How does a fungus eat without a mouth?

A fungus eats by digesting its food outside its body, then soaking up the result. It does not swallow its food. Instead, it releases enzymes straight onto its food, and the enzymes cut that food into small pieces the fungus can absorb. Biologists call this external digestion. A saprotroph is an organism that feeds on dead organic matter or waste this way, by excreting chemicals to digest it rather than eating it directly.

The work happens through the mycelium, the fungus's web of hyphae. First the hyphae secrete enzymes onto or into the food, which break large molecules into smaller units. Then those small units are absorbed into the mycelium. Different enzymes handle different foods: proteases break proteins into amino acids, lipases break fats into fatty acids and glycerol, amylases break starch into simple sugars, and cellulose, the main material of plant cells, ends up as glucose.

The shape of a fungus suits this. Hyphae grow at their tips, and each one is only about 4 to 6 micrometres across, so a mycelium can thread through a log or a heap of leaves and touch food everywhere. Fungi are also very wet: 80 to 90 percent of their mass is water, and they need excess water to absorb their food.

You can see it yourself. Turn a backyard compost pile and you will commonly expose visible networks of mycelia growing through the decaying material. Mycelia are vital in land and water ecosystems for decomposing plant material, quietly turning dead leaves back into nutrients.