Premed · Premed · General Biology 2
Lecture 11: Fungi
General Biology II — Organismal, Evolution & Ecology
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the general characteristics of fungi, including cell structure, nutrition, and growth
- Explain the life cycles and reproductive strategies of major fungal phyla
- Distinguish among Chytridiomycota, Zygomycota, Ascomycota, and Basidiomycota
- Describe the ecological roles of fungi as decomposers, mutualists, and pathogens
- Explain the structure and significance of lichens and mycorrhizae
- Discuss the medical, agricultural, and industrial importance of fungi
Lecture Content
I. General Characteristics of Fungi
Fungi are eukaryotic, heterotrophic organisms classified in the domain Eukarya, kingdom Fungi. Despite a superficial resemblance to plants in their sessile lifestyle, fungi are in fact more closely related to animals -- both belong to the supergroup Opisthokonta.
Several features distinguish fungi from all other organisms. Their cell walls are made of chitin, a tough polysaccharide also found in arthropod exoskeletons, rather than the cellulose found in plant cell walls. Fungi are heterotrophic by absorption: they secrete extracellular digestive enzymes (exoenzymes) into their environment to break down complex organic molecules, then absorb the resulting small molecules through their cell membranes. Most fungi are multicellular, with bodies composed of filaments called hyphae (singular: hypha) that form an interconnected network called a mycelium -- the main body of the fungus, which is typically hidden underground or within a substrate. Some fungi, however, are unicellular yeasts, such as Saccharomyces. Hyphal structure varies: septate hyphae are divided by cross-walls (septa) perforated by pores that allow cytoplasm and organelles to flow between cells, while coenocytic (aseptate) hyphae lack septa entirely, creating a continuous multinucleate tube. Like animals, fungi store carbohydrates as glycogen rather than starch.
Fungal growth occurs at the hyphal tips (apical growth), allowing rapid colonization of substrates. The filamentous body plan creates a remarkably high surface area-to-volume ratio that maximizes absorptive efficiency -- a single mycelium can extend for kilometers through soil or wood. Fungi exploit three major nutritional modes: saprotrophs (decomposers) feed on dead organic matter, parasites feed on living hosts, and mutualists form mutually beneficial relationships with partner organisms.
<image>A diagram showing fungal body structure. Panel A: A mushroom (basidiomycete) shown above and below ground — the visible mushroom (fruiting body or basidiocarp) is above ground, while an extensive branching mycelium of hyphae extends through the soil below. Panel B: Magnified view of septate hyphae — showing septa with pores, nuclei in each compartment, and the cell wall made of chitin. Panel C: Magnified view of coenocytic hyphae — no septa, many nuclei scattered throughout the continuous cytoplasm. Panel D: A yeast cell (unicellular fungus) showing budding reproduction — a smaller daughter cell emerging from the parent cell. Labels identify all key structures.</image>
II. Fungal Reproduction
Fungi reproduce by spores -- microscopic propagules produced in enormous numbers, often airborne, that serve as the primary agents of dispersal and survival. Spores can be generated either asexually or sexually. Asexual reproduction involves the production of spores by mitosis (called conidia), budding in yeasts, or simple fragmentation of hyphae.
Sexual reproduction in most fungi follows a distinctive pattern unlike that of any other kingdom. First, plasmogamy occurs: the cytoplasm of two compatible hyphae fuses, bringing two genetically distinct haploid nuclei together within the same cell. Rather than fusing immediately, these nuclei typically coexist for an extended period in a heterokaryotic (dikaryotic) stage (n + n). Eventually, karyogamy occurs: the two nuclei fuse to form a diploid (2n) zygote nucleus. This is followed almost immediately by meiosis, which produces haploid spores. The diploid stage in fungi is therefore remarkably brief -- most of the fungal life cycle is spent in the haploid or dikaryotic state. Fungi do not have male and female sexes; instead, they possess mating types (designated as + and -, or by specific mating type alleles) that determine compatibility.
