Fungi, together with bacteria, are the major decomposers of organic matter on Earth. Most fungi are saprobes; that is, they digest nonliving organic matter, such as wood, fallen leaves, and dead animals. However, some fungi are parasites that attack living things and cause disease. Fungi cause many agricultural diseases, as well as several human diseases.
A unique physical structure and the method by which they obtain nutrients distinguish fungi from the other kingdoms. Fungi secrete enzymes into the environment and break down organic matter, and then absorb the small nutrient particles through their cell membranes. This process is called extracellular digestion.
Except for unicellular yeast, fungi are multicellular organisms, and the body of a fungus consists primarily of cells joined in filaments. Each microscopic filament of a fungus is a hypha (the plural is hyphae). Hyphae may form a huge tangled interwoven network called a mycelium (the plural is mycelia). A mycelium is a visible structure. Fungi cells are unique because they possess a polysaccharide called chitin. In some cases, the cell walls also contain cellulose. Fungi live in environments that are generally acidic, and they prefer carbohydrate-rich foods.
Reproduction in fungi can be sexual or asexual. In some cases, filaments break from the main mycelium and grow into new individuals. Alternately, a fungus may produce spores by an asexual or sexual process. The spores disperse, germinate, divide, and produce genetically identical fungi. The hyphae and spores are usually haploid (containing one set of chromosomes), and the haploid phase dominates the life cycle of a fungus. Spores can withstand extreme dryness and cold to produce a new fungus when conditions permit.
Fungi can also reproduce by a sexual process, which results in a short-lived diploid cell (with two sets of chromosomes) that soon produces haploid cells through meiosis. The spores develop into cells that divide by mitosis to form a new hypha, and then a new mycelium.
In the most accepted classification scheme, fungi are placed in four major divisions: Zygomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes.