ÷ 100 = 0.25, so the actual width = 60 ÷ 0.25 = 240 mm.” Student B writes: “actual width = 240 µm.”
Both students got the number 240. Only one is right. What went wrong, and what is the correct answer?
Student A mixed units mid-calculation
Dividing 25 mm by 100 µm without converting gives a meaningless “magnification” of 0.25 — which would mean the microscope makes things 4 times smaller. The correct route is 100 µm = 0.1 mm, so M = 25 ÷ 0.1 = ×250.
The correct actual width
A = 60 ÷ 250 = 0.24 mm = 240 µm. Student B is right. Student A landed on the same digits by accident and then attached “mm”, which claims the cell is 24 cm wide — roughly the length of a school ruler.
Biology Connection
The right digits with the wrong unit is a wrong answer, and it is the single most common way to lose the accuracy mark. Always finish by asking whether a cell that size could be seen with the naked eye.
2
A microscope has an eyepiece lens marked ×10 and three objective lenses marked ×4, ×10 and ×40. A student views an onion cell using the ×40 objective and estimates that the cell fills one twentieth of the field of view, which is 0.45 mm across.
What total magnification is she using, and roughly how long is the cell in µm?
Total magnification multiplies
Total magnification = eyepiece × objective = 10 × 40 = ×400. The two lenses each enlarge, one after the other, so their effects multiply. Adding them (“×50”) is a common and badly wrong answer.
Reading a size from the field of view
The field of view is 0.45 mm across in real terms. One twentieth of that is 0.45 ÷ 20 = 0.0225 mm, which is 0.0225 × 1000 = 22.5 µm. Notice the magnification is not needed here at all — the field of view was already given as an actual size.
Biology Connection
Estimating from the field of view is how biologists size cells at the bench without any photograph. Read carefully whether a length you are given is an image length or an actual length — that decision comes before any arithmetic.
3
A journal prints an electron micrograph of a chloroplast at ×25 000. A magazine reprints the same picture but shrinks it to exactly half its printed width and forgets to change the caption, which still says ×25 000.
What is the true magnification of the magazine image, and what would a reader calculate for the actual length of a 50 mm chloroplast in that image if they trusted the caption?
Shrinking the print halves the magnification
Magnification is image size ÷ actual size. Halving every image length while the specimen stays the same halves the magnification: the true value is ×12 500.
The reader's error
Trusting the caption: A = 50 ÷ 25 000 = 0.002 mm = 2 µm. Using the true magnification: A = 50 ÷ 12 500 = 0.004 mm = 4 µm. The reader underestimates the chloroplast by a factor of two.
Biology Connection
This is exactly why serious micrographs carry a scale bar rather than a stated magnification: a scale bar is resized along with the picture, so it stays correct no matter how the image is scaled. A printed magnification does not.
Check Yourself: 2.2 Size of Specimens
20 multiple choice questions. Every one is a calculation you could meet on a real paper.
Your Score
0 / 20
Question 1
A drawing of a cell is 60 mm long. The actual cell is 0.2 mm long. The magnification of the drawing is
M = image ÷ actual = 60 ÷ 0.2 = 300. Option A is the fraction upside down (actual ÷ image), which is the most common error of all — and it would mean the drawing is smaller than the cell.
Question 2
The unit of magnification is
Magnification is one length divided by another, so the units cancel and it is a pure number, written ×300. Writing “300 mm” loses the mark even when the arithmetic is right.
Question 3
5 µm expressed in millimetres is
1 mm = 1000 µm, so µm → mm means divide by 1000: 5 ÷ 1000 = 0.005 mm. Moving to a larger unit must make the number smaller, which rules out 5000 mm immediately.
Question 4
0.35 mm expressed in micrometres is
mm → µm means multiply by 1000: 0.35 × 1000 = 350 µm. A micrometre is a smaller unit, so you need more of them and the number must get bigger.
Question 5
A photomicrograph has a magnification of ×200. A cell in it measures 30 mm across. The actual width of the cell is
A = I ÷ M = 30 ÷ 200 = 0.15 mm = 150 µm. Option B comes from multiplying by the magnification instead of dividing, which would make the real cell bigger than its own photograph.
