Fungal Cell Structure and Composition
Explore the fundamentals of Fungal Cell Structure and Composition.
FUNGI: DETAILED KEYPOINTS
I. General Overview and Characteristics
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Species Diversity: Between 80,000 to 120,000 fungal species have been described, but the total number is estimated at approximately 1.5 million.
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Cell Wall Composition: Fungi have cell walls made of chitin (not cellulose like plants).
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Body Structure: The fungal body is composed of long, thread-like filaments or tubes called hyphae (singular: hypha).
II. Ainsworth's (1973) Characteristics of Fungi
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Nutrition: Heterotrophic, feeding by absorption rather than ingestion. No photosynthesis.
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Vegetative State: Typically a non-motile mycelium of hyphae on or in the substratum, showing internal protoplasmic streaming. Motile reproductive state may occur.
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Cell Wall: Typically present, based on glucans and chitin.
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Nuclear Status: Eukaryotic, uni- or multinucleate. Thallus may be homo- or heterokaryotic, haploid, dikaryotic, or diploid (diploid usually short duration, with exceptions).
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Life Cycle: Simple or, more usually, complex.
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Reproduction Events: May include sexual (nuclear fusion and meiosis), parasexual (nuclear fusion followed by gradual de-diploidization), and/or asexual (purely mitotic nuclear division).
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Propagules: Typically microscopically small spores produced in high numbers. Motile spores are confined to certain groups.
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Sporocarps: Microscopic or macroscopic, showing characteristic shapes but only limited tissue differentiation.
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Habitat: Ubiquitous in terrestrial and freshwater habitats; less common in marine environments.
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Ecology: Important roles as saprotrophs, mutualistic symbionts, parasites, or hyperparasites.
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Distribution: Cosmopolitan (worldwide).
III. Elemental Requirements of Fungal Cells
| Element | Common Sources | Cellular Functions |
|---|---|---|
| Carbon | Sugars | Structural element (with H, O, N); energy source |
| Hydrogen | Protons from acidic environments | Transmembrane proton motive force for nutrition; intracellular acidic pH (5–6) for metabolism |
| Oxygen | Air, O₂ | Substrate for respiratory and oxidative enzymes; essential for ergosterol and unsaturated fatty acid synthesis |
| Nitrogen | NH₄⁺ salts, urea, amino acids | Organic amino nitrogen in proteins and enzymes |
| Phosphorus | Phosphates | Energy transduction; nucleic acid and membrane structure |
| Potassium | K⁺ salts | Ionic balance; enzyme activity |
| Magnesium | Mg²⁺ salts | Enzyme activity; cell and organelle structure |
| Iron (Fe) | Ferric salts (chelated by siderophores, released as Fe²⁺) | Heme-proteins; cytochromes |
| Manganese | Mn²⁺ salts | Enzyme activity |
| Zinc | Zn²⁺ salts | Enzyme activity |
| Nickel | Ni²⁺ salts | Urease activity |
IV. Hyphae and Mycelium
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Not all fungi grow as hyphae – some grow as discrete yeast cells.
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Yeast Reproduction: Divide by fission or more frequently by budding.
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Yeast Habitats: Common where efficient substratum penetration is not required (e.g., plant surfaces, animal digestive tracts).
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Dimorphic Fungi: Species capable of switching between hyphal and yeast-like growth forms (e.g., some human/animal pathogens).
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Pseudo-hyphae: Intermediate stages between yeast cells and true hyphae.
V. Classification of Fungi (Five True Phyla)
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Naming Convention: "-mycota" designates a phylum; "-mycetes" designates a class or informally refers to phylum members.
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The five true phyla: Chytridiomycota, Zygomycota, Ascomycota, Basidiomycota, and Glomeromycota.
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Deuteromycota: An older, no-longer-used group for fungi with only asexual reproduction. Molecular analysis shows these belong to Ascomycota or Basidiomycota.
VI. Phylum Chytridiomycota (Chytrids)
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Class: Only Chytridiomycetes.
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Status: Simplest and most primitive Eumycota (true fungi).
