Fungi: Hyphae, Spores and Digestion Outside the Body

Fungi are eukaryotes, which means their cells have a nucleus, and they have lived on land for more than 450 million years. They cannot make their own food. They have no chlorophyll and no chloroplasts, so they feed on dead and decaying matter instead. An organism that feeds this way is called a saprobe.

A fungus digests its food outside its body. It releases enzymes into its surroundings, and those enzymes break down whatever the fungus is growing on. The small molecules that result are then absorbed back through the fungal surface. This is why fungi are built as huge networks of thin threads called hyphae. More thread surface means more places to release enzymes and absorb food.

Because of this feeding style, fungi are the great decomposers of the living world. They break down dead plants and animals and release essential elements, like carbon and nitrogen, back into the environment so other organisms can use them again. Without fungi, dead matter would pile up and nutrients would stay locked inside it.

The same machinery that makes fungi good recyclers can also make them dangerous. Some fungi release their enzymes into living plants or animals, digesting living tissue and causing disease. Whether a fungus is a helpful partner, a recycler, or a pathogen depends on what it is digesting and how its host responds.

Here are some useful words for talking about fungi.

Hyphae are the thin, branching threads that make up the body of most fungi. Together, a mass of hyphae is called a mycelium.

A spore is a tiny reproductive cell that a fungus releases to spread to new places. Spores are light enough to travel on wind or water.

A mycorrhiza is a partnership between a fungus and the roots of a plant. The fungus helps the plant absorb water and minerals from the soil, and the plant gives the fungus sugars it made by photosynthesis. In an arbuscular mycorrhiza, the fungal hyphae actually enter the root cells and form branching networks inside them, pressing against the cell membrane without breaking through it.

An ascocarp is the fruiting body of the sac fungi, the phylum Ascomycota, which store their spores in a little sac called an ascus. A basidiocarp is the fruiting body that pokes out of the ground in the club fungi, the phylum Basidiomycota. A typical mushroom is a basidiocarp. These structures exist only to make and release spores; the real body of the fungus stays hidden in the soil or wood as a mycelium.

When you picture a fungus, you probably picture a mushroom. But the mushroom is only the reproductive structure, the part the fungus pushes above ground to release its spores. Most of the fungus is out of sight, spread through the soil or inside a log as a web of microscopic threads.

Fungi come in many forms. Some produce the familiar mushroom. Some, like the coral fungi, grow brightly colored fruiting bodies that look like tiny undersea reefs. Others, like Aspergillus, are molds found mostly in soil and on plants, visible only under a microscope as tufts of spore-bearing stalks. Some molds are useful to us, giving us medicines like penicillin, while others are toxic or cause disease.

What unites all of them is how they live. A fungus is a surface. Its threadlike body is built to touch as much of its food source as possible, because it feeds by releasing digestive enzymes outward and soaking up what dissolves. Keep that one idea in mind, and the rest of fungal biology starts to make sense: why they rot our food, why they recycle nutrients in every forest, why they partner with plant roots, and why they can also be stubborn pathogens.

We spend a lot of time looking at what is inside a cell. But what is on the outside? That depends on the kind of cell.

Plants and fungi both wrap their cells in a tough cell wall that gives protection and support. The walls are made of different materials, though. Plant cell walls are built mostly from cellulose, while fungal cell walls are built from chitin, the same sturdy material found in insect shells.

Animal cells take a different approach. They have no wall at all. Instead, they secrete materials into their surroundings to form a meshwork of large molecules called the extracellular matrix, which holds cells together and helps them communicate.

The fungal cell wall matters for medicine. To kill a fungus living in a human body, a drug has to attack something the fungus has that our cells do not. Bacteria are easy targets because their cells work very differently from ours. But fungi are eukaryotes, just like us, so their cells share much of our machinery. Drugs that target the chitin wall or the fungal membrane can be selective, but there are far fewer safe targets than for bacteria. That is why antifungal drugs are harder to design and often have more side effects.