lec 09:Fungi: Structure, Nutrition, and Reproduction

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Fungi: The Microbial Eukaryotes

What Are Fungi?

Fungi are a diverse kingdom of eukaryotic organisms, distinct from plants, animals, and bacteria. They include yeasts, molds, and mushrooms, and are found in a wide range of terrestrial and aquatic environments. Fungi play essential roles in nutrient cycling, symbiosis, and disease.

Photograph of mushrooms, a type of fungus

Structural Characteristics of Fungi

Cell Walls: Fungal cell walls contain chitin, a strong, flexible polysaccharide, rather than cellulose (as in plants).
Hyphae: Most fungi grow as long, branching filamentous cells called hyphae. Interconnected hyphae form a mycelium, which is the main vegetative body of the fungus.
Yeasts: Some fungi, such as yeasts, exist as unicellular forms.

Diagram of mushroom structure: cap, gills, stalk, hyphae Diagram of mycelium and hyphae

Fungal Nutrition

Fungi are heterotrophs that use absorptive nutrition. They secrete digestive enzymes (exoenzymes) into their environment to break down complex organic matter, then absorb the resulting small molecules.

Exoenzymes: Examples include cellulases, proteases, and lipases.
Absorption: The extensive surface area of the mycelium allows efficient uptake of nutrients by diffusion or active transport.

Diagram showing excretion of digestive enzymes and absorption by hyphae Diagram of enzyme-substrate interaction and breakdown

Modes of Fungal Nutrition

Saprotrophs: Decompose dead organic matter, recycling nutrients in ecosystems.
Parasites: Infect living hosts, using specialized structures (haustoria) to absorb nutrients, often causing disease.
Mutualists: Form beneficial symbioses, such as mycorrhizae with plant roots, aiding nutrient and water uptake in exchange for sugars.

Diagram of white-rot fungi decomposing lignin in wood

Fungi as Pathogens: Athlete’s Foot

Dermatophyte Infections

Athlete’s foot (Tinea pedis) is a common fungal infection caused by dermatophytes that digest keratin in the skin, hair, and nails. These fungi thrive in warm, moist environments and spread by direct contact.

Keratin Digestion: Dermatophytes secrete proteases (e.g., fungalysin) to break down keratin, enabling absorption of nutrients.
Symptoms: Redness, scaling, and cracking of the skin, especially between the toes.

Feet with symptoms of athlete's foot Red, irritated skin between toes due to athlete's foot Diagram of fungal cell wall and keratin degradation in skin

Fungal Hyphae and Mycelium

Structure and Function

Hyphae are microscopic, thread-like filaments that collectively form the mycelium. Hyphae grow by apical extension, allowing fungi to efficiently explore and absorb nutrients from their environment.

Roles: Decomposition, symbiosis, pathogenesis, and reproduction.
Composition: Primarily chitin, providing rigidity and protection.

Microscopic view of fungal hyphae Diagram comparing septate and coenocytic hyphae

Types of Hyphae

Septate Hyphae: Divided by cross-walls (septa) with pores for cytoplasmic flow. Common in many fungi (e.g., Aspergillus).
Coenocytic (Aseptate) Hyphae: Lack septa, forming continuous filaments with multiple nuclei (e.g., Rhizopus).
Pseudohyphae: Chains of elongated yeast cells, seen in Candida albicans.

Diagram of septate and coenocytic hyphae Diagram of septate, coenocytic, and pseudohyphae

Fungal Cell Wall Structure

Inner Layer: Chitin and β-glucans provide mechanical strength and rigidity.
Outer Layer: Mannoproteins facilitate cell–cell recognition, adhesion, and immune evasion.

Diagram of mushroom, mycelium, hypha, and cell wall structure

Fungal Reproduction

Asexual Reproduction

Most fungi reproduce asexually under favorable conditions, producing genetically identical offspring. Common mechanisms include:

Sporogenesis: Formation of mitospores (conidia) on specialized hyphae.
Budding: Common in yeasts; a new cell forms as a bud on the parent.
Fragmentation: Mycelium breaks into fragments, each growing into a new colony.

Diagram of asexual spore formation in fungi Diagram of yeast budding

Sexual Reproduction

Sexual reproduction is typically triggered by harsh or changing conditions, generating genetic diversity. Key stages include:

Plasmogamy: Fusion of cytoplasm from two parent cells.
Karyogamy: Fusion of haploid nuclei to form a diploid zygote.
Meiosis: Diploid nucleus divides to restore haploid state, producing genetically diverse spores.

Generalized fungal life cycle showing sexual and asexual cycles

Classification of Fungi

Fungi are classified based on sexual reproduction, molecular phylogeny, and hyphal structure. Major phyla include:

Phylum	Main Features	Examples
Chytridiomycota	Aquatic, motile zoospores with flagella	Batrachochytrium
Zygomycota (historical)	Coenocytic hyphae, zygospore formation	Rhizopus (bread mold)
Glomeromycota	Arbuscular mycorrhizal symbionts	Plant root symbionts
Ascomycota	Sexual spores in asci (sacs)	Yeasts, molds, Aspergillus
Basidiomycota	Basidiospores on basidia (clubs)	Mushrooms, rusts, smuts
Deuteromycota (obsolete)	No observed sexual stage	Reclassified as Ascomycota/Basidiomycota

Review Questions

Which feature best distinguishes fungi from plants? C. Use of absorptive nutrition
Why is fungal digestion considered “external”? B. Enzymes act outside the fungal body
Haustoria are specialized fungal structures primarily involved in: C. Host penetration and nutrient absorption
The primary structural polymer in fungal cell walls is: C. Chitin
Which hyphal type lacks septa and contains multiple nuclei within a shared cytoplasm? C. Coenocytic hyphae
The outer layer of the fungal cell wall is primarily involved in: C. Environmental and host interactions
Which reproductive strategy produces genetically identical offspring? C. Asexual reproduction
Which asexual mechanism is most common in yeasts? C. Budding
Why do fungi often switch to sexual reproduction under harsh conditions? C. It increases genetic variation