The big idea: A unicellular organism is made of just one cell — yet that single cell must carry out every function of life on its own.
There is no division of labour into tissues or organs: the one cell feeds, reacts, grows, removes waste and reproduces all by itself.
Examples include Paramecium and Amoeba (animal-like protists), Chlamydomonas (a green alga) and bacteria.
The functions of life
- Nutrition — obtaining nutrients and energy
- Metabolism — the chemical reactions inside the cell
- Growth — getting larger / making more cell material
- Response — reacting to a stimulus in the surroundings
- Excretion — getting rid of the waste products of metabolism
- Homeostasis — keeping internal conditions stable
- Reproduction — producing offspring
A memory hook: To recall the seven functions, group them by what the cell is doing:
- Staying alive day to day: nutrition (feeding), metabolism (its reactions), excretion (removing waste) and homeostasis (staying balanced).
- Dealing with the outside world: response (reacting to its surroundings).
- Getting bigger and making more: growth and reproduction.
Any memory trick works — what matters is that you can name all seven functions.
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To answer exam questions you must be able to match a feature you can see in a single-celled organism to the function of life it demonstrates.
First, lock down what the key terms mean.
- Unicellular organism
- A living organism whose whole body is a single cell.
- Function of life
- A life process that every living organism must carry out to stay alive (e.g. nutrition, response, reproduction).
- Contractile vacuole
- A structure in many freshwater single-celled organisms that collects excess water and pumps it back out of the cell.
- Binary fission
- Reproduction in which one cell divides into two identical cells.
Now connect each function to a clue you might observe under a microscope or in a description. This table is the tool you use in the exam:
| Function of life | What it means | Clue in a single cell |
|---|---|---|
| Nutrition | Getting the nutrients/energy it needs | Engulfing food, a food vacuole, or chloroplasts to photosynthesise |
| Metabolism | Chemical reactions inside the cell | Enzymes working / energy being released or used |
| Growth | Getting larger or making more cell material | Cell increases in size over time |
| Response | Reacting to a stimulus | Moving toward food or away from light/touch |
| Excretion | Removing waste products of metabolism | Releasing CO₂ or emptying a contractile vacuole |
| Homeostasis | Keeping internal conditions stable | A contractile vacuole pumping out excess water |
| Reproduction | Producing offspring | Dividing into two cells (binary fission) |
A single-celled organism carrying out all functions of life.
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Homeostasis vs excretion — keep them separate: A contractile vacuole is the classic source of confusion.
Pumping out excess water keeps the cell's internal water balance stable, so this is homeostasis.
Excretion is removing the waste products of metabolism, such as CO₂ — water entering by osmosis is not a metabolic waste, so the contractile vacuole is best matched to homeostasis, not excretion.
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How this is tested: On Paper 1A (multiple choice) you are often shown images or a description of a unicellular pond organism and asked to identify which functions of life it is demonstrating.
On Paper 2 a short question can ask you to describe how a single cell carries out a named function, or to state that one cell performs all the functions of life.
The skill is always the same: read off an observable clue and name the matching function.
IB-style question — a Paramecium under the microscope
A student watches a single Paramecium (a unicellular pond organism). They observe it (i) sweeping food particles into a food vacuole, and (ii) steadily emptying a contractile vacuole. State the function of life shown by each observation. [2]
How to score both marks
- Read clue (i). Sweeping food into a food vacuole is taking in nutrients — this is nutrition.
- Read clue (ii). Emptying a contractile vacuole removes excess water and keeps the internal water balance stable — this is homeostasis (osmoregulation).
- Answer the command term. (i) Nutrition; (ii) Homeostasis. Two correct functions = 2 marks.
Final answer
(i) Nutrition (taking in food via the food vacuole); (ii) Homeostasis (the contractile vacuole pumps out excess water to keep internal conditions stable).
Why this scores: Each mark comes from matching one observed clue to one function. Naming excretion for the contractile vacuole would lose the mark — that water is not a metabolic waste, so it is homeostasis.