Question 6
A scale bar labelled 100 µm measures 20 mm on the page. The magnification of the image is
Convert first: 100 µm = 0.1 mm. Then M = 20 ÷ 0.1 = 200. Dividing 20 by 100 without converting gives 0.2 — the error that produces “×5” and “×20” style answers.
Question 7
A structure is 4 µm long. Drawn at a magnification of ×5000, how long will the drawing be?
I = A × M = 4 µm × 5000 = 20 000 µm = 20 mm. Image size is the only quantity you ever multiply for — it sits alone on the top of the formula triangle.
Question 8
Which of these lengths is the largest?
Put them all in µm: 0.5 mm = 500 µm; 400 µm; 0.02 mm = 20 µm; 900 µm. Comparisons like this are only safe once every value is in the same unit — guessing from the digits alone picks 400 or 900 at random.
Question 9
A student calculates that the actual size of a cell is larger than its size in the photomicrograph. This tells you that
A microscope enlarges, so the actual specimen must be smaller than its image. An actual size bigger than the image means the formula was used upside down — use this as an automatic check on every answer.
Question 10
A microscope has a ×10 eyepiece lens and a ×40 objective lens. The total magnification is
The two lenses enlarge one after the other, so their magnifications multiply: 10 × 40 = ×400. Adding them to get ×50 is a very common slip.
Question 11
A drawing made at ×300 measures 90 mm. The actual length of the specimen is
A = 90 ÷ 300 = 0.3 mm, and 0.3 × 1000 = 300 µm. Option D divides correctly but then converts the wrong way; option A multiplies instead of divides.
Question 12
An electron micrograph shows a bacterium 30 mm long. Its actual length is 2 µm. The magnification is
2 µm = 0.002 mm, so M = 30 ÷ 0.002 = 15 000. Forgetting the conversion gives 30 ÷ 2 = 15, which is out by a factor of 1000 — the signature of a missed unit change.
Question 13
A cell measures 48 mm on an image taken at ×1200. Its actual width in micrometres is
48 ÷ 1200 = 0.04 mm, and 0.04 × 1000 = 40 µm — a very believable size for an animal cell. Option A and option C are the same digits shifted by one conversion step.
Question 14
Which statement is correct?
1 mm = 1000 µm. Options C and D have the relationship backwards, which would make a micrometre bigger than a millimetre — a mistake that turns bacteria into objects you could hold.
Question 15
A micrograph carries a scale bar labelled 50 µm which measures 10 mm. A nucleus in the same image measures 6 mm across. The actual diameter of the nucleus is
From the bar, M = 10 ÷ 0.05 = 200. Then A = 6 ÷ 200 = 0.03 mm = 30 µm. Quicker route: the nucleus is 6/10 of the bar, so it is 0.6 × 50 µm = 30 µm.
Question 16
A magnification of ×1 would mean that
M = image ÷ actual, so M = 1 means the two sizes are equal. It is a useful anchor: anything above 1 is enlarged, anything below 1 is reduced — which is why a magnification of 0.25 should always look suspicious.
Question 17
A question asks for an actual length “in µm”. A student correctly calculates 0.006 mm and writes that as the final answer. The examiner will
Correct working still earns the method mark, but the answer must be 6 µm to satisfy the instruction. Read the last line of a calculation question before you start — it tells you which unit to finish in.
Question 18
On one micrograph, structure X measures 80 mm and structure Y measures 5 mm. How many times longer is X than Y in real life?
Both were enlarged by the same factor, so it cancels in a ratio: 80 ÷ 5 = 16. Option D is tempting but wrong — the magnification is only needed when you want an absolute size, not a comparison.
Question 19
Which is the most realistic actual diameter for a human red blood cell?
Red blood cells are about 7 µm across. The other three are between 100 and 1000 times too big — 7 mm would be visible on your fingertip. Knowing a few typical sizes lets you reject an impossible answer instantly.
