The fungi are organisms that have cells with nuclei (eukaryotes) and that require other living things from which to obtain their food (they are heterotrophic). Their cells have a thick wall of a compound (polysaccharide) called chitin, which gives them strength and rigidity. Chitin is also the main constituent of the exoskeleton of arthropods. Most fungi are multicellular and their bodies are made up of microscopic tubular filaments, called hyphae, which branch and intersect. A group of hyphae is called mycelium. What we see on the surface in various forms, sometimes with a "hat" and called mushrooms, are the reproductive organs of one of the groups.
To feed, fungi first break down their food into small molecules that are then absorbed through their cell membranes. Most feed on dead organic matter (saprobes), while others are parasites and some are predators.
During sexual or asexual reproduction, fungi produce spores allowing their dispersal to new sites and helping them survive adverse conditions such as dehydration or freezing. They can also develop from any piece of mycelium, however small, although this is rare.
Fungi are essential in the recycling of nutrients in all terrestrial habitats; they help to regulate the populations of plants, animals and insects that they parasitize, and form partnerships with other organisms, such as lichens and mycorrhizae.