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Campbell Biology - Fungi Study Notes

Chapter 31: Fungi

Overview of Fungi

  • Nutritional Mode:

    • Fungi are heterotrophs that absorb nutrients from their environment. They utilize hydrolytic enzymes to break down complex organic molecules from a variety of sources, both living and dead, into smaller, absorbable organic compounds.

  • Ecosystem Roles:

    • Decomposers: Break down and absorb nutrients from nonliving organic material.

    • Parasitic Fungi: Absorb nutrients from living hosts, often harming them.

    • Mutualistic Fungi: Absorb nutrients from hosts while reciprocating with beneficial actions for the host.

Body Structure of Fungi

  • Forms of Fungi:

    • Fungi typically exist in two forms: multicellular filaments and single-celled yeasts. Most species grow primarily as filaments.

    • Yeasts thrive in moist environments rich in soluble nutrients, particularly sugars and amino acids.

  • Hyphae:

    • The fungal body is composed of hyphae, which are networks of tiny filaments.

    • Hyphae possess tubular cell walls made of chitin, a polymer that provides structural strength and prevents cell lysis due to osmotic pressure during nutrient absorption.

  • Septa and Coenocytes:

    • In many fungi, hyphae are divided by septa (cross-walls) that contain pores for organelle movement.

    • Coenocytic fungi lack septa, resulting in a multinucleated cytoplasmic mass.

  • Mycelium:

    • The interwoven mass of hyphae is termed mycelium, which infiltrates food sources and optimizes the surface-to-volume ratio for efficient absorption.

    • Hyphae grow primarily in length through cytoplasmic streaming, moving materials toward the tips.

    • This structure is non-motile but can colonize new territories by growing.

Specialized Hyphae in Mycorrhizal Fungi

  • Feeding on Live Animals:

    • Some fungi have specialized hyphae for parasitizing live animals.

  • Haustoria:

    • Specialized hyphae called haustoria enable the extraction of nutrients from plants.

  • Mycorrhizae:

    • Mycorrhizae are beneficial partnerships between fungi and plant roots, aiding in nutrient exchange.

      • Functions: Mycorrhizal fungi supply phosphate ions and minerals to plants; in return, plants provide organic nutrients to the fungi.

      • Most vascular plants depend on these relationships.

      • Types of Mycorrhizae:

        • Ectomycorrhizal fungi: Form sheaths over root surfaces and extend into root cortex spaces.

        • Arbuscular mycorrhizal fungi: Extend arbuscules through root cell walls into invaginations of the root plasma membrane.

      • Mycorrhizal fungi spread through spores, which are haploid reproductive cells.

Reproduction in Fungi

  • Spores Production:

    • Fungi reproduce by generating large quantities of spores through sexual or asexual cycles.

    • Spores can travel long distances via wind or water and will germinate into new mycelia under suitable moist conditions.

  • Nuclei Forms:

    • Fungal nuclei and spores are typically haploid, although transient diploid nuclei may form during sexual reproduction.

  • Sexual Reproduction:

    • Involves fusion of hyphae from different mating types (e.g., Plasmogamy) - the cytoplasmic fusion from two parent mycelia creates a heterokaryon with mixed nuclei.

    • Karyogamy: The fusion of haploid nuclei occurs, resulting in a diploid zygote that later undergoes meiosis to produce haploid spores, promoting genetic variation.

  • Asexual Reproduction:

    • Fungi may also reproduce asexually by mitosis, where molds produce haploid spores, or single-celled yeasts that divide without forming spores (termed deuteromycetes if no sexual stage is known).

Evolutionary Origins of Fungi

  • Ancestry:

    • The common ancestor of fungi was an aquatic, single-celled, flagellated protist.

    • Fungi and animals are more closely related than either is to plants. The opisthokont clade includes fungi, animals, and their protistan relatives.

  • History:

    • Fungi likely originated in aquatic environments, colonizing land around 505 million years ago. The earliest accepted fossils are from terrestrial species dated 440 million years ago.

Diversity of Fungal Lineages

  • Total Species:

    • Approximately 145,000 species of fungi are known, yet estimates range from 2.2 to 3.8 million.

  • Cryptomycetes and Microsporidians:

    • Both represent a basal lineage of fungi. Cryptomycetes include around 30 species, many suspected to be diverse and found in soils/aquatic habitats. Microsporidians, around 1,300 species, are unicellular parasites with reduced mitochondria and small genomes.

Specific Groups of Fungi

  • Chytrids:

    • Comprising about 1,000 species, found in diverse habitats and are significant as decomposers, parasites, and mutualists. They notably feature flagellated zoospores.

  • Zoopagomycetes:

    • Approximately 900 species act as parasites or mutual symbionts, reproducing via nonflagellated spores and forming zygosporangium during sexual reproduction.

  • Mucoromycetes:

    • Around 750 species, notable for their roles as decomposers (e.g., Rhizopus stolonifer) and mutualists with plants, particularly mycorrhizae. They can reproduce sexually when conditions are poor.

  • Ascomycetes:

    • About 90,000 species, ranging from yeast to complex fungi like morels, known for producing ascocarps containing asci that produce sexual spores. They reproduce asexually via conidia. They play roles in symbiotic associations with plants (e.g., lichens).

  • Basidiomycetes:

    • Comprising 50,000 species, including mushrooms and plant parasites. They form fruiting bodies called basidiocarps and play significant roles in decomposing wood.

Ecological and Practical Importance of Fungi

  • Nutrient Cycling:

    • Fungi are critical decomposers in ecosystems, breaking down complex organic compounds, including cellulose and lignin, aiding in nutrient recycling with bacteria.

  • Mutualistic Relationships:

    • Form partnerships with plants and animals, such as endophytes that live inside plants without harming them. Many endophytes are ascomycetes.

  • Fungus-Animal Mutualisms:

    • Certain fungi aid in the digestion of plant material in herbivores, while leaf-cutter ants farm fungi for nutrients.

  • Lichens:

    • Symbiotic relationships between fungi and photosynthetic microorganisms (e.g., algae, cyanobacteria) which are important pioneers in new environments.

    • Reproductive strategies include fragmentation and producing soredia.

  • Parasitism:

    • Fungi can be pathogenic, causing significant agricultural losses (10% to 50% of fruit harvest). For instance, Claviceps purpurea produces harmful toxins affecting rye crops.

    • Animals are less affected, although certain chytrids cause amphibian declines.

    • Fungal infections (mycosis) can be severe; conditions like Coccidioidomycosis can be life-threatening.

Human Uses of Fungi

  • Culinary Uses:

    • Fungi are consumed widely as food and are essential in producing cheese, alcohol, and bread.

  • Medical Uses:

    • Fungi like Penicillium have produced antibiotics, while other compounds help regulate health conditions.

  • Biotechnological Applications:

    • Genetic research on fungi has potential medical applications (e.g., genetically modified yeast for human proteins) and biofuels (e.g., Gliocladium roseum produces hydrocarbons similar to diesel).