Put a drop of pond water under a microscope and you will see a crowd of single cells darting, crawling and spinning. Most of them are protists. Now think of the white fuzz on old bread, the mushrooms on a lawn after rain, or the yeast that makes bread rise: those are fungi. Both groups are eukaryotes, so their cells have a nucleus and membrane-bound organelles, yet neither is a plant or an animal. Protists show us what early eukaryotic life may have looked like, and they include the algae that make much of the oxygen we breathe. Fungi quietly run the recycling system of every forest, feed most plants through their roots, spoil food, cause disease, and gave us the first antibiotic. This lesson looks at how each group is built and how it lives.
Protists: the eukaryotes that do not fit elsewhere
Kingdom Protista is defined mostly by what its members are not: a protist is any eukaryote that is not a fungus, plant or animal. That makes it a very mixed group. Most protists are unicellular, though some, such as kelp, are huge multicellular organisms. They live wherever there is water, including ponds, oceans, damp soil and the bodies of other organisms. Biologists often sort them into three informal groups by how they get food, because nutrition is easy to observe and links to their other features. This grouping is practical rather than evolutionary: some plant-like protists are more closely related to animal-like ones than to each other.
| Group | Nutrition | Examples | How they move | Notable features |
|---|---|---|---|---|
| Animal-like (protozoans) | Heterotrophic; ingest food | Amoeba, Paramecium, Plasmodium (malaria) | Pseudopods, cilia, or none (parasites) | Food vacuoles; contractile vacuoles pump out excess water |
| Plant-like (algae) | Autotrophic; photosynthesize | Euglena, diatoms, kelp (brown alga) | Flagella, or none | Chloroplasts; diatoms have glassy silica shells; kelp is multicellular |
| Fungus-like | Heterotrophic; absorb food from decaying matter | Plasmodial and cellular slime moulds, water moulds | Creeping, amoeba-like stages | Produce spores like fungi but have no chitin cell walls |
How protists move, feed and reproduce
An Amoeba moves by pushing out pseudopods ("false feet"): cytoplasm flows into a bulge, and the rest of the cell follows. The same pseudopods wrap around a bacterium or smaller protist and enclose it in a food vacuole, a process called phagocytosis; enzymes then digest the prey inside the vacuole. A Paramecium is covered in thousands of hair-like cilia that beat in coordinated waves, driving it forward and sweeping food into an oral groove. Freshwater protists face a constant problem: water keeps entering them by osmosis, so a contractile vacuole collects the excess and squirts it out, preventing the cell from bursting. Euglena whips a long flagellum to swim, and a red eyespot helps it steer toward light so its chloroplasts can photosynthesize. In the dark, Euglena can absorb food instead, which is why it was once argued over as plant or animal. Diatoms are single algal cells in two-part silica shells, and they are among the most important phytoplankton producers of oxygen in the oceans. Kelp is a giant multicellular brown alga anchored by a holdfast, forming underwater forests off Canada's Pacific coast. Slime moulds creep over rotting logs as a slimy mass, engulfing bacteria, then form stalked spore-producing structures when food or water runs short.
- The cell copies its DNA, then divides into two identical daughter cells.
- Fast: a population doubles every generation when conditions are good.
- No new combinations of genes; offspring are clones.
- Used by Amoeba, Paramecium, Euglena and most protists.
- Two Paramecia join side by side and swap some of the DNA in their small micronuclei.
- No new cells are made at that moment; each partner leaves with a new mix of genes.
- Creates genetic variation, which helps a population survive a changing environment.
- Often happens when conditions become stressful.
Fungi: built from hyphae
Apart from single-celled yeasts, a fungus is made of hyphae, thin threads of cells surrounded by walls of chitin, the same tough polysaccharide found in insect exoskeletons. The tangled network of hyphae is the mycelium, and it usually hides inside soil, wood or food. A mushroom is only the fruiting body, a temporary structure that the mycelium builds to release spores. Fungi are heterotrophs that feed by absorption: hyphae release digestive enzymes into their surroundings, break food down outside the body (extracellular digestion), and then absorb the small molecules. The thread shape gives an enormous surface area for absorbing, which is why a mycelium can digest a whole log. Fungi reproduce mainly with spores, tiny, light cells that spread on wind, water or animals and can survive dry conditions. Most fungi can make spores both asexually (by mitosis) and sexually (after two compatible hyphae fuse), and the structure that makes the sexual spores is used to sort them into groups.
| Group | Sexual spore structure | Examples | Importance |
|---|---|---|---|
| Zygomycetes (common moulds) | Thick-walled zygosporangium, formed where two hyphae fuse | Rhizopus stolonifer (black bread mould) | Spoil bread and fruit; asexual spores form in round sporangia on upright hyphae |
| Ascomycetes (sac fungi) | Spores form in tiny sacs called asci | Yeast (Saccharomyces), Penicillium, morels, Dutch elm disease fungus | Bread, beer, penicillin; also many plant diseases. Yeast reproduces asexually by budding. |
| Basidiomycetes (club fungi) | Spores form on club-shaped basidia, often on the gills of a mushroom | Mushrooms, puffballs, shelf fungi, rusts and smuts | Major wood decomposers; edible and poisonous mushrooms; rusts damage wheat crops |
Fungi as decomposers, partners and pathogens
- Decomposers — with bacteria, fungi break down dead leaves, wood, dung and carcasses, returning carbon, nitrogen and phosphorus to the soil and air. Many fungi can digest lignin, the tough material in wood that few other organisms can break down. Without them, forests would be buried in their own litter.
- Lichens — a partnership between a fungus and a photosynthetic partner (a green alga or a cyanobacterium). The alga makes food; the fungus provides shelter, water and minerals. Lichens grow on bare rock and are pioneer species, and in northern Canada caribou rely on lichens as winter food.
- Mycorrhizae — associations between fungal hyphae and plant roots. The hyphae act like extra roots, delivering water and phosphorus, and the plant pays with sugars. Most land plants form mycorrhizae, and fossils suggest the partnership helped the first plants colonize land.
- Disease — fungi cause athlete's foot, ringworm and yeast infections in humans. In plants they cause rusts, smuts and Dutch elm disease, which killed many of the elm trees that once lined Ontario streets.
- Medicine and food — in 1928 Alexander Fleming noticed that a Penicillium mould killed the bacteria around it, leading to penicillin, the first widely used antibiotic. Yeasts raise bread and ferment beverages, and fungi give us cheeses such as blue cheese.