Fungi: An Overview

Introduction 

Fungi are heterotrophic, eukaryotic organisms which are different from plants, animals and bacteria. They’re unable to photosynthesize because they don’t have the pigment that they need. They are important decomposers in the ecosystem and are also economically important in the areas of medicine and industry. The study of fungi is known as Mycology. 

General Characteristics of Fungi

• Eukaryotic & Heterotrophic: Fungi have a well defined nucleus and membrane bound organelle. They are heterotrophs, as saprophytes or parasites.
• Cell Wall Composition: Fungal cell walls differ from plant cell walls (cellulose) in their main composition, which is chitin and glucans.
• Nutrition (Absorptive Heterotrophy): They release digestive enzymes outside the body into the environment, break down the complex organic material and absorb the nutrient. Similar to animals, fungi store energy as a glycogen reserve and oil bodies (not starches).
• Habitat: They are found in warm humid places. 

Structure of Fungi

• The body of a multicellular fungus is composed of slender thread-like filaments known as hyphae, except the unicellular yeast.
• The vegetative part of the fungus is called mycelium, which is a network, or bundle of hyphae.
• Septate vs. Coenocytic: If hyphae have cross-walls then they are called septate. They may be coenocytic if they are lacking in cross-walls and have numerous nuclei.
• Reproductive Structure: Mushrooms are the reproductive structure of fungi, which is used to produce spores. 

Classification of Fungi

The morphology, mode of spore formation and structure of the mycelium are conventional basis for the classification of fungi. There are four major classes:

• Phycomycetes (Algal Fungi): Aquatic or on rotting wood. They have aseptate and coenocytic mycelium. Rhizopus (bread mould), Albugo (parasitic fungi on mustard).
• Ascomycetes (Sac Fungi): Most are multicellular, some unicellular (yeast). The are saprophytic or parasitic, producing spores (ascospores) in a sac-like structure (ascus). Examples: Penicillium, Aspergillus, Neurospora (for biochemical research).
• Basidiomycota (Club Fungi): Fungi with a septate and branched mycelium. They make sexual spores (basidiospores) on a club-shaped structure (basidium). Examples: Agaricus (mushroom) and Ustilago (smut).
• Deuteromycetes (Fungi Imperfecti): These fungi are only known by their vegetative or asexual stages. If a sexual stage is found, they are then grouped into other classes. Examples: Trichoderma, Alternaria. 

Reproduction in Fungi

Fungi reproduce vegetative, asexually and sexually:

• Vegetative: Fragmentation, fission and budding (common in yeast).
• Asexual: Spores formed (conidia, sporangiospores, or zoospores).
• Sexual: three processes: 1) Plasmogamy (fusions of protoplasm), 2) Karyogamy (fusions of nuclei), 3) Meiosis and the formation of spores. 

Ecological and Economic Importance

• Decomposers: Fungi play an important role in nutrient cycling, decomposition of organic matter, carbon and nitrogen recycling.
• Symbiotic Associations: Mycorrhizae are symbiotic relationships between plant roots and fungi, and Lichens are symbiotic relationships between fungi and algae.
• Agriculture and Food Industry: Yeasts are employed in the production of bread, alcoholic beverages, and mushrooms are a good source of protein.
• Medicine: Antibiotics such as Penicillin are produced by fungi, as well as other drugs (e.g., cyclosporin, statins).
• Toxic Effects: Toxic fungi are responsible for diseases in man (ringworm, candidiasis) and bad spoilage of food and crops.

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