What If? Lab

What if there were no fungi?

Mushrooms, molds and yeast are all fungi. They help make bread, they tidy up old logs, and they help plants grow. What if they were all gone? Let's imagine it with Olly!

Fungi are a whole kingdom of living things, with about 144,000 known species, from tiny yeasts to big mushrooms. They puff up our bread, ripen some cheeses, rot dead wood and team up with plant roots. Imagine a world with no fungi at all and follow what might change.

Fungi are heterotrophs that digest food outside their bodies with enzymes, and they are more closely related to animals than to plants. This thought experiment removes the whole kingdom and follows three threads: the foods and medicines people get from fungi, the recycling of dead wood, and the root partnerships that nearly 90 percent of vascular plant species rely on.

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The big question

A mushroom on a log is a fungus. The yeast that puffs up bread is a fungus too.

What if there were no fungi anywhere? Let's think about it together.

When you think of fungi, you might picture mushrooms. But fungi also include molds, mildews and yeasts, and most of a fungus is hidden: a web of tiny threads spreading through soil, wood or food.

What if every fungus on Earth disappeared? In this thought experiment you can follow three paths: the bread and cheese in your kitchen, the dead logs on the forest floor, and the hidden partners that live on plant roots.

Fungi form their own kingdom, with about 144,000 known species according to Britannica, including yeasts, rusts, smuts, mildews, molds and mushrooms. Their bodies are mostly mycelia, networks of thread-like hyphae that grow at their tips.

This thought experiment deletes the entire kingdom and asks what would go missing. It follows three documented roles of fungi: in human food and medicine, as decomposers of dead wood, and as mycorrhizal partners of plant roots. The outcomes for the imagined world are ideas, built from what each role does today.

Explore the idea

Make a prediction

Which of these comes from a fungus?

Which of these do we get thanks to a fungus?

Which of these depends on a fungus?

The gas bubbles that make bread rise
The salt in seawater
The colors of a rainbow

Established fact
Fungi break down dead things and help plant roots.

Fungi recycle nutrients from dead matter, and nearly 90 percent of plant species with roots and stems have fungal partners (OpenStax).

Fungi release nitrogen, phosphorus and trace elements from decaying matter, and nearly 90 percent of vascular plant species form mycorrhizae (OpenStax).
An idea scientists discuss
The world without fungi is pretend. We imagine what might happen.

The fungus-free world in this experiment is imagined. What might happen in it is an idea built from what fungi really do today.

The imagined world combines effects documented one at a time; real ecosystems have many decomposers and partners, so its outcomes might differ greatly from any simple guess.
Established fact
Mushrooms, molds and yeasts are all fungi.

Fungi are a kingdom of their own, with about 144,000 known species, including yeasts, molds, mildews and mushrooms.

Britannica counts about 144,000 known species in the kingdom Fungi, including yeasts, rusts, smuts, mildews, molds and mushrooms.
Established fact
Fungi cannot make food from sunlight like plants do.

Fungi can't make food from sunlight. Like animals, they get their food from other living or once-living things, and they are more closely related to animals than to plants.

Fungi are heterotrophs that cannot photosynthesize, and molecular analysis shows they are more closely related to animals than to plants (OpenStax).
Established fact
Yeast makes gas bubbles that puff up bread dough.

Yeast feeds on sugar and makes alcohol and carbon dioxide. In baking, the carbon dioxide makes the dough rise, and the alcohol is driven off in the oven.

Yeasts ferment sugars to ethanol and carbon dioxide; bakers want the gas, the ethanol is driven off during baking, and the strains used for bread, beer and wine are selected strains of Saccharomyces cerevisiae (Britannica).
Established fact
Some cheeses get their blue lines from a mold.

Molds of the genus Penicillium ripen many cheeses. In Roquefort, France, sheep milk cheeses are stacked in caves to pick up the molds that make their blue veins.

Penicillium molds ripen many cheeses; in the caves of Roquefort, France, wheels of sheep milk cheese are stacked to capture the molds responsible for the blue veins and pungent taste (OpenStax).
Established fact
Long ago, a scientist saw a mold stop germs from growing.

In 1928 Alexander Fleming saw that a green mold had stopped bacteria growing on a dish. The mold made a substance he named penicillin.

In 1928 Fleming observed that Staphylococcus aureus failed to grow where a culture was contaminated by Penicillium notatum; Chain and Florey purified penicillin in the late 1930s, and it was in therapeutic use by 1941 (Britannica).
An idea scientists discuss
Without fungi, bread might be flat and blue cheese might not exist.