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Cell Wall: Contains chitin; one group has both cellulose and chitin.
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Structure: Mostly unicellular; a few form multicellular organisms and hyphae with no septa (coenocytic).
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Motility: Produce gametes and diploid zoospores that swim using a single flagellum.
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Habitat: Usually aquatic; some live on land. Ecological habitat and cell structure similar to protists.
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Ecology: Some are parasites on plants, insects, or amphibians; others are saprobes.
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Model Organism: Allomyces – well characterized, with both asexual and sexual reproductive phases. Produces diploid or haploid flagellated zoospores in a sporangium.
VII. Phylum Zygomycota (Conjugated Fungi)
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Size: Relatively small group.
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Examples: Rhizopus stolonifer (bread mold) on bread, fruits, and vegetables.
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Ecology: Most are saprobes on decaying organic material; a few are parasites (especially of insects).
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Commercial Role: Metabolic products of Rhizopus species are intermediates in semi-synthetic steroid hormone synthesis.
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Hyphal Structure: Thallus of coenocytic hyphae (no septa); nuclei are haploid in vegetative stage.
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Asexual Reproduction: Produces sporangiospores in swollen sporangia (black tips of bread mold).
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Sexual Reproduction (Conjugation) : Occurs when conditions become unfavorable. Requires two opposing mating strains (+ and –) in close proximity. Gametangia from hyphae fuse, leading to karyogamy. Develops diploid zygospores with thick protective coats against desiccation.
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Zygospore Dormancy: May remain dormant until favorable conditions return.
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Germination: Zygospore undergoes meiosis and produces haploid spores that grow into new organisms.
VIII. Phylum Ascomycota (Sac Fungi)
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Characteristic Feature: Formation of an ascus (plural: asci) – a sac-like structure containing haploid ascospores.
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Abundance: Majority of known fungi belong to this phylum.
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Beneficial Commercial Examples: Yeasts for baking, brewing, wine fermentation; truffles and morels (gourmet delicacies).
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Harmful Effects: Infest and destroy crops; produce poisonous secondary metabolites making crops unfit for consumption.
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Hyphal Structure: Filamentous ascomycetes produce hyphae divided by perforated septa allowing cytoplasmic streaming.
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Separation of Reproductive Structures: Conidia (asexual) and asci (sexual) are separated from vegetative hyphae by blocked (non-perforated) septa.
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Asexual Reproduction: Frequent; involves production of conidiophores that release haploid conidiospores.
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Sexual Reproduction – Structures: "Male" strain produces antheridium; "female" strain develops ascogonium.
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Plasmogamy: Antheridium and ascogonium combine without nuclear fusion.
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Ascogenous Hyphae: Special hyphae arise in which pairs of nuclei migrate – one from male strain, one from female strain.
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Karyogamy: In each ascus, two or more haploid ascospores fuse their nuclei.
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Meiosis: Diploid nucleus gives rise to haploid nuclei by meiosis.
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Ascocarp: Fruiting body containing thousands of asci.
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Ascospore Release: Ascospores are released, germinate, and form hyphae disseminated in the environment.
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Life Cycle Predominance: Haploid phase is the predominant phase.
IX. Phylum Basidiomycota (Club Fungi)
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Characteristic Feature: Club-shaped fruiting bodies called basidia (singular: basidium) – swollen terminal cells of hyphae.
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Familiar Examples: Mushrooms (after rain, on lawns, supermarket shelves).
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Alternative Name: "Gill fungi" due to gill-like structures on the underside of the cap (gills are compacted hyphae bearing basidia).
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Other Members: Shelf fungi (on tree bark), smuts, rusts (important plant pathogens), toadstools.
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Edibility: Most edible fungi belong to Basidiomycota; however, some produce deadly toxins (e.g., Cryptococcus neoformans causes severe respiratory illness).
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Life Cycle: Includes alternation of generations.
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Spore Production: Generally through sexual reproduction, not asexual.
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Basidiospores: Spores carried by the club-shaped basidium.
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Karyogamy in Basidium: Nuclei of two different mating strains fuse, giving rise to a diploid zygote.
