Cells and organisation · Contrast
Unicellular organisms
Two single cells, one much smaller. One is a whole animal. One is a piece of you. What tells them apart?
Start here
A quarter of a millimetre long, and it hunts.
A Paramecium swims, finds food, eats it, senses obstacles, reverses, grows and splits in two. A cheek cell is four times smaller and lines your mouth. Neither is more alive than the other.
One is an organism. One is part of one. What settles it?
Three of those four are true of a Paramecium and settle nothing. Telling which fact does the work is the whole of this lesson.
At the bench · two slides
Pond water, then your own cheek
Commit first. At which magnification will you first be able to tell one pond organism from another?
Make your prediction first — then the lab runs.
Side by side
The same seven processes. One cell does all of them; one does one.
- MovementBeats hundreds of cilia and swims, reversing when it hits something.Does not move. It is held in a sheet by the cells around it.
- RespirationIts own mitochondria, releasing energy for its own swimming and feeding.Mitochondria too — but the oxygen is delivered to it by blood.
- SensitivitySenses an obstacle and backs away from it.Senses nothing. Nerve cells do that for the whole of you.
- GrowthGrows until it is big enough to divide.Does not grow. It is replaced when it wears out.
- ReproductionSplits in two, and then there are two organisms.Divides to replace itself, and there is still only one of you.
- ExcretionVacuoles collect the water that keeps seeping in and squeeze it back out.Waste passes into the blood, and your kidneys deal with it.
- NutritionSweeps food into an oral groove and digests it in a vacuole inside itself.Is fed. Glucose arrives dissolved in blood, already digested.
Key fact
One cell doing all seven life processes is an organism. One cell doing one job is part of one.
Your turn · four mystery cells
Does that settle it?
Mystery cell 1
About 0.05 mm long. It is green, and it swims using one long whip at the front.
It can swim.
Settles nothing. A sperm cell swims. So does a white blood cell, through tissue. Moving on your own tells you nothing about whether you are an organism.
It is green, and makes its own food from light.
Settles it. Nothing in an animal body makes its own food. A cell feeding itself is not being fed by an organism — so it is one.
It has mitochondria.
Settles nothing. Every cell in both columns has mitochondria. A fact that is true of everything discriminates nothing.
It is only 0.05 mm long.
Settles nothing. Size never settles it. Your cheek cell is 0.06 mm and a Paramecium is 0.25 mm — the bigger one is the organism here, and that is a coincidence.
It is an organism
Euglena — a single-celled organism.
Green, so it makes its own food; a flagellum, so it can go and find light. One fact out of four did the work.
Mystery cell 2
It has a long tail and swims fast in one direction. It never feeds.
It has a tail and swims.
Settles nothing. So does Euglena. Swimming is the most misleading fact on this page.
It never feeds.
Settles it. An organism must take in nutrition. A cell that never feeds is being supplied by something larger — and everything it will ever spend was loaded before it set out.
It carries half a set of chromosomes.
Settles it. Half a set is only useful if it is going to be added to another half. Nothing that lives on its own has half its instructions.
It is packed with mitochondria.
Settles nothing. True, and it tells you the cell works hard. A Paramecium is packed with them too.
It is part of an organism
A sperm cell — one cell of a multicellular organism.
Two facts settled it, and neither was the swimming. Some cases have more than one discriminator; the skill is spotting which facts qualify, not counting them.
Mystery cell 3
It has no nucleus at all. In warm conditions it divides about every twenty minutes.
It has no nucleus.
Settles nothing. A red blood cell has no nucleus either, and that is a part of you. Missing a nucleus settles nothing on its own.
It divides on its own, over and over.
Settles it. A red blood cell can never divide again. A cell that makes more of itself unaided is reproducing — the sixth life process — and so it is an organism.
It is very small — about 0.002 mm.
Settles nothing. Size again. It is the smallest thing on the slide and it is still a complete organism.
Its DNA sits in a loose loop in the cytoplasm.
Settles it. No cell in your body does this. Yours keep their DNA inside a nucleus, always. Loose DNA is a different kind of cell entirely.