In a world without fungi, bakers might need other ways to make bread rise, and people might have had to find medicines like penicillin some other way.

Without fungi, leavening might rely only on non-fungal methods, and the history of antibiotics might have taken a very different path; how different is unknowable.
Established fact
Some fungi can eat the tough glue that holds wood together.

Lignin is like a glue that holds wood cells together and makes them stiff. White rot fungi break down lignin as well as the rest of the wood, which leaves the wood looking bleached.

White rot fungi attack cellulose and lignin, the 'glue' that binds wood cells and stiffens their walls, giving a bleached look; brown rot fungi attack the cellulose and modify the lignin, leaving brown wood that cracks into cubes (Penn State Extension).
Established fact
When fungi break down dead things, they make food for new plants.

Living things need nitrogen and phosphorus, but these are hard to find. When fungi break down dead matter, they set these elements free for other living things to use.

Fungi release nitrogen and phosphorus from decaying matter, and trace elements would remain tied up in rotting organic matter if fungi and bacteria did not return them to the environment (OpenStax).
An idea scientists discuss
Without fungi, fallen logs might stay on the ground for a long, long time.

Penn State Extension asks us to imagine how forests would look with no fungi to rot dead trees. Logs might pile up, and the nutrients locked inside them might reach new plants much more slowly.

With no fungi, dead wood might accumulate and nutrient return might slow; bacteria would still decompose some material, so how much slower is uncertain.
Established fact
Most plants have fungus helpers on their roots.

Nearly 90 percent of all vascular plant species, the plants with roots, stems and tubes for water, have fungal partners on their roots.

Nearly 90 percent of all vascular plant species have mycorrhizal partners; the hyphae's large surface area channels water and minerals from the soil into the plant (OpenStax).
Established fact
Some orchid seeds need a fungus to start growing.

Some orchids need a fungus right from the start: their seeds will not sprout without a fungal partner.

Orchids rely on a distinct type of mycorrhiza; their seeds will not germinate without a mycorrhizal partner, usually a basidiomycete (OpenStax).
Established fact
Fungus helpers matter a lot for trees and grasses.

The biology textbook OpenStax says that without their fungal partners, 80 to 90 percent of trees and grasses would not survive.

OpenStax: the mycorrhizal relationship is essential for the productivity of farmland, and without the fungal partner 80–90 percent of trees and grasses would not survive.
An idea scientists discuss
Without fungi, many plants might grow less well, and some might not grow at all.

In a world without fungi, many plants might struggle to get water and minerals, and the animals that eat those plants might feel the change too.

Losing mycorrhizae might change which plants could grow almost everywhere, with effects passing up food webs; such knock-on effects are ideas, not predictions.

In our world

Fungi are not plants. They cannot make food from sunlight.

Instead, they grow tiny threads into their food and soak it up. Some live in soil, some on logs, and some on bread!

Fungi are not plants. They can't make their own food from sunlight the way plants do. Instead, a fungus grows tiny threads called hyphae into its food. The threads let out special chemicals that break the food down outside the fungus, and then the threads soak it up.

Scientists who compare fungus genes with those of other living things have found that fungi are more closely related to animals than to plants. Their cell walls even contain chitin, the same tough material found in insect shells.

Fungi cannot photosynthesize. Like animals, they are heterotrophs: they take carbon and energy from complex organic compounds. A fungus secretes exoenzymes from its hyphae that break large molecules into small ones outside the cell, then carries them in through protein carriers in its membrane (OpenStax).

Molecular analysis shows fungi are more closely related to animals than to plants. Their cell walls are built from chitin and glucans; chitin is also found in the exoskeletons of arthropods. A mycelium can be huge: OpenStax describes an Armillaria solidipes in eastern Oregon spreading under more than 2,000 acres of soil, estimated to be at least 2,400 years old.

Imagine a world without fungi

In our pretend world, Olly takes away every fungus.

Bread might stay flat. Old logs might stay on the ground for a long time. Many plants might grow less well.

Now imagine that every fungus vanished. Bakers would lose the yeast that puffs up bread, and blue cheeses would lose the molds that ripen them. Penicillin, one of the first antibiotics, came from a mold, so it would never have been found that way.

In forests, dead trees might rot much more slowly. And because nearly 90 percent of plant species with roots and stems have fungal partners, many plants might struggle to get enough water and minerals. These are ideas, but each one follows from something fungi really do today.