Question 20
An image is 25 mm long and the specimen is 500 µm long. The magnification is
500 µm = 0.5 mm, so M = 25 ÷ 0.5 = 50. Dividing 25 by 500 without converting gives 0.05; inverting the fraction gives 0.02. Both errors are caught by asking whether the image should be bigger than the specimen.
2.3 Exam Technique & Precise Vocabulary
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Biology Pays for the Exact Word
You already know from Physics and Chemistry that examiners are fussy. Biology is fussier. In Physics an equation can rescue a clumsy sentence; in Biology the sentence is the answer, and mark schemes list the specific words that earn credit. Two answers can say the same thing in your head and score 3 and 0.
The table below is the Topic 2 vocabulary that mark schemes accept and reject. Learn the right-hand column as phrases, not as individual words.
| If you are tempted to write… | Write this instead | Why the examiner insists |
|---|---|---|
| “the cell wall controls what goes in and out” | “the cell membrane controls what enters and leaves the cell” | The wall is fully permeable. Only the membrane is partially permeable, so only the membrane controls anything. |
| “mitochondria make energy” | “mitochondria are the site of aerobic respiration, which releases energy” | Energy cannot be created. “Produces energy” is explicitly rejected in mark schemes. |
| “chloroplasts make food” | “chloroplasts contain chlorophyll, which absorbs light energy for photosynthesis” | Two separate marking points hide here: the pigment and the process. Naming only one gets one mark. |
| “the nucleus is the brain of the cell” | “the nucleus contains the genetic material and controls the activities of the cell” | Analogies score nothing. Cambridge wants the two functions stated plainly. |
| “bacteria have no DNA” | “bacteria have no nucleus; their DNA is a circular loop free in the cytoplasm” | Absence of a nucleus is not absence of DNA — a distinction examiners test on purpose. |
| “cells multiply” / “the body makes new cells” | “new cells are produced by division of existing cells” | This is the syllabus statement, almost word for word, and it is marked as such. |
| “a tissue is a group of cells” | “a tissue is a group of cells with similar structures working together to perform a shared function” | Without “similar”, your definition also describes an organ, so it cannot be credited. |
| “an organ is a body part” | “an organ is a structure made of a group of different tissues working together” | The word different is the entire distinction from a tissue. |
| “root hairs suck up water” | “the extension gives a large surface area for absorption of water and mineral ions” | The mark is for the adaptation and its consequence, not for the outcome alone. |
| “the magnification is 400 mm” | “the magnification is ×400” | A magnification is a ratio and has no units. Adding one is treated as an error. |
Command Words — What Each One Is Buying
The command word tells you exactly how much writing is wanted. Getting this right is free marks and saves time.
| Command word | What to do | Topic 2 example |
|---|---|---|
| Name / State | One or two words. No explanation, no sentence needed. | “Name structure X.” → mitochondrion. Do not write a paragraph. |
| Identify | Pick out the named thing from a diagram or list. | “Identify the structure that contains the genetic material.” → nucleus. |
| Describe | Say what happens or what something is like. No reasons required. | “Describe the structure of a bacterial cell.” → list wall, membrane, cytoplasm, ribosomes, circular DNA, plasmids. |
| Explain | Say why. Every explain mark needs a reason word: because, so that, this means. | “Explain why a root hair cell has no chloroplasts.” → because there is no light underground, so photosynthesis cannot occur. |
| Suggest | Apply what you know to an unfamiliar situation. There may be more than one acceptable answer. | “Suggest why this cell contains many mitochondria.” |
| Calculate | Show working, give a unit unless the quantity has none. | “Calculate the magnification.” → working plus ×400. |
| Compare | Give linked statements covering both things in the same sentence. | “A plant cell has a cell wall whereas an animal cell does not.” Two separate lists usually score less. |
The comparison sentence that always scores
When asked to compare, never write two paragraphs. Write linked sentences using whereas or but: “A plant cell has a large permanent vacuole whereas an animal cell has only small temporary vacuoles.” One sentence, both organisms, one clear point of difference — that is what a comparison mark looks like. And make sure each point is genuinely comparative: “a plant cell has a cell wall” on its own is a description, not a comparison.
Attacking an Unfamiliar Micrograph
The hardest Topic 2 questions show you a real photograph or an electron micrograph, not the tidy cartoon you revised from. The cell will be an odd shape, the organelles will be grey blobs, and something will be cut in half. Here is a routine that works every time.
| Step | What to look for | What it tells you |
|---|---|---|
| 1. Look at the outline first | Straight edges and sharp corners, or a soft rounded blob? | Angular with a thick double boundary = cell wall = plant or bacterial. Soft and rounded = animal. |
| 2. Find the biggest dark round structure | A large dense sphere, often with a darker spot inside. | That is the nucleus (the spot is the nucleolus). If there is none at all, think bacterial — or red blood cell, if the cell is small and disc-shaped. |
| 3. Find the biggest empty-looking space | One huge pale region taking up most of the cell. | A large permanent vacuole — strong evidence for a plant cell. |
| 4. Look for repeated small ovals | Sausage shapes with internal folds; or oval bodies with stacked internal layers. | Folded interior = mitochondria. Stacked layers and green in a light image = chloroplasts. |
| 5. Look for a scale bar | A short line with a length written beside it. | Use it to size anything in the picture — and to check your identification. A structure 2 µm long could be a mitochondrion or a bacterium; one 80 µm long is a whole plant cell. |
Size is evidence, not decoration
If a scale bar tells you the whole object is about 2 µm long, it cannot be a plant cell (about 100 µm) and it cannot be a nucleus inside one. It can be a bacterium or a mitochondrion. Candidates almost never use the scale bar as an identification tool, and it is often the cleanest evidence in the whole question. When a question says “using the scale bar, explain why structure P cannot be a plant cell”, this is exactly what it wants.
How to Lay Out a Calculation So It Cannot Lose Marks
Four lines. Always the same four lines. Do this even when the arithmetic is easy, because the working line is often worth as much as the answer.
formula → conversion → substitution → answer + unit
Line 1 — formula. magnification = image size ÷ actual size. Writing it down costs three seconds and often earns a mark on its own.
Line 2 — conversion. 50 µm = 0.05 mm. Show it explicitly. This is where the examiner sees that you understood the units.
Line 3 — substitution. M = 20 ÷ 0.05. Numbers in, before the calculator gets involved.
Line 4 — answer with unit. = ×400 (no unit for magnification) or = 90 µm (unit essential for a size). Underline it.
Significant figures and rounding in Topic 2
Unless the question says otherwise, give your answer to the same number of significant figures as the data you were given, or to 2–3 significant figures for an awkward decimal. If a division gives 53.333…, write 53. Do not round in the middle of a multi-step calculation — carry the full value and round only at the end, or your final answer can drift far enough to fall outside the accepted range.
Four Habits That Are Worth Marks in Every Topic 2 Question
| Habit | What it looks like in practice |
|---|---|
| Answer the number of marks | A [3] question needs three separate creditable points. Count them on your fingers before moving on. Three ways of saying the same thing is one mark, not three. |
| Never repeat the question | “The cell is adapted because it is adapted for absorption” earns nothing. Add information the question did not give you. |
| Link feature to function | Use so that or because at least once in every adaptation answer. Without it you have described, not explained. |
| Check the unit last | Before turning the page, look at every number you wrote and confirm it carries the unit the question asked for — and that a magnification carries none. |
Check Yourself: 2.3 Exam Technique and Vocabulary
20 multiple choice questions on wording, command words and calculation layout.
Your Score
0 / 20
Question 1
Which answer would gain the mark for the function of a mitochondrion?
Energy is released from glucose by aerobic respiration, never produced or created. “Powerhouse” is an analogy and scores nothing on a Cambridge mark scheme.
Question 2
A question says “Explain why a red blood cell has no nucleus. [2]”. Which answer scores both marks?
An explain question needs a reason chain. Two linked steps — more room for haemoglobin, therefore more oxygen carried — is exactly two marks. Option D describes what happened without explaining any advantage.
Question 3
The command word “suggest” usually means
“Suggest” signals that you have not been taught this exact case and must transfer a principle you do know. There is often more than one acceptable answer, so a sensible reasoned response is credited.
