What is a protist?
A protist is a living thing that has a true nucleus but is not a plant, an animal or a fungus. A true nucleus is a little bag that keeps the DNA inside. Living things with this are called eukaryotes. Bacteria do not have it.
Most protists are just one cell. That one cell must do every job: eat, move, breathe, get rid of waste and make more of itself. A few protists live in groups (colonies), and some seaweeds are big and many-celled.
Scientists put protists into three easy groups by the way they get food:
- Animal-like (protozoa): catch and eat food. Examples: amoeba, paramecium, Plasmodium.
- Plant-like (algae): make food from light with chloroplasts. Examples: euglena, diatoms, Chlamydomonas, seaweeds.
- Fungus-like: soak up food from dead matter. Example: slime moulds, water moulds.
Today biologists know protists are a mixed group, not one family. The word is still used because it is handy.
How protists move, feed, breathe and keep water balance
Moving
- Pseudopods (false feet): the amoeba pushes its jelly-like inside forward and the rest follows.
- Cilia: thousands of short hairs beat together like oars. Paramecium is fast.
- Flagella: one or two long whips spin or lash. Euglena uses one.
Feeding
An amoeba surrounds food with pseudopods and closes it in a food vacuole. Enzymes digest it there. Waste is pushed out of the cell. Paramecium sweeps food into its oral groove (mouth groove) with cilia. Euglena makes food in light with chloroplasts; its red eyespot helps it swim towards light. In the dark, euglena takes in food from the water. So it can feed both ways (mixotroph).
Breathing and waste
Oxygen comes in and carbon dioxide goes out through the cell membrane by diffusion (spreading from more to less). Ammonia waste leaves the same way.
Water balance (osmoregulation)
Pond water has less salt than the cell. So water keeps leaking in. A contractile vacuole collects the extra water and squirts it out, again and again. Without it, the cell would swell and burst. In sea water the vacuole works slowly or is missing.
Reproduction, life cycles and cysts
Most protists multiply by binary fission: the nucleus copies itself and the cell splits into two daughter cells. Some split into many cells at once (multiple fission), as Plasmodium does inside red blood cells. Paramecium can also swap nucleus material with a partner (conjugation), which mixes genes.
Many protists have life cycles with two phases: a phase with one set of chromosomes (haploid) and a phase with two sets (diploid). Plasmodium switches between a human host and a mosquito host.
When the pond dries or gets cold, many protists build a thick wall and become a cyst. Inside, life slows down. When water returns, the cell comes out. Cysts are also how some disease protists pass from dirty water to people.
Try it: put a handful of dry grass (hay) in a jar of pond or tap water, cover loosely and leave it in a warm, bright place for 5–7 days. A drop under a school microscope will show fast-moving protists. This is called a hay infusion culture. Wash your hands afterwards.
Protists that cause disease, and their good side
- Malaria: caused by Plasmodium, carried by the female Anopheles mosquito. Fever comes in cycles when the parasites burst out of red blood cells. Prevent with nets, repellents and removing still water.
- Amoebic dysentery: Entamoeba histolytica from dirty water or food; causes bloody diarrhoea.
- Giardiasis: Giardia cysts in untreated water; causes stomach cramps and diarrhoea.
- Toxoplasmosis: Toxoplasma from cat faeces or undercooked meat; risky in pregnancy.
- Trichomoniasis: Trichomonas, passed by sexual contact; treated with medicine.
- Sleeping sickness: Trypanosoma, spread by the tsetse fly in Africa.
Good side: plant-like protists in the sea (phytoplankton, diatoms) make a huge share of Earth's oxygen and are the base of ocean food chains. Diatom shells form diatomaceous earth used in filters. Seaweeds give agar (used in labs and desserts) and food. Protozoa clean sewage by eating bacteria.
Key formulas and definitions
- Protist = eukaryote (true nucleus) that is not a plant, animal or fungus
- Pseudopod = false foot (amoeba); cilia = tiny hairs (paramecium); flagellum = whip (euglena)
- Contractile vacuole = pump that removes extra water (osmoregulation)
- Binary fission: 1 cell → 2 cells; multiple fission: 1 cell → many
- Cyst = resting stage with a hard wall
Worked examples
1. A paramecium is moved from pond water into slightly salty water. What happens to its contractile vacuole?
Less water leaks into the cell because the outside water is now closer to the cell's own saltiness. So the vacuole fills more slowly and pumps less often.
2. Why can euglena be called both plant-like and animal-like?
In light it uses chloroplasts to make food like a plant. In the dark it takes food from the water like an animal, and it swims actively with a flagellum.
3. A village gets diarrhoea after drinking from a stream. Which protists could be the cause, and how can it be stopped?
Giardia or Entamoeba, spread as cysts in dirty water. Boiling water for one minute, filtering it and keeping toilets away from water sources stops them.
Common mistakes
- Calling protists bacteria. Bacteria have no nucleus; protists do.
- Thinking all protists are harmful. Most are harmless, and many make oxygen and food for sea life.
- Saying the contractile vacuole digests food. It only pumps out water; food is digested in the food vacuole.
- Thinking the mosquito causes malaria by itself. The cause is the protist Plasmodium; the mosquito only carries it.