The big idea: Cell theory is one of the foundations of biology. It says three things:
- 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 seven functions of life (MRS GREN)
- M — Metabolism: all the chemical reactions inside the cell
- R — Reproduction: making new cells / new organisms
- S — Sensitivity: responding to changes (stimuli) in the surroundings
- G — Growth: getting larger or making more living material
- R — Respiration: releasing energy from food to power the cell
- E — Excretion: removing the waste products of metabolism
- N — Nutrition: taking in (or making) the food and nutrients it needs
A single-celled Paramecium: each visible structure is labelled with the function of life it carries out — one cell does all seven (MRS GREN) 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 seven functions of life above — remember them with MRS GREN.
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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).
Louis Pasteur disproved this. 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 seven activities every living organism must carry out: metabolism, reproduction, sensitivity, growth, respiration, excretion and nutrition (MRS GREN).
- Unicellular organism
- An organism made of a single cell that carries out all seven 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 one of the seven functions of life: it feeds, respires, grows, responds, reproduces, and removes its own waste.
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How this is tested: On Paper 1A (multiple choice) you may be asked to identify a function of life from a description (e.g. a Paramecium ingesting algae = nutrition), or to recognise the evidence that disproved spontaneous generation.
On Paper 2 a short Explain question often asks 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 seven 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.