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Meiosis in Basidium: Diploid zygote undergoes meiosis; haploid nuclei migrate into basidiospores.
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Primary Mycelium: Basidiospores germinate and generate monokaryotic hyphae (one nucleus per cell).
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Secondary Mycelium (Dikaryotic Stage) : Mycelia of different mating strains combine, producing a mycelium with haploid nuclei of two different mating strains. This dikaryotic stage is the dominant stage of the basidiomycete life cycle.
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Basidiocarp: Fruiting body that protrudes from the ground (the mushroom). Bears developing basidia on the gills under its cap.
X. Phylum Glomeromycota
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Size: Newly established phylum with approximately 230 species.
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Habitat: All live in close association with the roots of trees.
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Evolutionary History: Fossil records indicate trees and their root symbionts share a long evolutionary history.
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Arbuscular Mycorrhizae: All members form these – hyphae interact with root cells in a mutually beneficial association.
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Exchange in Mutualism: Plants supply carbon source and energy (carbohydrates) to the fungus; fungus supplies essential minerals from soil to the plant.
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Sexual Reproduction: Do not reproduce sexually.
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Survival Requirement: Cannot survive without the presence of plant roots.
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Hyphal Structure: Have coenocytic hyphae (like zygomycetes) but do not form zygospores.
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Phylogeny: DNA analysis shows all glomeromycetes likely descended from a common ancestor – a monophyletic lineage.
XI. Deuteromycota (Imperfect Fungi – No Longer a Formal Phylum)
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Definition: Fungi that do not display a sexual phase.
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Current Status: Classification group no longer used. Molecular analysis shows members belong to Ascomycota or Basidiomycota.
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Description: Less well described than other members due to absence of sexual structures used for classification.
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Habitat: Most live on land; few aquatic exceptions.
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Appearance: Form visible mycelia with fuzzy appearance – commonly known as mold.
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Reproduction: Strictly asexual, mostly by production of asexual conidiospores.
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Genetic Recombination: Some hyphae may recombine and form heterokaryotic hyphae; genetic recombination occurs between different nuclei.
XII. Ecology of Fungi
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Environmental Preference: Prefer dark, moist conditions.
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Extreme Habitats: Can thrive in hostile environments like tundra through symbiosis with photosynthetic organisms (lichens).
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Ecological Role: Along with bacteria, fungi are the major decomposers of nature. Their versatile metabolism breaks down organic matter that would otherwise not be recycled.
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Habitat Diversity: From seawater to human skin and mucous membranes.
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Example – Dry Habitats: Coccidioides immitis (causes pneumonia when spores are inhaled) thrives in dry, sandy soils.
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Primary Terrestrial Habitat: Forest floor (dark, damp, rich in decaying debris).
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Nutrient Recycling: Fungi release nitrogen and phosphorus from decaying matter, making them available to other organisms. Trace elements tied up in rotting matter are returned to the environment via fungal metabolic activity.
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Digestion Mechanism: Digestion precedes ingestion. Fungi produce exoenzymes released into the substrate or bound to the outside of the fungal cell wall.
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Absorption: Large molecules broken into small molecules, transported into the cell by protein carriers embedded in the cell membrane.
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Moisture Requirement: Active growth depends on relatively high environmental moisture because movement of small molecules and enzymes requires water.
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Saprobes: Help maintain sustainable ecosystems by replenishing nutrients.
XIII. Mutualistic Relationships
A. Fungus/Plant Mutualism (Mycorrhizae)
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Definition of Mycorrhiza: Association between vascular plant roots and symbiotic fungi (Greek: myco = fungus, rhizo = root).
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Prevalence: 80–90% of all plant species have mycorrhizal partners.
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Exchange: Fungal mycelia channel water and minerals from soil to plant; plant supplies photosynthesis products to fuel fungal metabolism.
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Ectomycorrhizae: Fungus envelops roots in a sheath (mantle) and forms a Hartig net of hyphae extending into roots between cells (not penetrating cells). Fungal partner from Ascomycota, Basidiomycota, or Zygomycota.
