What Are Fungi?

Fungi constitute a distinct kingdom of life, separate from plants, animals, and bacteria. With an estimated 2.2 to 3.8 million species [2], they represent one of the most diverse groups of organisms on Earth.

Key Points

Fungi form their own kingdom, distinct from plants and animals
They feed via absorption, not photosynthesis or ingestion
Mycelium is the true body; the visible 'mushroom' is the reproductive organ
Cell walls contain chitin, not cellulose
Evolutionarily closer to animals than to plants

Neither Plant Nor Animal

Unlike plants, fungi cannot make their own food through photosynthesis—they lack chlorophyll. Unlike animals, they cannot ingest their food. Instead, fungi secrete enzymes to break down organic matter outside their bodies, then absorb the nutrients. This unique feeding strategy is called absorptive nutrition [3].

Fungal Structure

What we commonly call a "mushroom" is only the reproductive part (the fruiting body) of a much larger organism. The main body of the fungus consists of microscopic filaments called hyphae, which together form a network called mycelium. This mycelium can extend over vast areas—some individuals cover multiple hectares [1].

Cell Walls of Chitin

A distinctive feature of fungi is that their cell walls contain chitin, the same material found in insect exoskeletons. Plants, by contrast, have cell walls made of cellulose. This biochemical feature is one of many pieces of evidence that fungi are actually more closely related to animals than to plants from an evolutionary perspective [3].

Reproduction by Spores

Fungi primarily reproduce through spores—microscopic structures that can travel through air, water, or on animals. A single fruiting body can release billions of spores. Unlike plant seeds, spores do not contain an embryo or significant nutrient reserves.

Illustrated Concepts

Mycelium

The underground network of filaments (hyphae) that makes up the main body of the fungus.

Fruiting Body

The visible reproductive structure, like this Fly Agaric mushroom.

Spores under Microscope

Spores are the reproductive units of fungi, often invisible to the naked eye.

Yeasts

Some fungi are single-celled, such as the yeasts used in baking and brewing.

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