Fungi and Protists

Animals, plants, fungi and protists are all eukaryotes: their cells share the same basic organization (see UCMP). The Classification lesson shows where they sit in the tree of life, and Microbiology and Immunity looks at the smallest of them as microbes.

A fungus is built from thin threads called hyphae. The whole tangle of hyphae is the mycelium, the body of the fungus. A mushroom is only the part that makes spores, called a sporocarp (see UCMP).

Fungal cell walls contain chitin, the same tough material found in the outer skeletons of insects and spiders (see UCMP).

Fungi cannot make their own food like plants, nor swallow it like animals. Their hyphae grow through wood, soil or cheese, give out digestive enzymes and absorb the nutrients that are freed (see UCMP).

Fungi spread by spores. In a mushroom, spores are made by meiosis on its gills, and each spore can grow into new hyphae (see UCMP).

A typical life cycle runs like this (see UCMP):

• hyphae from two different fungi meet and fuse, but their nuclei stay separate, so each cell has two nuclei;
• this two-nucleus stage can grow for years;
• at last the nuclei fuse and undergo meiosis, making new spores.

Hyphae can also make spores without meiosis, and some fungi have lost sex altogether. Yeasts mostly reproduce asexually rather than by spores (see UCMP).

By breaking down dead material, fungi return nutrients to the ecosystem (see UCMP). Most fungi feed on dead or decaying matter (see UCMP).

Most plants rely on a fungus living in their roots. Root and fungus together are a mycorrhiza, meaning fungal root. The fungus soaks up water and nutrients over a wide area, and the plant pays it in sugars from photosynthesis (see UCMP). The Plant Biology lesson follows that water up through the plant.

A lichen is not a single organism. It is mostly fungal threads, with cells of a green alga or a cyanobacterium living among them (see UCMP).

Baker's yeast, Saccharomyces cerevisiae, is a single-celled fungus. When it ferments sugar it makes alcohol and carbon dioxide (see Parapouli and others). Fungi give us the bubbles in bread, champagne and beer (see UCMP). In bread the gas makes the dough rise; in beer the alcohol is the point.

Yeast is sensitive to temperature. One study found that S. cerevisiae grows best at about 32 °C and stops growing above about 45 °C (see Salvadó and others).

Fungi give us drugs such as penicillin and other antibiotics (see UCMP). Penicillin comes from the mold Penicillium, which Alexander Fleming noticed killing bacteria in 1928 (see Nobel biography).

Fungi also cause disease. In people they cause ringworm, athlete's foot and some more serious illnesses. Fungi are chemically and genetically closer to animals than other organisms are, which makes fungal infections hard to treat without harming the patient. In crops, fungi cause rusts, smuts and rots (see UCMP).

A protist is any eukaryote that is not a true animal, plant or fungus. Most are single cells (see Britannica). Many scientists no longer treat protists as a real natural group: it is a convenient label for what is left over (see Britannica).

The microscopic forms were long split into two informal groups (see Britannica):

• algae, the “minute plants”, which photosynthesize;
• protozoa, the “first animals”, such as amoebas and the malaria parasite.

Slime molds were long filed with the fungi, but they are now known to be quite unrelated to them. Plasmodial slime molds are giant single cells with thousands of nuclei. Cellular slime molds live as separate amoeba-like cells until a chemical signal makes them gather into a swarm (see UCMP).

Malaria is caused by Plasmodium parasites, spread by the bites of infected female Anopheles mosquitoes. It does not spread from person to person. Five species infect people; P. falciparum and P. vivax are the greatest threat (see WHO).

In 2024 there were an estimated 282 million cases and 610 000 deaths in 80 countries. The WHO African Region carried about 95% of both. There, children under five made up about 76% of malaria deaths (see WHO). Yet malaria is preventable and curable (see WHO).

The parasite needs two hosts, a person and a mosquito (see CDC). Inside the mosquito, its part of the cycle takes about 9 to 18 days (see CDC).

Diatoms are algae that live inside glass-like shells of silica. They are very common in ocean and freshwater plankton, and because they photosynthesize they are an important food for sea life (see UCMP).

Scientists estimate that roughly half of Earth's oxygen production comes from the ocean, mostly from plankton: drifting algae, plants and some bacteria. The exact share is hard to measure because plankton numbers keep changing, and ocean life uses up roughly as much oxygen as it makes (see NOAA). So be wary of any exact figure for how much of your breath comes from diatoms.

Much of this story turned on microscopes, and on people willing to argue that two organisms could live as one.