Fungi - Definition, Examples, Characteristics (2024)

Fungi (singular: fungus) are one of the kingdoms of life in biology, along with animals, plants, protists, bacteria, and archaebacteria. Examples of fungi include yeast, mushrooms, toadstools (poisonous mushrooms), and molds. The scientific study of fungi is called mycology. Around 150,000 species of fungi are known, but there may be between 2.2 and 3.8 million species worldwide.

Fungi Definition

Fungi are eukaryotic organisms that have cell walls containing chitin. They absorb nutrients from their surroundings and reproduce via spores. Fungi live in a wide variety of habitats. They include both unicellular and multicellular organisms.

Examples of Fungi

Fungi include yeasts, mushrooms, and molds. Here are some specific examples:

• Saccharomyces cerevisiae (Baker’s yeast): A unicellular fungus used in baking and brewing.
• Aspergillus niger: A filamentous fungus that produces citric acid and enzymes.
• Penicillium chrysogenum: The source of the antibiotic penicillin.
• Amanita muscaria (Fly agaric): A toadstool or poisonous mushroom species with a distinctive red cap with white spots.
• Rhizopus stolonifer (Black bread mold): A common saprophytic mold.

Distinguishing Characteristics

Fungi have characteristics that distinguish them from other life forms:

1. Eukaryotic Cells: Fungi have a true nucleus enclosed in a membrane and membrane-bound organelles like mitochondria.
2. Cell Wall Composition: Their cell walls contain chitin. In contrast, animals and protists lack cell walls. Plant cell walls contain cellulose, which bacterial cell walls contain peptidoglycan.
3. Heterotrophic Nutrition: Fungi are heterotrophs that absorb nutrients through extracellular digestion. This is one of the characteristics that distinguish fungi from plants, which are autotrophs.
4. Absorptive Mode of Nutrition: They secrete digestive enzymes into the environment that break down complex organic materials so that they absorb the simpler products.
5. Growth Form: Fungi are unicellular (yeasts) or multicellular (molds and mushrooms). Multicellular fungi consist of hyphae, which form a mycelium.
6. Reproduction: Fungi reproduce through spores, which they produce either sexually or asexually.

Key Differences Between Fungi and Plants

The main characteristics that distinguish fungi from plants are their cell wall chemical composition and their mode of nutrition:

• Fungal cell walls contain chitin, while plant cell walls contain cellulose.
• Fungi are heterotrophs that break down and absorb nutrients around them. In contrast, plants are autotrophs that produce their own food using photosynthesis.
• Plant cells contain chloroplasts and chlorophyll, which is a green pigment that plays a key role in photosynthesis. Some fungi are green (e.g., green molds), but fungal cells do not contain chloroplasts or chlorophyll.

Lichens sort of bridge the gap between fungi and plants. A lichen is a symbiotic relationship between fungi and either photosynthetic algae or cyanobacteria.

Structure of a Multicellular Fungus

A typical multicellular fungus like a mold or mushroom consists of:

• Hyphae: Tubular, thread-like structures that form the basic unit of a fungus.
• Mycelium: A network of hyphae that forms the vegetative part of the fungus. It is responsible for nutrient absorption.
• Fruiting Body: The reproductive structure, often visible above ground (e.g., a mushroom cap). It contains spore-producing structures like basidia or asci.
• Septum: Cross-walls that divide hyphae into cells. Septa sometimes have pores that allow cytoplasmic flow.
• Spores: Reproductive cells that develop into a new organism.

Classifying Fungi

The two main methods of classifying fungi are by their mode of nutrition and by the way they form spores

Classification by Mode of Nutrition

Fungi either decompose dead or decaying matter, obtain nutrients as parasites, or else obtain nutrients via a mutualistic symbiotic relationship:

• Saprophytic Fungi
  • Description: Obtain nutrients by decomposing dead organic matter.
  • Examples:
    • Rhizopus stolonifer (black bread mold)
    • Aspergillus niger
• Parasitic Fungi
  • Description: Extract nutrients from living hosts, often causing disease.
  • Examples:
    • Candida albicans (causes thrush and yeast infections)
    • Puccinia graminis (causes wheat rust)
• Mutualistic (Symbiotic) Fungi
  • Description: Form beneficial relationships with other organisms.
  • Examples:
    • Mycorrhizal fungi (form mutualistic relationships with plant roots)
    • Lichens (a symbiotic association between fungi and algae)

This classification method is not clear-cut because some fungi are opportunistic parasites that live on their own when conditions are suitable.

