A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, protists, and bacteria.
The defining feature of all fungi is the presence of chitin in their cell walls. Chitin is a type of polysaccharide that gives fungi rigidity and structure; it also allows them to absorb nutrients from their environment. In addition to chitin, fungi also contain other types of cell wall components such as glucans and mannans.
Fungi are heterotrophic organisms; they obtain their food by breaking down organic matter externally or through absorption. This process is known as decomposition, and it is an important ecological role played by fungi in many ecosystems. Many species of fungi are capable of breaking down complex carbohydrates such as cellulose and lignin – substances that most other organisms cannot digest. As a result, fungi play an important role in recycling dead plant and animal material back into the ecosystem. Some fungi even form symbiotic relationships with plants, helping them to obtain nutrients from the soil in exchange for sugars produced by photosynthesis.
Fungi reproduce both sexually and asexually. Sexual reproduction generally involves the fusion of two haploid cells (cells with half the usual number of chromosomes) to form a diploid zygote (a cell with two sets of chromosomes). This zygote then undergoes meiosis (a type of cell division) to produce four haploid spores – each containing one set of chromosomes from each parent organism. These spores are typically dispersed by wind or water and can germinate under suitable conditions to produce new fungal individuals. Asexual reproduction usually takes place via budding or fragmentation – processes whereby small pieces break off an existing individual fungal organism and develop into new ones