III. Major Fungal Phyla
A. Chytridiomycota (Chytrids)
The chytrids are the most ancient lineage of fungi and the only group that produces flagellated spores (zoospores), a trait that reflects their aquatic origins. They inhabit freshwater, marine, and moist terrestrial environments and include both saprotrophs and parasites. The chytrid Batrachochytrium dendrobatidis (Bd) causes chytridiomycosis in amphibians, a disease that has emerged as a major driver of amphibian population declines and extinctions worldwide. The basal position of chytrids on the fungal phylogenetic tree makes them important for understanding how fungi evolved from aquatic protist ancestors.
B. Zygomycota (Zygomycetes)
The zygomycetes are primarily terrestrial saprotrophs that decompose organic matter. They possess coenocytic hyphae and reproduce asexually by producing sporangiospores within sporangia at the tips of upright sporangiophores. Sexual reproduction involves the fusion of gametangia from two compatible hyphae, producing a thick-walled, resistant zygospore within a zygosporangium. The zygospore can remain dormant through unfavorable conditions and undergoes meiosis upon germination. The common black bread mold Rhizopus stolonifer is a familiar example, as is the remarkable Pilobolus, which launches its sporangia toward light sources with remarkable precision.
C. Ascomycota (Sac Fungi)
The ascomycetes are the largest phylum of fungi, with approximately 64,000 described species. Their defining feature is the ascus (plural: asci), a sac-like structure in which sexual spores called ascospores are produced -- typically eight per ascus, the result of one round of meiosis followed by one round of mitosis. Asci are often organized within a fruiting body called an ascocarp, which may take the form of a cup, a flask, or the distinctive honeycomb structure of a morel. Asexual reproduction occurs through conidia produced at the tips of specialized hyphae called conidiophores.
The ascomycetes are ecologically and economically diverse. Saprotrophic members include Aspergillus and Penicillium. Plant pathogens include Claviceps purpurea (ergot of rye) and Ophiostoma (Dutch elm disease). Animal pathogens include Candida albicans (responsible for yeast infections) and Aspergillus species (causing aspergillosis). On the beneficial side, Saccharomyces cerevisiae -- baker's and brewer's yeast -- drives fermentation in baking and alcohol production. Penicillium chrysogenum is the source of penicillin, the first antibiotic. Truffles and morels are prized edible fungi. And Neurospora crassa served as the model organism through which Beadle and Tatum established the one gene-one enzyme hypothesis.
D. Basidiomycota (Club Fungi)
The basidiomycetes are defined by the basidium (plural: basidia), a club-shaped structure that produces sexual spores called basidiospores -- typically four per basidium, each the direct product of meiosis. Basidia line the gills (or pores) of the basidiocarp, the familiar mushroom that is merely the reproductive fruiting body of the organism. The vast majority of the fungal body remains hidden as mycelium beneath the surface.
The basidiomycete life cycle features an extended dikaryotic stage. Two compatible haploid mycelia fuse (plasmogamy) to produce a dikaryotic mycelium that can persist for years, spreading through soil or wood. Under favorable conditions, this dikaryotic mycelium produces a basidiocarp, and within the basidia, karyogamy finally unites the two nuclei into a brief diploid state before meiosis yields four haploid basidiospores. The basidiomycetes include mushrooms, puffballs, shelf fungi, and coral fungi, as well as the rusts and smuts that are important plant pathogens (wheat rust, corn smut). The genus Amanita contains some of the deadliest poisonous species known, including the death cap and the destroying angel, while Agaricus bisporus is the common button mushroom of grocery stores. Many basidiomycetes play irreplaceable roles in forest ecosystems by producing enzymes that degrade lignin (white-rot fungi) and cellulose (brown-rot fungi), making them the primary agents of wood decomposition and carbon cycling.
