Fungi are defined as eukaryotic organisms with chitinous cell walls and no chloroplasts; they are heterotrophs, usually deriving their carbon from dead matter (as a decomposer) or parasitically from other living things. Fungi also require a source of moisture, be it humidity in the air, water from the cells they attack, or the sweat that comes out of the skin on your feet. The fungal life cycle is somewhat complex; all true fungi go through a haploid (1n) stage, a diploid (2n) stage, and a dikaryotic (n+n) stage. (In some fungi, the dikaryotic stage is reduced; in some, it is largely extended.)

Originally, the fungi were grouped with Kingdom Plantae. Recently, it has been shown that if anything, fungi are more closely related to animals than plants.

Kingdom Fungi is broken into 4 major phyla: Deuteromycota, Basidiomycota, Ascomycota, and Zygomycota. Chytridiomycota are sometimes included in this kingdom as a 5th phylum, and some molecular biologist group the Deuteromycota with their closest relatives in the other phyla rather than separating them into their own. According to current accepted theories, slime molds and water molds (Oomycota, Myxomycota, and Acrasiomycota are not phylogenetically related to the fungi, and therefore, they will not be covered here).

These "fungi imperfecti" generally do not exhibit a sexual reproductive function. If they are found to do so, they are then grouped with their proper phylum.

The "club fungi" include mushrooms, smuts, puffballs, shelf fungi, and many others. Beginning with the haploid spore, the life cycle starts by the spore germinating into a hypha, or long filament. The hypha divides and grows, and when it meets with another hypha, some of the cells in the hyphae will fuse by plasmogamy. The dikaryotic cells will continue to form hyphae, and at the tip, they will form basidiocarps, or fruiting bodies. Certain cells in the fruiting bodies can have their nuclei fuse (karyogamy) to make diploid cells, which can then go through meiosis to make 4 basidiospores. The spores are released, and the cycle repeats.

These "cup fungi" include the common molds and mildews that spoil food and attack plants, yeasts, truffles, and morels. An ascomycete Claviceps purpurea is also the cause of ergot, which is the source of lysergic acid diethylamide, everyone's favourite LSD. The life cycle of the ascomycete begins with the release of the haploid spore. The spore then germinates, forming a hypha. Several hyphae often come together, forming a mycelium. In this mycelium, many hyphae branch through asexual reproduction, but some of the hyphae may go through plasmogamy to become dikaryotic. The dikarya can then go through karyogamy to form a zygote, which then goes through meiosis to form between 2 and 8 ascospores encased in an ascus. The spores are released, and the cycle starts over.
In the single-celled ascomycetes, such as the yeasts, the cycle is the same, save the formation of hyphae.

The zygomycetes include the common black bread mold Rhizopus. The life cycle of zygomycetes is simpler than the other fungi. The spore, once liberated, germinates and forms a mycelium of hyphae. Some of the hyphae can grow erect and form sporangia filled with haploid spores. This is its more common asexual reproductive stage. If conditions are adverse, and compatible opposite mating types are near each other, the hyphae will form gametangia. The gametangia fuse, forming a zygote, which immediate goes through meiosis to form four zygospores.

The chytrids are primitive, single-celled, colonial, or mycelial fungi that appear to reproduce asexually most of the time, only reproducing sexually in times of dire need.

Diseases caused by fungi

Some of the more well-known diseases caused by fungal infections include:

  • Ringworm -- actually several different types of infections of the hair, scalp, skin, beard, or nails -- usually caused by Trichophyton spp.
  • jock itch (tinea cruris) -- caused by Epidermophyton or Trichophyton spp.
  • athlete's foot (tinea pedis) -- caused by Trichophyton or Epidermophyton spp.
  • histoplasmosis -- affects the phagocytes, which are white blood cells -- caused by Histoplasma spp.
  • candidiasis -- opportunistic; affects the mouth (thrush), the vagina (yeast infection), or the skin on the butt (diaper rash) most often -- caused by Candida albicans
  • pneumocystis pneumonia -- while most pneumonia is bacterial or mycoplasmic, this is the only fungal pneumonia -- caused by Pneumocystis carinii

Fungi in Biochemistry and Molecular Biology

Several fungi are important as models for genetic and biochemical phenomena, including: