Knowing the three statements of cell theory gets you a couple of marks. Being able to say what evidence supports them, and how strong that evidence is, gets you the rest. This page is about turning an observation into an answer.
📚 What you need to know
Evidence for cell theory comes from observations of many different organisms, not from one lucky experiment.
A theory is accepted when repeated, independent observations keep agreeing with it.
Micrographs are evidence: you describe what you can see, then say what it shows.
Pasteur’s experiment supported the idea that cells only come from pre-existing cells, and helped end the idea of spontaneous generation.
A good experiment has a control that differs by only one thing.
Answers should follow claim → evidence → link, and should say what the evidence cannot show.
A few organisms are atypical and do not fit the theory neatly. Scientists adjust or qualify a theory rather than dumping it.
What counts as evidence
Students often write “scientists proved it” and stop. That earns nothing. Evidence in biology is something specific that was seen or measured, described clearly enough that someone else could go and check it.
For cell theory, the evidence falls into three types:
Type of evidence
An example
What it supports
Direct observation under a microscope
Tissue from plants, animals, fungi and bacteria all show cells
Statement 1: organisms are made of cells
Watching a process happen
Cell division has been filmed under a microscope
Statement 3: cells come from pre-existing cells
Controlled experiments
Sterile sealed broth stays clear; broth open to the air goes cloudy
Statement 3, and the rejection of spontaneous generation
The strength of cell theory is not one killer experiment. It is that thousands of separate observations, made by different people, on organisms that are nothing like each other, all point the same way. Write that sentence in an evaluation question and you will pick up a mark.
Reading a micrograph as evidence
If a question hands you an image, it wants two things from you: what you can see, and what that means. Keep them separate in your answer.
Steps 1 and 2 are where most marks are lost, because students jump straight to the conclusion without saying what they saw.
So if you are shown a micrograph of onion tissue and asked how it supports cell theory:
Describe: the tissue is divided into repeating units, each with a boundary around it and a darker structure inside.
Interpret: those units are cells, and the whole tissue is made of them with nothing in between that is not a cell.
Link: this supports the statement that living organisms are made of cells.
Limit: one sample from one plant cannot on its own support a claim about all organisms.
The experiment that settled statement three
People used to think living things could appear from non-living matter. Maggots on meat, mould on bread — it looked like life starting from nothing. This idea is spontaneous generation.
The problem was that both explanations fitted what people saw. Meat left out does end up covered in maggots. To choose between the explanations you need an experiment where only one thing differs.
The bend is the whole point. It lets air through but traps falling dust and the microorganisms riding on it, so the flask is open yet still protected.
Here is why that design is so clever, and it is worth understanding rather than memorising:
Both flasks contain the same nutrient broth, and both are boiled to kill anything already living in it.
Both are open to the air. That matters, because supporters of spontaneous generation argued that sealing a flask kept out some vital ingredient in air.
The only difference is the shape of the neck. Curved neck: dust and the microorganisms on it settle in the bend and never reach the broth. Broken neck: they drop straight in.
Result: the curved flask stays clear for months. Snap the neck and it goes cloudy within days.
So the growth was not the broth generating life. It was cells arriving from outside and then dividing. Life came from pre-existing life.
Why the control matters. If the flasks had differed in two ways — say one sealed and one open — you could not tell which difference caused the cloudiness. Changing only the neck shape means only one explanation is left standing. That is the point of a control, and it is a mark in any experimental-design question.
Writing the answer
Evaluation questions are predictable once you see the pattern. Use this order:
The shape of a good answer
claim → the evidence for it → how the evidence links to the claim → what the evidence still cannot tell you
Worked into a real sentence
“Cells come from pre-existing cells (claim). In Pasteur’s experiment, boiled broth in a swan-neck flask stayed clear while broth in a broken-neck flask turned cloudy (evidence). Since both flasks were open to air, the difference must be that microorganisms could physically reach the broth in one and not the other, so the cells came from outside rather than forming in the broth (link). However, this shows only that cells do not form under these conditions, not that it could never happen anywhere (limit).”
