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Chapter 31

BIO153H5 Chapter Notes - Chapter 31: Microsporidia, Cyanobacteria, Glomeromycota

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Christoph Richter

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Chapter 31: Fungi
Key Concepts
o Fungi are important in part because many species live in close association with land plants. They
supply plants with key nutrients and decompose dead wood. They are the master recyclers of
nutrients in terrestrial environments.
o All fungi make their living by absorbing nutrients from living or dead organisms. Fungi secrete
enzymes so that digestion takes place outside their cells. Their morphology provides a large amount
of surface area for efficient absorption.
o Many fungi have unusual life cycles. It is common for species to have a long-lived heterokaryotic
stage, in which cells contain haploid nuclei from two different individuals. Although most species
reproduce sexually, very few species produce gametes.
Why Do Biologists Study Fungi?
Fungi nourish the plants that nourish us. They affect global warming, because they are critical to the
carbon cycle on land. A handful of species can cause debilitating diseases in humans and crop plants.
Fungi Provide Nutrients for Land Plants
Mycorrhizal associations between fungi and plant roots allow faster plant growth (Figure 31.1).
Fungi Provide Nutrients for Land Plants
Saprophytes are fungi that make their living by digesting dead plant material.
They play a key role in carbon cycling.
The carbon cycle has two basic components: (1) the fixation of carbon by land plants, and (2) the
release of CO2 from plants, animals, and fungi as the result of cellular respiration (Figure 31.3).
For most carbon atoms, fungi connect the two components.
Fungi Provide Nutrients for Land Plants
Although parasitic fungi cause athlete’s foot, vaginitis, diaper rash, ringworm, pneumonia, and
thrush in humans, the incidence of fungal infections in humans is very low.
Their major destructive impact is on crops (Figure 31.4).
Fungi Are Key Model Organisms in Eukaryotic Genetics
The yeast Saccharomyces cerevisiae is very important in basic research on cell biology and
molecular genetics.
It is easy to culture and manipulate in the lab, grows rapidly, and mutants can easily be created and
transferred among individuals.
How Do Biologists Study Fungi?

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About 80,000 species of fungi have been described and named; about 1000 more are discovered each
The fungi are so poorly studied that the known species are widely regarded as a tiny fraction of the
real total, estimated at 1.65 million species or even higher.
Analyzing Morphological Traits
Fungi have very simple bodies. Two growth forms exist: (1) singlecelled forms (yeasts) and (2)
multicellular filamentous forms (mycelia) (Figure 31.5).
Some fungi adopt both lifeforms.
The Nature of the Fungal Mycelium
Mycelia grow out in the direction of food sources and die back in areas where food is running out.
The filaments that make up a mycelium are called hyphae, and most are haploid
or heterokaryotic, with two haploid nuclei (Figure 31.6).
Each filament is separated by celllike compartments called septa.
Because mycelia are composed of branching hyphae, the body of a fungus has a high surfacearea
tovolume ratio.
This makes absorption extremely efficient but also makes fungi prone to drying out.
Mycelia are an adaptation to the absorptive lifestyle of fungi. Thus, reproductive organsnot
feeding structuresare the only thick, fleshy structures that fungi produce.
Reproductive Structures
There are four major groups of fungi based on reproductive structures (Figure 31.7): the
Chytridiomycota, the Zygomycota, the Basidiomycota, and the Ascomycota.
Chytridiomycota (chytrids) live primarily in water and have spores and gametes with flagella.
Zygomycota have haploid hyphae of different mating types. Hyphae of different mating types may
become yoked together and the cells of the hyphae fuse to form a sporeproducing structure called
a zygosporangium.
Basidiomycota, or club fungi, have basidia that form at the ends of hyphae and produce spores.
Mushrooms, bracket fungi, and puffballs are among the complex reproductive structures this group
Ascomycota, or sac fungi, produce complex reproductive structures. The tips of hyphae inside these
structures produce distinctive saclike cells, called asci, that generate spores.
Evaluating Molecular Phylogenies
Fungi are more closely related to animals than land plants (Figure 31.8).
The Chytridiomycota, Zygomycota, Ascomycota, and Basidiomycota have traditionally been
recognized as separate phyla (Figure 31.9).
Chytrids are the most basal group of fungi.
Chytridiomycota and Zygomycota are paraphyletic. A single common ancestor did not give rise to
all species within each phylum.
Microsporidians are phylogenetically within the fungi.
The Basidiomycota and Ascomycota are monophyletic.
Experimental Studies of Mutualism
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