Fungi: Diversity, Structure, and Ecological Roles

Fungi are heterotrophic eukaryotes, meaning they absorb food after digesting it outside their bodies.
Unlike plants, fungi have cell walls made of chitin, while plants have cell walls made of cellulose.
Fungi consist mainly of hyphae, which form a network known as mycelium.
Mycorrhiza: Symbiotic relationship between fungi and plant roots, critical for nutrient absorption.

There are two primary forms of hyphae:
- Septate hyphae: Contain septa or cross-walls between cells.
- Coenocytic fungi: Lack septa, resulting in a continuous cytoplasmic mass.
Haustoria: Specialized structures for nutrient absorption from host organisms.

Fungi digest food externally by secreting enzymes into the surrounding environment, then absorbing the resulting nutrients.
This process contrasts with humans, who digest food internally.

Fungi can reproduce both sexually and asexually, producing spores.
Plasmogamy: The fusion of cytoplasm from two parent mycelia, which leads to a heterokaryotic stage (unfused haploid nuclei).
After karyogamy (fusion of nuclei), meiosis occurs, producing haploid spores.
Imperfect fungi (Deuteromycetes) reproduce only asexually using mitosis to create haploid spores.

Chytrids (Chytridiomycota):
- Found in terrestrial, freshwater, and marine habitats, they are important decomposers, mutualists, or parasites.
- Flagellated spores known as zoospores are a characteristic feature.
Zygomycetes (Zygomycota):
- Fast-growing molds, including black bread mold (Rhizopus stolonifer).
- They possess coenocytic hyphae and reproduce via asexual sporangia.
Glomeromycetes (Glomeromycota):
- Form mutualistic relationships with about 90% of plants through arbuscular mycorrhizae.
Ascomycetes (Ascomycota):
- Known as sac fungi, they produce sexual spores called ascospores in sac-like asci.
- Many species form symbiotic relationships with algae or cyanobacteria to create lichens.
Basidiomycetes (Basidiomycota):
- Commonly include mushrooms, puffballs, and shelf fungi.
- These fungi have a club-like structure called a basidium, where spores (basidiospores) are produced.

Fungi play essential roles in ecosystems:
- Decomposers breaking down organic matter, recycling nutrients back into the environment.
- Form symbiotic relationships with plants (mycorrhizae), enhancing plant nutrient uptake.
- Potential use in bioremediation to clean up pollutants like oil spills.

Humans utilize fungi in several ways:
- Culinary uses: Mushrooms and cheeses.
- Beverage Production: Yeasts in alcohol production and in leavening bread.
- Pharmaceuticals: Fungi produce antibiotics like penicillin and other medically significant compounds.
- Biotechnology: Yeasts are valuable in molecular biology studies and potential biofuel production.

Many fungi are plant pathogens, posing significant agricultural risks; approximately 10-50% of fruit harvests are lost annually due to fungal infections.
Fungi also form mutualistic partnerships with animals, aiding in the digestion of complex plant fibers, as seen in herbivorous mammals.

Lichens: Symbiotic association between a fungus (usually an ascomycete) and a photosynthetic partner (alga or cyanobacteria).
- Play a crucial role in ecological succession, colonizing new habitats.
- Lichen types include crustose (flat), foliose (leaf-like), and fruticose (shrub-like).