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Arbuscular Mycorrhizae (Endomycorrhizae) : Formed by Glomeromycete fungi. Fungi form arbuscules (shrub-like structures) that penetrate root cell walls (but not cell membranes). Arbuscules are the site of metabolic exchanges.
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Endophytes: Fungi living inside plant tissue without damaging the host. Release toxins repelling herbivores or confer resistance to environmental stress (microorganisms, drought, heavy metals).
B. Lichens
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Definition: Mutualism between a fungus (Ascomycota or Basidiomycota) and a photosynthetic organism (eukaryotic alga or prokaryotic cyanobacterium).
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Survival Dependency: Neither organism typically survives alone outside the symbiotic relationship.
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Thallus: Body of a lichen formed of hyphae wrapped around the photosynthetic partner.
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Photosynthetic Partner's Role: Provides carbon and energy as carbohydrates. Some cyanobacteria fix nitrogen, contributing nitrogenous compounds.
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Fungal Partner's Role: Supplies minerals and protection from dryness and excessive light by encasing algae in mycelium. Also attaches the symbiotic organism to the substrate.
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Forms: Crust-like, hair-like, or leaf-like.
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Survival Ability: Can survive extended drought (complete desiccation) and rapidly become active when water returns.
C. Fungus/Animal Mutualism
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Arthropod Partners: Fungi have mutualisms with numerous insects in Phylum Arthropoda.
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Benefit to Arthropods: Depend on fungus for protection from predators and pathogens.
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Benefit to Fungi: Obtain nutrients and a way to disseminate spores to new environments.
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Basidiomycota and Scale Insects: Fungal mycelium covers and protects insect colonies; scale insects foster nutrient flow from parasitized plant to fungus.
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Leaf-Cutting Ants: "Farm" fungi by cutting leaf disks and piling them in gardens. Fungi digest cellulose in leaves that ants cannot break down. Fungi produce smaller sugar molecules, then become food for the ants.
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Ant Garden Maintenance: Ants patrol their garden, preying on competing fungi.
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Mutual Benefit: Fungus receives steady supply of leaves and freedom from competition; ants feed on the cultivated fungi.
XIV. Fungivores and Spore Dispersal
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Animal Dispersal Importance: Animals may carry spores considerable distances from the source.
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Spore Survival in Digestion: Fungal spores are rarely completely degraded in animal gastrointestinal tracts; many germinate when passed in faeces.
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Dung Fungi Requirement: Some dung fungi actually require passage through herbivore digestive systems to complete their lifecycle.
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Truffles: Fruiting bodies of underground mushrooms. Almost all are ectomycorrhizal, found in close association with trees. Animals eat truffles and disperse the spores.
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Truffle Hunting: In Italy and France, female pigs sniff out truffles because the fungus releases a volatile compound closely related to a pheromone produced by male pigs.
XV. Key Definitions (Glossary from Lecture)
| Term | Definition |
|---|---|
| Arbuscular mycorrhizae | Mycorrhizae involving Glomeromycetes where fungal hyphae penetrate plant root cell walls (but not cell membranes) |
| Ascocarp | Fruiting body of ascomycetes |
| Ascomycota | Sac fungi; phylum that stores spores in a sac called ascus |
| Basidiocarp | Fruiting body that protrudes from the ground and bears the basidia |
| Basidiomycota | Club fungi; phylum that produces club-shaped basidia containing spores |
| Basidium | Club-shaped fruiting body of basidiomycetes |
| Chytridiomycota | Chytrids; primitive phylum of fungi that live in water and produce gametes with flagella |
| Deuteromycota | Former form phylum of fungi without a known sexual reproductive cycle (now members of Ascomycota and Basidiomycota) |
| Ectomycorrhizae | Mycorrhizae where fungal hyphae do not penetrate plant root cells |
| Glomeromycota | Phylum of fungi that form symbiotic relationships with the roots of trees |
| Mold | Tangle of visible mycelia with fuzzy appearance |
| Zygomycota | Conjugated fungi; phylum that forms a zygote contained in a zygospore |
| Zygospore | Structure with thick cell wall that contains the zygote in zygomycetes |