It is an organism
A bacterium — a single-celled organism with no nucleus.
Its instructions are present; only the container is missing. That is exactly why it can divide when a red blood cell cannot.
Mystery cell 4
It changes shape constantly, flows around a smaller cell, and takes it inside itself.
It changes shape as it moves.
Settles nothing. A white blood cell squeezes out of a blood vessel and crawls through tissue by changing shape. This is not a giveaway.
It flows around another cell and engulfs it.
Settles nothing. This is the hardest one. A white blood cell engulfs bacteria in exactly this way — and it is part of you. Behaviour is not evidence.
Nothing else feeds it and nothing else positions it.
Settles it. It is on its own. Whatever it eats, it found; wherever it is, it went. That is an organism.
It has a vacuole that keeps filling with water and squeezing it out.
Settles it. Your body holds the fluid around its cells steady, so no cell of yours needs to bail out. A cell that must manage its own water is not inside a body.
It is an organism
Amoeba — a single-celled organism.
The two most vivid facts — shape-changing and engulfing — are the two that settle nothing, because a white blood cell does both. The quiet facts did the work.
Now look back at your four marks. Did you have it?
Nobody marks this but you. The facts you marked wrongly are the ones to read again before the next cell.
Think again
“No nucleus means no instructions — so a bacterium cannot divide.”
Two lessons ago that was exactly right, about a red blood cell. It is wrong here, and the difference is worth being precise about. A red blood cell had a nucleus and destroyed it, and the DNA went with it. A bacterium never had one: its DNA is there, in a loose loop in the cytoplasm, with no membrane around it. Instructions present, container absent. That is why a bacterium can divide every twenty minutes and a red blood cell can never divide again.
“One cell means simple.”
Compare what you have just looked at. Your cheek cell has a membrane, cytoplasm, a nucleus and mitochondria, and it is fed, cleaned, positioned and defended by the rest of you. A Paramecium has all of that plus cilia to swim, an oral groove to feed, food vacuoles to digest, and vacuoles that bail out water it cannot stop taking in. It has more machinery than your cell, not less, because there is nobody to do any of it for it.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Discriminate
A single cell moves on its own, has mitochondria, and is about 0.05 mm long. Which of these would settle whether it is an organism or part of one?
Rung 2 · The one that catches people
A red blood cell has no nucleus and can never divide. A bacterium has no nucleus and divides every twenty minutes. What explains the difference?
Rung 3 · Explain both sides
A Paramecium and one of your cheek cells are both single cells. Explain why the Paramecium needs cilia, an oral groove and a contractile vacuole, and why your cheek cell needs none of the three. Your answer must say something about both cells.
Rung 4 · Take it somewhere new
A student finds a single cell in a drop of water from a plant pot. It has a wall, changes shape a little, has no chloroplasts, and buds off smaller copies of itself. They say it must be part of a plant because it has a wall. Say whether they are right, which fact you would use instead, and what you would still want to check.
Key note
A unicellular organism is one cell that does all seven life processes for itself. Its adaptations — cilia, an oral groove, a contractile vacuole, a loose loop of DNA — are what a whole organ system does for one of your cells.
Going further
Euglena is the awkward one. It is green, traps light and makes its own food like a plant; it also swims after food with a flagellum and can feed like an animal in the dark. Biologists spent a century arguing about which kingdom it belonged in, and the answer they settled on was that the question was wrong — it belongs to neither, and single-celled life does not sort neatly into plant and animal at all. Being hard to classify is not a fault in the organism. It is information about the categories.
Before this lesson
Connects to
At GCSE this becomes
- Bacterial cell structure, culturing microorganisms, and how single-celled organisms cause and fight disease.
Where to next
- Next: What the skeleton does
Movement: skeleton and muscles
- Previous: Levels of organisation
Not sure which fact settles it?
Pond water is not sterile. Keep it away from your mouth and cuts, seal slides with a coverslip, and wash your hands before you leave the lab.
Lesson content © MrBadmusAI.