IV. Ecological Roles of Fungi
Fungi are the primary decomposers of plant material on Earth. Only fungi can efficiently degrade lignin, the toughest structural component of wood, making them essential for the return of carbon, nitrogen, and phosphorus to the soil.
Mycorrhizae, the mutualistic associations between fungi and plant roots (described in detail in Lecture 10), connect approximately 90% of all plant species to fungal partners. Beyond their role in individual plant nutrition, mycorrhizal networks -- sometimes called the "wood wide web" -- can connect trees across a forest, facilitating the sharing of nutrients and chemical warning signals between individuals.
Lichens are mutualistic partnerships between a fungus (usually an ascomycete) and a photosynthetic partner (a green alga or cyanobacterium). The fungus provides structural support, protection from desiccation, and mineral absorption, while the photosynthetic partner provides organic carbon through photosynthesis. Lichens are pioneer organisms that colonize bare rock and other inhospitable surfaces, initiating the process of soil formation. They are sensitive to air pollution, making them valuable bioindicators of air quality. Lichens grow in three forms: crustose (flat, crust-like, adhering tightly to the substrate), foliose (leafy, with lobed edges), and fruticose (shrub-like or hanging).
Endophytes are fungi that live inside plant tissues without causing visible disease. They may confer significant benefits to their hosts, including the production of toxic alkaloids that deter herbivores, enhanced drought tolerance, and increased resistance to disease.
As pathogens, fungi cause enormous agricultural losses through rusts, smuts, blights, and wilts. In humans and animals, fungal infections range from superficial conditions like athlete's foot, ringworm, and candidiasis to life-threatening systemic mycoses like histoplasmosis and aspergillosis, particularly in immunocompromised patients. Cordyceps species are parasitic fungi that manipulate insect behavior -- the so-called "zombie fungi" that control their hosts' movements to optimize spore dispersal. On a more cooperative note, leaf-cutter ants and certain termites cultivate fungal gardens as their primary food source, representing some of the most sophisticated examples of agriculture in the animal kingdom.
<image>A diagram showing the structure of a lichen and its ecological role. Panel A: Cross-section of a foliose lichen — the upper cortex (dense fungal hyphae), the algal layer (photosynthetic green algae or cyanobacteria interspersed with fungal hyphae), the medulla (loosely packed fungal hyphae), and the lower cortex with rhizines (attachment structures). Arrows show the algae providing sugars to the fungus and the fungus providing water, minerals, and protection to the algae. Panel B: Three growth forms of lichens shown on tree bark and rock — crustose (flat, paint-like), foliose (leafy, with lobed edges), and fruticose (branching, three-dimensional). Panel C: A lichen colonizing bare rock — acids secreted by the lichen break down the rock surface, beginning soil formation (primary succession).</image>
V. Fungi and Human Affairs
The relationship between fungi and human civilization is deep and multifaceted. In food production, edible mushrooms, truffles, and morels are culinary delicacies, while Penicillium roqueforti produces blue cheese, Aspergillus contributes to soy sauce production, and Rhizopus is used to make tempeh. In fermentation, Saccharomyces cerevisiae is indispensable for brewing beer and wine, baking bread, and producing bioethanol. In medicine, penicillin from Penicillium revolutionized the treatment of bacterial infections, cyclosporine from Tolypocladium made organ transplantation practical by providing immunosuppression, and statins such as lovastatin from Aspergillus lower cholesterol. In biotechnology, fungi produce industrial enzymes (amylases, cellulases), recombinant proteins, and citric acid (from Aspergillus niger). As research model organisms, Saccharomyces cerevisiae and Neurospora crassa have been central to our understanding of the cell cycle, genetics, and gene regulation. But fungi also pose dangers: mycotoxins such as aflatoxin from Aspergillus can cause liver cancer, ergot alkaloids from Claviceps cause the devastating condition known as ergotism, and consumption of poisonous mushrooms causes hundreds of deaths annually worldwide.
