The big idea: Cell theory is one of the foundations of biology. It says three things.
The three parts of cell theory
- All living things are made of one or more cells.
- The cell is the basic unit of structure and function in life.
- New cells only come from existing cells (by cell division).
So: There is no smaller living unit than a cell — and a cell never appears from non-living material.
The functions of life
- Homeostasis: keeping conditions inside the cell steady, such as its water content
- Metabolism: all the chemical reactions inside the cell
- Nutrition: taking in (or making) the food and nutrients it needs
- Movement: moving itself, or moving things around inside the cell
- Excretion: removing the waste products of metabolism
- Growth: getting larger or making more living material
- Response to stimuli: reacting to changes in the surroundings
- Reproduction: making new cells / new organisms
A single-celled Paramecium: each visible structure is labelled with the function of life it carries out — one cell carries out every function of life by itself.
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Why this matters: Because a cell is the smallest living thing, every cell must carry out all the jobs needed to stay alive by itself.
Those jobs are the functions of life above.
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For a long time people believed in spontaneous generation — the idea that living things could appear from non-living matter (for example, maggots simply forming on meat).
How Louis Pasteur disproved it
- He boiled broth in swan-neck flasks.
- The broth stayed clear because microbes from the air were trapped in the bend.
- When the neck was broken off, microbes reached the broth and it turned cloudy.
- This showed that cells (microbes) only come from other cells, not from the broth itself.
- Cell theory
- Living things are made of cells; the cell is the basic unit of life; cells come only from pre-existing cells.
- Spontaneous generation
- The disproved idea that living organisms can arise from non-living material.
- Functions of life
- The processes every living organism carries out: homeostasis, metabolism, nutrition, movement, excretion, growth, response to stimuli and reproduction.
- Unicellular organism
- An organism made of a single cell that carries out all the functions of life by itself.
- Excretion
- The removal of the waste products of metabolism from the cell or organism.
A single cell must do it all: A unicellular organism — such as Paramecium or a yeast cell — is one cell that is a whole organism.
It has no other cells to help it, so that one cell must perform every function of life: it feeds, respires, grows, responds, reproduces, and removes its own waste.
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The big idea: Cell theory lets us predict before we look: any newly discovered organism will be made of one or more cells. That is deductive reasoning.
The points to remember
- A theory explains many observations; cell theory says all living things are made of one or more cells.
- Deductive reasoning goes from a general theory to a prediction about a particular case.
- Deduced from cell theory: a newly discovered organism will consist of one or more cells.
- A prediction can be tested, for example by looking at the organism under a microscope.
- Each time a prediction is supported, confidence in the theory grows; one clear failure would challenge it.
Remember it as: Theory first, prediction next, then look.
Real example: a microbe found by its DNA: Scientists first found DNA of a new group of archaea in deep-sea mud. Cell theory predicted it came from cells. In 2020, after 12 years of trying, a Japanese team grew one: tiny round cells with long branches.
Keep the direction of the reasoning right:
Deduction starts from the theory: Deductive reasoning goes from the general theory to a prediction about one organism. Going from many observations to a general rule is induction.
How this is tested: This sub-topic comes up as a quick identify question and as a short Explain question.
Where it comes up
- Paper 1A (multiple choice): identify a function of life from a description (e.g. a Paramecium ingesting algae = nutrition).
- Paper 1A: recognise the evidence that disproved spontaneous generation.
- Paper 2: a short Explain question on why a single cell that performs all the functions of life must also carry out one named function — such as excretion.
IB-style question — why a single cell must excrete
A unicellular alga lives in pond water and carries out all the functions of life. Explain why this single cell must also carry out excretion. [2]
How to score both marks
- Start from metabolism. The cell's chemical reactions (metabolism), such as respiration, produce waste products — for example carbon dioxide (CO₂).
- Say why the waste is a problem. If these waste products are not removed, they build up and become toxic / harmful, which would damage or kill the cell.
- Answer the command term — explain. So the cell must carry out excretion (removing metabolic waste) to stay alive, because no other cell can do it for a single-celled organism.
Final answer
Metabolism (e.g. respiration) makes waste products such as CO₂; if not removed they build up and become toxic, so the cell must excrete to survive.
What earns the marks: A full answer needs both ideas: (1) metabolism produces waste (name one, e.g. CO₂), and (2) the waste would build up and harm the cell if it were not excreted.
Just writing 'to remove waste' on its own is usually only worth one mark.