Fungi: Structure, Function, and Reproduction (Campbell Biology Ch. 31)

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Fungi: Structure, Function, and Reproduction

Concept: Fungi

Fungi are a diverse group of heterotrophic eukaryotes that play essential roles in ecosystems as decomposers, mutualists, and parasites. Their unique biology and reproductive strategies distinguish them from other kingdoms.

Heterotrophic Eukaryotes: Fungi obtain nutrients by absorbing organic material from their environment. They generally possess haploid nuclei and are the main decomposers in many ecosystems.
Mutualists: Some fungi form symbiotic relationships that benefit both partners. Examples include endophytes (fungi living inside plants) and lichens (associations between fungi and algae or cyanobacteria).
Symbioses: Fungi can have obligate (required) or facultative (optional) symbiotic relationships.
Parasites: Many fungi infect plants and animals, causing diseases known as mycoses.
Decomposers: Fungi digest plant material (cellulose and lignin), moving carbon through the carbon cycle. Saprophytes feed on dead plant matter.
Yeast: Unicellular fungi important in food production (bread, beer).

Morel mushrooms, an example of fungi Amanita mushrooms, another example of fungi

Fungal Mutualisms

Fungi often form mutualistic relationships with other organisms, providing benefits such as nutrient exchange and protection.

Lichens: Symbiotic association between a fungus and a photosynthetic partner (algae or cyanobacteria).
Endophytes: Fungi living within plant tissues, often enhancing plant resistance to stress.

Diagram of lichen structure Lichen growing on tree bark

Fungal Structure

The basic structural unit of fungi is the hypha, a filamentous cell surrounded by a cell wall. Fungi grow as networks of hyphae called mycelia, which maximize surface area for nutrient absorption.

Hypha: Long, filamentous structure; can be divided by septa (cross walls) or be coenocytic (continuous cytoplasm).
Chitin: Main component of fungal cell walls, a polymer of modified β-glucose.
Mycelium: Interwoven mass of hyphae forming the main body of the fungus.
Extracellular Digestion: Fungi release enzymes to break down organic matter and absorb nutrients.

Microscopic view of fungal hyphae Mycelium growing in soil Diagram of fungal hypha structure

Specialized Fungal Structures

Fungi have evolved specialized structures for nutrient acquisition and symbiosis with plants.

Haustoria: Specialized hyphal tips that penetrate plant cells to extract nutrients.
Mycorrhizae: Symbiotic association between fungi and plant roots, enhancing nutrient uptake.
Ectomycorrhizae: Fungal hyphae form a sheath around roots but do not penetrate cell walls.
Arbuscular (Endomycorrhizal) Fungi: Hyphae penetrate root cortical cells.

Diagram of ectomycorrhizal association Mycelium growing in soil

Fungal Reproduction

Fungi reproduce both sexually and asexually, always using spores. Their reproductive cycles are complex and involve unique cellular processes.

Spores: Reproductive cells produced in both sexual and asexual cycles.
Fruiting Body: Structure that produces spores (e.g., mushrooms).
Deuteromycetes: Fungi that reproduce only asexually.
Mating Types: Fungi have many mating types, identified by pheromones rather than morphological differences.
Plasmogamy: Fusion of cytoplasm from two parent cells (without nuclear fusion).
Heterokaryotic: Cells with two or more genetically distinct nuclei (n + n).
Dikaryotic: Cells with two nuclei.
Karyogamy: Fusion of nuclei to form a diploid zygote, which produces spores.

Diagram of fungal sexual cycle

Fungal Groups and Their Reproduction

Major fungal groups differ in their reproductive structures and cycles.

Chytrids: Only fungi with alternation of generations; produce flagellated spores (zoospores).
Zygomycetes: Plasmogamy leads to formation of zygosporangium, where karyogamy occurs. Asexual reproduction via sporangiophores.
Basidiomycota: Dikaryotic hyphae form basidia, where karyogamy and spore production occur. Fruiting body is called basidiocarp.
Ascomycota: Dikaryotic hyphae form asci, each producing eight haploid ascospores. Fruiting body is called ascocarp. Conidia are asexual spores.

Diagram of zygomycete sexual cycle Diagram of basidiomycete sexual cycle Diagram of ascomycete sexual cycle

Summary Table: Major Fungal Groups

Group	Key Reproductive Structure	Spore Type	Unique Features
Chytrids	Zoosporangium	Zoospores (flagellated)	Alternation of generations
Zygomycetes	Zygosporangium	Sporangia, zygospores	Coenocytic hyphae
Basidiomycota	Basidiocarp (fruiting body)	Basidiospores	Dikaryotic hyphae, basidia
Ascomycota	Ascocarp (fruiting body)	Ascospores, conidia	Asci, eight spores per ascus

Key Terms and Definitions

Hypha: Filamentous cell of fungi.
Mycelium: Mass of hyphae forming the fungal body.
Chitin: Structural polysaccharide in fungal cell walls.
Plasmogamy: Fusion of cytoplasm from two parent cells.
Karyogamy: Fusion of nuclei to form a diploid zygote.
Haustoria: Specialized hyphal tips for nutrient extraction.
Mycorrhizae: Symbiotic association between fungi and plant roots.
Ascocarp: Fruiting body of ascomycetes.
Basidiocarp: Fruiting body of basidiomycetes.

Equations and Formulas

Heterokaryotic Stage:
Dikaryotic Stage:
Diploid Zygote:

Additional info: Academic context was added to clarify fungal reproductive cycles, mutualisms, and structural terminology.