Classification by Spore Formation

Grouping of the major phyla of fungi is according to the reproductive structures and means of spore formation. Here are some example phyla:

1. Phylum Ascomycota (Sac Fungi)
   • Characteristics: Spores (ascospores) are produced in sac-like structures called asci.
   • Examples:
     • Penicillium species
     • Saccharomyces cerevisiae (yeast)
2. Phylum Basidiomycota (Club Fungi)
   • Characteristics: Spores (basidiospores) are borne on club-shaped structures called basidia.
   • Examples:
     • Mushrooms (e.g., Agaricus bisporus)
     • Puffballs
     • Shelf fungi
3. Phylum Zygomycota (Zygote Fungi)
   • Characteristics: Produce thick-walled resting spores called zygospores.
   • Examples:
     • Rhizopus stolonifer (black bread mold)
     • Mucor species
4. Phylum Chytridiomycota (Chytrids)
   • Characteristics: Produce motile spores with flagella called zoospores.
   • Examples:
     • Batrachochytrium dendrobatidis (causes chytridiomycosis in amphibians)
5. Phylum Glomeromycota
   • Characteristics: Form arbuscular mycorrhizae with plants.
   • Examples:
     • Glomus species

Other phyla are Opisthosporidia, Neocallimastigomycota, Blastocladiomycota, Zoopagomycotina, Mucoromycota. Note, however, the taxonomy changes as new molecular and genetic data becomes available.

Reproduction

Fungi reproduce both sexually and asexually. Here are the various methods:

Asexual Reproduction

Asexual reproduction results in offspring that are genetically identical to the parent.

• Sporangiospores: Spores form inside sac-like sporangia (e.g., Rhizopus stolonifer).
• Conidiospores: Spores form externally on special hyphae called conidiophores (e.g., Aspergillus).
• Blastoconidia (Budding): Buds develop from parent cells (e.g., yeasts like Saccharomyces cerevisiae).
• Chlamydospores: Cells form thick-walled resting spores (e.g., Candida albicans).

Sexual Reproduction

Sexual reproduction in fungi combines genetic material from two individuals, resulting in greater genetic diversity. The process generally involves three stages:

1. Plasmogamy: The cytoplasm of two haploid cell fuse and form a dikaryon, without the fusion of nuclei.
2. Karyogamy: Nuclei fuse and form a diploid zygote.
3. Meiosis: Cells divide and form haploid spores.
4. Formation of Sexual Spores:
   • Ascospores: Form in asci (e.g., Penicillium).
   • Basidiospores: Form in basidia (e.g., Agaricus bisporus).
   • Zygospores: Form by the fusion of specialized cells called gametangia (e.g., Rhizopus stolonifer).

Beneficial Uses of Fungi

Fungi are important as food and medicine, plus they have other uses:

• Food Production:
  • Yeasts (Saccharomyces cerevisiae) are essential in bread, beer, and wine production.
  • Mushrooms like Agaricus bisporus are food.
• Medicine:
  • Production of antibiotics like penicillin (from Penicillium chrysogenum).
  • Immunosuppressants like cyclosporine (from Tolypocladium inflatum).
• Biotechnology:
  • Production of enzymes like amylases and lipases.
  • Genetically engineered yeasts for biofuel production.
• Agriculture:
  • Mycorrhizal fungi enhance nutrient uptake in plants.
  • Biological control agents (e.g., Trichoderma species).

Harmful Effects of Fungi

While fungi are useful, they are also important pathogens. Also, some produce potent toxins.

• Pathogens in Humans and Animals:
  • Candidiasis: Caused by Candida albicans.
  • Aspergillosis: Caused by Aspergillus species.
  • Ringworm: Caused by dermatophytes.
• Plant Pathogens:
  • Rusts and Smuts: (e.g., Puccinia and Ustilago species).
  • Late Blight: Caused by Phytophthora infestans.
• Food Spoilage:
  • Mold growth on stored grains, fruits, and bread (e.g., Aspergillus and Rhizopus species).
• Mycotoxins:
  • Aflatoxins: Produced by Aspergillus flavus.
  • Ergot Alkaloids: Produced by Claviceps purpurea.
• Structural Damage:
  • Wood decay by fungi like Serpula lacrymans.

References

• Alexopoulos, C.J.; Mims, C.W.; Blackwell, M. (1996). Introductory Mycology. John Wiley & Sons. ISBN 978-0-471-52229-4.
• Cavalier-Smith, T. (1998). “A revised six-kingdom system of life”. Biological Reviews. 73 (3): 203–66. doi:10.1111/j.1469-185X.1998.tb00030.x
• Deacon, J. (2005). Fungal Biology. Cambridge, Massachusetts: Blackwell Publishers. ISBN 978-1-4051-3066-0.
• Karlson-Stiber, C.; Persson, H. (2003). “Cytotoxic fungi–an overview”. Toxicon. 42 (4): 339–49. doi:10.1016/S0041-0101(03)00238-1
• Naranjo-Ortiz, M.A.; Gabaldon, T. (2019). “Fungal evolution: Diversity, taxonomy and phylogeny of the Fungi”. Biological Reviews. 94 (6): 2101–2137. doi:10.1111/brv.12550

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