Question 4
Which of these is a correct comparative statement?
A comparison must mention both things in the same statement and state the difference. Option A is a description of one cell only, and option C says there is a difference without naming it.
Question 5
A question asks for the actual length of a structure “in µm”. The correct calculation gives 0.045 mm. The answer should be written as
0.045 mm × 1000 = 45 µm. Leaving the answer in millimetres, or converting the wrong way, loses the accuracy mark even though the working was right.
Question 6
On an unfamiliar micrograph, the clearest single piece of evidence that the cell is bacterial rather than plant is
Bacteria and plants both have walls, ribosomes and cytoplasm, so those cannot separate them. Only the absence of a nucleus (with DNA free in the cytoplasm) does.
Question 7
A scale bar shows that an object in a micrograph is about 2 µm long. The object is most likely to be
Two micrometres is far too small for a whole plant cell (about 100 µm) or an animal nucleus (about 5–10 µm for the whole cell to be 20 µm). Mitochondria and bacteria are both a couple of micrometres long. Using the scale bar as identification evidence is a skill few candidates use.
Question 8
Which definition of a tissue would be accepted?
The marking word is similar. Without it, option A also describes an organ, an organ system and an organism, so it cannot be credited.
Question 9
In a [3] mark question, a student writes the same idea in three different ways. The likely result is
Marks are awarded for distinct creditable points. Rephrasing one idea three times is still one point. Count the marks available and plan that many separate statements.
Question 10
Which is the best way to begin a magnification calculation on paper?
Writing magnification = image ÷ actual, then the conversion, then the substitution, means a slip on the calculator still leaves visible working — and working usually carries its own mark.
Question 11
“Describe the structure of a bacterial cell.” A good answer would
Describe asks what something is like, not why. Explaining, comparing or calculating are different command words — and answering the wrong command word is one of the most expensive mistakes in Biology.
Question 12
Which phrase should never appear in an answer about the cell wall?
The wall is fully permeable and controls nothing; the cell membrane does the controlling. This single swap is the most frequently penalised error in the whole topic.
Question 13
A student answers “the root hair cell is adapted because it absorbs water”. This scores no marks because it
The answer restates what the question already implied. A creditable answer names the feature (the long extension, giving a large surface area) and the consequence (faster absorption).
Question 14
How should a magnification of four hundred be written?
A magnification is a ratio of two lengths, so it has no unit and is written ×400. Attaching a unit is marked as an error even when the number is right.
Question 15
An answer to a “compare” question written as two separate lists, one for each cell type,
Comparison marks need linked statements — use whereas or but so each point covers both organisms. Parallel lists often leave the examiner unable to award a comparative point.
Question 16
Which observation would let you conclude a cell came from a green leaf rather than a root?
Root cells and leaf cells both have walls, vacuoles and nuclei. Only chloroplasts separate them, because roots receive no light and so have none.
Question 17
In a multi-step calculation you should round
Rounding early lets errors accumulate, and your final answer can fall outside the range the examiner accepts. Carry the full value through and round once, at the end.
Question 18
“State the function of the nucleus. [2]” Which answer earns both marks?
Two marks means two distinct points: contains genetic material (DNA) and controls the cell's activities. Options C and D are true statements that answer a different question.
Question 19
A question shows a cell with a soft rounded outline, a nucleus and several mitochondria, but no wall and no large vacuole. The safest identification is
No wall and no large permanent vacuole, but a nucleus is present, so it is an animal cell. Plant cells always have a wall, whichever part of the plant they come from, and bacteria have no nucleus.
Question 20
The most reliable final check on any Topic 2 calculation is to ask
A plausibility and unit check catches almost every real error: upside-down fractions, missed ×1000 conversions and forgotten units all produce answers that are absurd once you compare them with a real cell.
Before you move on. Cover this page and see if you can do four things from memory: draw a plant cell and an animal cell with every structure labelled; list the six structures of a bacterial cell and say which two are DNA; give the feature and the function of all six specialised cells; and calculate a magnification from a scale bar, finishing in µm. When all four are automatic, go to the Challenge Prep page, then sit the challenge papers.