Remove the kingdom and several things might follow. Yeast leavening and Penicillium-ripened cheeses would be gone, and penicillin would never have been isolated from a mold. Fungi break down organic matter that would otherwise not be recycled (OpenStax), so nitrogen, phosphorus and trace elements might stay locked in dead wood far longer, though bacteria, which OpenStax also names as major decomposers, would still be at work.

Nearly 90 percent of vascular plant species have mycorrhizal partners, and OpenStax states that without them 80 to 90 percent of trees and grasses would not survive. The imagined world might also lose some harms: fungi cause crop rusts and smuts and diseases such as athlete's foot. Which effects would dominate is an open question, not a model result.

Why fungi matter so much

Fungi are great at breaking things down. They turn old leaves and wood back into food for the soil.

Some fungi hug plant roots and bring them water. The plants share their food back.

Fungi are some of nature's best recyclers. Living things need elements such as nitrogen and phosphorus, but these are hard to find in the environment. When fungi break down dead leaves and wood, they set these elements free so new plants can use them.

Fungi also make deals with plants. Their threads spread widely through the soil, soaking up water and minerals and passing them into the roots. In return, the plant shares some of the food it makes from sunlight. This partnership is called a mycorrhiza.

Fungal nutrition is extracellular: digestion precedes ingestion. That lets fungi degrade large, insoluble molecules, releasing elements such as nitrogen and phosphorus, which organisms need in large quantities but which are scarce in the environment. Trace elements would remain tied up in rotting matter if fungi and bacteria did not return them (OpenStax).

Wood is a hard case. Penn State Extension describes lignin as the 'glue' that holds wood cells together and stiffens their walls. White rot fungi attack both cellulose and lignin, leaving bleached wood; brown rot fungi attack the cellulose and modify the lignin, leaving brown, cube-cracked wood.

In mycorrhizae, the large surface area of the hyphae channels water and minerals from the soil into the plant. In ectomycorrhizae the fungus sheathes the root; in arbuscular mycorrhizae it branches inside root cells.

Fungi at work today

Bakers use yeast every day to make bread rise.

Some cheeses get their blue lines from a mold. And forest fungi are busy turning old logs into soil.

Yeast is a fungus made of just one cell. It eats sugar in bread dough and gives off bubbles of carbon dioxide gas, which make the dough rise. A little alcohol is made too, but baking drives it off. Molds of the genus Penicillium ripen many cheeses, such as the blue cheese aged in caves at Roquefort, France.

In 1928 Alexander Fleming noticed that a green mold, later identified as Penicillium notatum, had stopped bacteria growing on a dish. By 1941 Howard Florey and Ernst Chain had turned its penicillin into a medicine. In forests, Penn State Extension calls wood-decaying fungi an important part of the nutrient cycle.

Bread, beer and wine use selected strains of Saccharomyces cerevisiae, which ferment sugars into ethanol and carbon dioxide; bakers want the gas, and the ethanol is driven off in the oven (Britannica). In Roquefort, France, wheels of sheep milk cheese are stacked in caves to capture the Penicillium molds that make the blue veins (OpenStax).

In 1928 Fleming saw that Staphylococcus aureus failed to grow where a culture had been contaminated by Penicillium notatum. Chain and Florey isolated and purified penicillin in the late 1930s, making it available for therapeutic use by 1941, and the three shared the 1945 Nobel Prize (Britannica). Fungi also yield the cephalosporins and the transplant drug cyclosporine (OpenStax).

OpenStax adds a caution: mycorrhizal inoculants are sold for gardens, but there is little evidence of their effectiveness.

Amazing facts

One fungus in Oregon is bigger than lots and lots of soccer fields.

A honey mushroom fungus in eastern Oregon spreads under more than 2,000 acres of soil and is thought to be at least 2,400 years old.

OpenStax describes an Armillaria solidipes (honey mushroom) in eastern Oregon spreading across more than 2,000 acres of underground soil, estimated to be at least 2,400 years old and considered the largest organism on Earth.
Some ants grow fungus gardens for food.

Leaf-cutter ants in Central and South America farm fungi. They carry leaves home, and the fungus digests the tough parts of the leaves that the ants can't.

Leaf-cutter ants of Central and South America cultivate fungi in gardens; the fungi digest the cellulose in the leaves, which the ants cannot break down (OpenStax).
Fungi are closer cousins to animals than to plants.

Fungi are more closely related to animals than to plants, and their cell walls contain chitin, the same tough stuff found in insect shells.

Molecular evidence places fungi closer to animals than to plants; their cell walls contain chitin, also found in the exoskeletons of arthropods (OpenStax).
One yeast cell can eat a lot of sugar.

A single yeast cell can ferment about its own weight of sugar in an hour.

