You have trillions of cells and each one does a narrow job. A Paramecium has one cell and it has to do everything — feed itself, move, get rid of waste, keep its water balanced and reproduce. That is the test of whether something is alive, and it is why a single cell counts as the smallest unit of life.
📚 What you need to know
Living things carry out seven functions of life: metabolism, response, homeostasis, growth, reproduction, excretion and nutrition.
A unicellular organism carries out all seven inside a single cell.
This is the evidence behind the second part of cell theory: a cell is the smallest unit that does all of them.
Paramecium and Chlamydomonas are the two named examples you should be able to describe.
Each function is linked to a specific structure in the cell — cilia for movement, a contractile vacuole for water balance, and so on.
Chlamydomonas makes its own food by photosynthesis, so it is an autotroph; Paramecium takes in food, so it is a heterotroph.
The seven functions
These are the jobs that separate a living thing from a rock or a machine. Something has to do all seven, not a few of them.
Metabolism is the umbrella term for every chemical reaction in the organism, so respiration and photosynthesis both sit under it rather than counting separately.
Function
What it means
Metabolism
All the chemical reactions happening inside the organism, including respiration
Response
Detecting a change in the surroundings and reacting to it
Homeostasis
Keeping internal conditions such as water content and pH within safe limits
Growth
A permanent increase in size or in the amount of living material
Reproduction
Producing offspring, either sexually or asexually
Excretion
Getting rid of the waste products made by its own reactions
Nutrition
Obtaining food, either by making it or by taking it in
🧠
MR H GREN
Metabolism, Response, Homeostasis, Growth, Reproduction, Excretion, Nutrition. Seven letters, seven functions, in the order most mark schemes list them.
One cell doing everything
Paramecium is a single-celled organism found in pond water. It is a good example because you can point at a structure for each function, which is exactly what an exam answer needs.
The purple oval is the nucleus. Paramecium actually has two, but at this level one is enough for a labelled drawing.
Function
How Paramecium does it
How Chlamydomonas does it
Metabolism
Enzymes in the cytoplasm control its reactions
Enzymes in the cytoplasm control its reactions
Response
Changes the beating of its cilia to move away from harm
An eyespot detects light and it swims towards it
Homeostasis
A contractile vacuole pumps out excess water
A contractile vacuole pumps out excess water
Growth
Gets bigger as food is digested and used
Gets bigger as glucose is made and used
Reproduction
Divides asexually by mitosis; can also exchange genetic material
Divides asexually, and can reproduce sexually too
Excretion
Waste diffuses out across the plasma membrane
Waste diffuses out across the plasma membrane
Nutrition
Sweeps food into the oral groove and digests it — a heterotroph
Makes glucose in its chloroplast by photosynthesis — an autotroph
The two organisms differ in only two rows: nutrition and response. If you remember that Chlamydomonas is the one with a chloroplast and an eyespot, you can rebuild the whole table from what you already know about Paramecium.
Why the contractile vacuole exists. Pond water is far more dilute than the inside of the cell, so water keeps moving in by osmosis. Without a way of pumping it back out, the cell would swell and burst. The vacuole fills up and then squeezes its contents out through the membrane, over and over. It is homeostasis in its simplest possible form.
Worked examples
WE 1
Link functions to structures
Outline how Paramecium carries out nutrition, homeostasis and movement. (3 marks)
Nutrition
Cilia sweep food particles into the oral groove, where they are enclosed in a food vacuole and digested.
Homeostasis
A contractile vacuole collects excess water that has entered by osmosis and pumps it back out.
Movement
The cilia covering the cell beat together to push it through the water.
Oral groove, contractile vacuole, ciliaeach mark needs a named structure, not just the name of the function
WE 2
Explain why a virus is not classed as living
Using the functions of life, explain why viruses are not considered to be living organisms. (3 marks)
Point 1: the standard
To be classed as living, something must carry out all the functions of life.
Point 2: what a virus cannot do
A virus has no metabolism of its own and cannot grow, respond or maintain internal conditions.
Point 3: reproduction
It can only reproduce by taking over a host cell, so it does not do this independently.
It fails most of the seven, so it is not classed as livingthe argument is about doing all seven, not about being small or simple
WE 3
Compare two unicellular organisms
Compare how Chlamydomonas and Paramecium obtain nutrients and respond to their surroundings. (4 marks)
Nutrition in Chlamydomonas
It makes its own glucose by photosynthesis in a chloroplast, so it is an autotroph.
Nutrition in Paramecium
It takes food in through the oral groove and digests it, so it is a heterotroph.
Response in Chlamydomonas
An eyespot detects light and the cell swims towards it, which helps photosynthesis.
Response in Paramecium
It changes the beating of its cilia to move away from obstacles or harmful conditions.
Autotroph with an eyespot, against a heterotroph with cilia“compare” needs both organisms in every point, not one then the other
💡 Exam tips
Learn all seven functions by name. Questions often ask for a set number of them.
Always name the structure that carries out a function in a unicellular organism.
Metabolism covers respiration and photosynthesis — do not list them as separate functions.
Excretion is removing waste made by the organism itself, which is not the same as removing undigested food.
Use the words autotroph and heterotroph when comparing nutrition.
Link this page back to cell theory: a cell is the smallest thing that does all seven.
⚠ Common mistakes
Confusing excretion with egestion. Excretion is metabolic waste; egestion is undigested material.
Saying growth means getting taller. It means a permanent increase in living material.
Treating movement as one of the seven. It is part of response, not a function on its own.
Forgetting homeostasis in unicellular organisms. The contractile vacuole is doing exactly that.
Calling Chlamydomonas a plant. It is a unicellular alga.
Writing that a virus is alive because it reproduces. It cannot do so without a host cell.
Up next: Eukaryotic Cells: Comparisons & Atypical Examples — how animal, plant and fungal cells differ, and the odd cells that refuse to follow the rules.
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