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CELL
Claim • Evidence • Link • Limit. Four steps, and you already know the word.
When something does not fit
A few organisms are awkward. Striated muscle fibres are enormous and have many nuclei sharing one membrane. Some fungi grow as long threads with nuclei scattered along them and no dividing walls. Certain algae are single cells you can see without a microscope.
These are called atypical examples. Notice what biologists did with them: they did not scrap cell theory, they described the exceptions and kept the theory, because everything else still fits. A theory that explains almost everything is far more useful than no theory at all, and this is exactly the kind of “nature of science” point an evaluation question is fishing for.
If a question asks whether an exception disproves cell theory, the safe answer is: it shows the theory does not describe every case perfectly, so it is qualified rather than rejected, because the huge weight of evidence still supports it.
Worked examples
WE 1
Explain how the swan-neck experiment supports cell theory
Explain how the results of the swan-neck flask experiment support the principle that cells arise from pre-existing cells. (4 marks)
Point 1: the set-up
Both flasks held the same boiled broth and both were open to the air; only the neck shape differed.
Point 2: the result
The curved-neck flask stayed clear, while the flask with the neck removed became cloudy with microorganisms.
Point 3: the reason
The bend trapped dust and the microorganisms carried on it, so no cells could reach the broth.
Point 4: the conclusion
Growth only happened when cells could get in from outside, so new cells came from existing cells rather than forming from the broth.
Growth needed cells to arrive — nothing was generated from non-living mattersay that both flasks were open to air, or you miss the control mark
WE 2
Judge whether one micrograph is enough
A student examines one micrograph of leaf tissue and concludes that all living things are made of cells. Evaluate this conclusion. (3 marks)
Point 1: what the evidence does show
The image does show that this leaf tissue is made of cells, which supports cell theory for this organism.
Point 2: why it is not enough
A conclusion about all living things needs evidence from many different organisms, not one sample of one species.
Point 3: what would strengthen it
Examining tissue from animals, fungi and prokaryotes as well, and repeating the observations, makes the conclusion far more reliable.
The observation is valid, but the conclusion is far too broad for the evidenceevaluate means say what is good and what is weak — give both sides
WE 3
Explain what happens when a theory meets an exception
Some organisms do not fit cell theory neatly. Outline how scientists respond to such observations. (3 marks)
Point 1: name an exception
Striated muscle fibres are very large and contain many nuclei inside one membrane, unlike a typical cell.
Point 2: the response
The theory is not rejected; the exceptions are described as atypical and the theory is qualified.
Point 3: the reason
A very large body of evidence from many organisms still supports the theory, so a few exceptions do not outweigh it.
Exceptions refine a theory rather than destroy italways name a specific exception — a vague “some organisms” rarely scores
💡 Exam tips
Describe before you interpret. Say what is in the image, then say what it means.
In any experiment question, name the one variable that changed and state that everything else was kept the same.
Use the word evidence, and make it specific: what was seen, in what organism, under what conditions.
Add a limitation when the command word is evaluate or discuss. It is usually worth a mark on its own.
Never write “proved”. Science supports, refutes or fails to reject — it does not prove.
Name a real exception if you mention exceptions, and say the theory is qualified, not disproved.
⚠ Common mistakes
Saying Pasteur sealed the flask. The point is that it was open — that is what killed the “air contains a vital force” argument.
Jumping to the conclusion without describing the observation first, and losing the description marks.
Writing “scientists proved cell theory”. Evidence supports it; nothing in science is proved.
Treating one observation as a general rule. The strength of cell theory is the number and variety of observations.
Claiming exceptions disprove the theory. They are atypical cases, and the theory is qualified.
Forgetting the boiling step when describing the experiment, which is what removes organisms already present.
Up next: Microscopes — the instruments behind every one of those observations, and the magnification and scale-bar calculations that come with them.
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