Britannica notes that one yeast cell can ferment approximately its own weight of glucose per hour.

Words to know

fungus (plural fungi): a living thing, such as a mushroom, mold or yeast, that cannot make its own food and absorbs food from what it grows on
hyphae: the tiny threads that make up the body of most fungi
yeast: a fungus made of a single cell; some kinds are used to make bread rise
decomposer: a living thing that breaks down dead plants and animals and returns their nutrients to the environment
lignin: a tough material that holds wood cells together and makes them stiff
mycorrhiza: a partnership between a fungus and a plant's roots in which each helps the other

Discovery challenge

Yeast makes gas bubbles that puff up bread. Which gas is it?

Which gas, made by yeast, makes bread dough rise?

During baking, which product of yeast fermentation leavens the dough?
- Carbon dioxide
- Oxygen
- Helium
Show the answer

Carbon dioxide

Yeast eats sugar and makes carbon dioxide bubbles.

Yeast feeds on sugar and gives off carbon dioxide gas, which puffs up the dough.

Carbon dioxide leavens the dough; the ethanol also produced is driven off in the oven (Britannica).
What do fungus helpers bring to plant roots?

What do mycorrhizal fungi do for plant roots?

In a mycorrhiza, what does the fungal partner supply to the plant?
- Water and minerals from the soil
- Sunlight for the leaves
- Seeds for new plants
Show the answer

Water and minerals from the soil

Fungus threads bring water and minerals to the roots.

The fungus threads reach far into the soil and pass water and minerals to the plant.

The hyphae's large surface area channels water and minerals from the soil into the plant (OpenStax).
Who saw a mold stop tiny germs from growing, in 1928?

In 1928, who noticed that a mold stopped bacteria from growing?

Who first observed, in 1928, that Penicillium notatum inhibited bacterial growth?
- Alexander Fleming
- Isaac Newton
- Marie Curie
Show the answer

Alexander Fleming

Alexander Fleming saw a mold stop germs growing on a dish.

Alexander Fleming saw a green mold stop bacteria growing, and he named the substance it made penicillin.

Alexander Fleming; Chain and Florey later purified penicillin, and it was in therapeutic use by 1941 (Britannica).
Some fungi can eat lignin. What is lignin?

White rot fungi can break down lignin. What is lignin?

White rot fungi degrade lignin. What role does lignin play in wood?
- A tough glue that holds wood cells together
- A sugar found in fruit
- The green color in leaves
Show the answer

A tough glue that holds wood cells together

Lignin is like glue that holds wood together.

Lignin is like a glue that holds wood cells together and makes them stiff.

Lignin binds wood cells together and gives their walls rigidity (Penn State Extension).

Experiments to try

Explore next

For parents & teachers

What children will learn

Fungi are a separate kingdom from plants and animals; they cannot photosynthesize and absorb food they digest outside their bodies.
Fungi are major decomposers that return nutrients such as nitrogen and phosphorus to the environment.
Most plant species rely on mycorrhizal fungi to take up water and minerals.
People use fungi for food (bread, cheese) and medicine (penicillin).

Recommended ages: 7 to 17 · About 15 minutes

Talk about it

Which foods in your kitchen might be different in a world without fungi?
Fungi also cause some plant and human diseases. Would a world without fungi be better or worse overall? Why is that hard to answer?
Why might a forest look different if nothing rotted the fallen trees?

Try at home or in class

With a grown-up, make a yeast balloon: put a packet of dried baker's yeast, a spoonful of sugar and some warm (not hot) tap water in a clean plastic bottle, stretch a balloon over the neck and check it every ten minutes. The balloon fills with carbon dioxide from the yeast.
On a walk with a grown-up, look for mushrooms or shelf-like fungi on logs and tree trunks. Look but don't touch, never pick or eat wild mushrooms, and wash your hands after the walk.

Key vocabulary: fungus (plural fungi), hyphae, yeast, decomposer, lignin, mycorrhiza

Where these facts come from

Fungus , Encyclopaedia Britannica
Yeast , Encyclopaedia Britannica
Penicillin , Encyclopaedia Britannica
Biology 2e, 24.1 Characteristics of Fungi , OpenStax
Biology 2e, 24.3 Ecology of Fungi (mycorrhizae) , OpenStax, Rice University
Biology 2e, 24.5 Importance of Fungi in Human Life , OpenStax
Brown Rot in Wood , Penn State Extension
Introduction to the Fungi , University of California Museum of Paleontology

Checked, not yet human-reviewed. Last updated October 26, 2026. Spotted a mistake? Tell us.