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Fungi
Chapter 31
By: Katie Quinlan
Absorptive nutrition enables fungi to live as
decomposers and symbionts
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Fungi are heterotrophs that acquire their nutrients by absorption.
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Three types of fungi:
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Small organic molecules are absorbed from the surrounding medium
Fungus digests food outside its body by secreting powerful hydrolytic enzymes
into the food
Enzymes decompose complex molecules to the simpler compounds that the
fungus can absorb and use
Saprobic fungi: absorb nutrients from nonliving organic material
Parasitic fungi: absorb nutrients from the cells of living hosts
Mutualistic fungi: absorb nutrients from a host organism, but they reciprocate
with functions beneficial to their partners
Most common in terrestrial habitats; some inhabit aquatic environment.
Extensive surface area and rapid growth adapt
fungi for absorptive nutrition
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The bodies of fungi are constructed of units called hyphae
 Hyphae are minute threads composed of tubular walls surrounding plasma
membranes and cytoplasm.
 Hyphae form a interwoven mat called a mycelium (the “feeding” network of a
fungus
Most fungi are multicellular with hyphae divided into cells by cross-walls, or septa.
 Generally have large pores large enough to allow ribosomes, mitochondris, and
nuclei to flow from cell to cell
Most build their cell walls of chitin (strong but flexible nitrogen-containing
polysaccharide)
Some fungi are aseptate (hyphae are not divided into cells by cross-walls)
 Known as coenocytic (consist of continuous cytoplasmic mass with hundreds or
thousands of nuclei)
Parasitic fungi usually have some of their hyphae modified as haustoria (nutrientabsorbing hyphal tips that penetrate the tissues of the host)
Characteristics of fungal hyphae
Fungi reproduce by releasing spores that are
produced either sexually or asexually
Chytrids may provide clues about
fungal origins
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Chytrids may be a link between fungi and protists
Mainly aquatic
 Some are saprobes, others parasitize protists, plants, and
aquatic invertebrates
 They are an absorptive mode of nutrition and cell walls
made of chitin
Form coenocytic hyphae, although some are unicellular
Have some key enzymes and metabolic pathways that are
common among fungi
Most primitive fungi ( they belong to the lineage that diverged
earliest in the phylogeny of fungi)
Phylogeny of fungi
Zygomycota
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These fungi are mostly terrestrial and live in soil or on decaying
plant and animal material
One group or major importance forms mycorrhizae (mutualistic
associations with the roots of plants)
Zygomycete hyphae are coenocytic, with septa found only
where reproductive cells are formed.
Common zygonycete is black bread mold
Life cycle of black bread mold
Ascomycota: Sac fungi produce sexual
spores in saclike asci
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Ascomycetes, or sac fungi, have been described from a wide
vafriety of marine, freshwater, and terrestrial habitats.
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Range in size and complexity
Include some of the most devastating plant pathogens
Many are important saprobes (particularly of plant material)
Half live with algae in symbiotic associations called lichens
Some form mycorrhizae with plants
Others live in leaves on the surface of mesophyll cells, where the
fungi apparently help protect these plant tissues from insects by
releasing toxic compounds.
Life cycle of an ascomycete
Basidiomycota: Club fungi have long-lived
dikaryotic mycelia and a transient diploid stage
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Name derives from the basidium, a transient diploid stage in the organism’s life
cycle
Clublike shape of the basidium also gives rise to the common name club fungus.
Improtant decomposers of wood and other plant material.
Division includes mycorrhiza-forming mutualists and plant parasites
Best at decomposing the complex polymer lignin, an abundant component of
wood
Two groups:
 Rusts
 Smuts
Reproduces sexually by producing elaborate friuting bodies called
basidiocarps
 An example is a mushroom
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Yeasts
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Yeasts are unicellular fungi that inhabit liquid or moist habitats.
Reproduce asexually, by simple cell division or by pinching of small
“bud cells” off a parent cell.
Some reproduce sexually, by forming asci or basidia (classified as
Ascomycota or Basidiomycota)
Lichens
Molds
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A rapidly growing, asexually reproducing fungus
Mycelia of these fungi grow as saprobes or parasites on a great
variety of substrates.
Mold produces asexual spores
Same fungi may reproduce, producing zygosporangia,
ascocarps, or basidiocarps.
Molds can be classified as zygomycetes, ascomycetes, or
basidiomycetes because of known sexual stages.
Those are collectively called deuteromycetes (imperfect fungi).
These reproduce asexually by producing spores.
Penicillium (fermenters)