What is a Fungus? The Intuition
Think of a mushroom you see in the forest, the mould on a forgotten slice of bread, or the yeast that makes dough rise. These are all fungi. But what makes them a kingdom of their own, separate from plants and animals?
The first and most important thing to grasp is this: fungi are not plants. They do not have chlorophyll, so they cannot make their own food by photosynthesis. They are heterotrophs — they must obtain organic carbon from other sources. But unlike animals, which ingest food and digest it inside their bodies, fungi do something completely different: they digest food outside their bodies and then absorb the nutrients.
Imagine throwing digestive juices onto your dinner, waiting for it to break down into a soup, and then drinking it through a straw. That is exactly how a fungus feeds. This mode of nutrition is called absorptive heterotrophy.
The Fungal Body: Hyphae and Mycelium
Most fungi are multicellular (yeasts are the major unicellular exception). But a fungus does not build tissues like a plant or an animal. Instead, its body is made of long, branching, thread-like structures called hyphae (singular: hypha).
A single hypha is a tiny tube, often just one cell wide. It is filled with cytoplasm and nuclei, and it grows at its tip, pushing forward into new food sources.
The entire network of hyphae that makes up the body of a fungus is called the mycelium. You have seen mycelium before — it is the white, fuzzy, cobweb-like growth you find under a rotting log or on old bread. The mycelium is the main body of the fungus; the mushroom you see above ground is just the reproductive structure, like the fruit of a plant.
The Cell Wall: A Key Distinction
Every fungal cell is surrounded by a cell wall. But here is the critical difference from plants:
- Plants have cell walls made of cellulose.
- Fungi have cell walls made of chitin.
Chitin is the same tough, flexible material that makes up the exoskeleton of insects and crustaceans. This is a fundamental biochemical difference that places fungi closer to animals than to plants in evolutionary terms.
Reproduction: The Power of Spores
Fungi reproduce primarily through spores. A spore is a tiny, often single-celled reproductive unit that can develop into a new fungus without needing to fuse with another cell (though sexual fusion also occurs).
Spores are produced in enormous numbers and can be dispersed by wind, water, or animals. They are incredibly resilient — some can survive extreme heat, cold, or drought for years, waiting for the right conditions to germinate.
When you see a puffball mushroom release a cloud of "smoke" when stepped on, that smoke is millions of spores being launched into the air. …