AP Biology
Notes: Fungus

Introduction

        Ecosystems would be in trouble without fungi to decompose dead organisms, fallen leaves, feces, 
and other organic materials. This decomposition recycles vital chemical elements back to the
 environment in forms other organisms can assimilate.

 Fungi

·        Fungi are eukaryotes and most are multicellular.

·        While once grouped with plants, fungi generally differ from other eukaryotes
 in nutritional mode, structural organization, growth, and reproduction.

·        Molecular studies indicate that animals, not plants, are the closest relatives of fungi.

 Absorptive nutrition: 

·        Fungi are heterotrophs that acquire their nutrients by absorption.

·        They absorb small organic molecules from the surrounding medium.

·        Exoenzymes, powerful hydrolytic enzymes secreted by the fungus, break
 down food outside its body into simpler compounds that the fungus can 
  absorb and use.

·        The absorptive mode of nutrition is associated with the ecological roles of fungi 
  as decomposers (saprobes), parasites, or mutualistic symbionts.

·        Saprobic fungi absorb nutrients from nonliving organisms.

·        Parasitic fungi absorb nutrients from the cells of living hosts.

·        Some parasitic fungi, including some that infect humans and plants, are pathogenic.

·        Mutualistic fungi also absorb nutrients from a host organism, but they 
 reciprocate with functions that benefit their partner in some way.

Extensive surface:

·        The vegetative bodies of most fungi are constructed of tiny filaments called hyphae 
  that form an interwoven mat called a mycelium.

·        Fungal mycelia can be huge, but they usually escape notice because they are subterranean.

·        One giant individual of Armillaria ostoyae in Oregon is 3.4 miles in diameter and covers 2,200 acres of forest,

·        It is at least 2,400 years old, and weighs hundreds of tons.

·        Fungal hyphae have cell walls.

·        These are built mainly of chitin, a strong but flexible nitrogen-containing polysaccharide, identical to that found in arthropods.

·        Most fungi are multicellular with hyphae divided into cells by cross walls, or septa.

·        These generally have pores large enough for ribosomes, mitochondria, 
  and even nuclei to flow from cell to cell.

·        Fungi that lack septa, coenocytic fungi, consist of a continuous cytoplasmic 
  mass with hundreds or thousands of nuclei.

·        This results from repeated nuclear division without cytoplasmic division.

·        Parasitic fungi usually have some hyphae modified as haustoria, 
 nutrient-absorbing hyphal tips that penetrate the tissues of their host.

·        Some fungi even have hyphae adapted for preying on animals.

·        The filamentous structure of the mycelium provides an extensive surface
 area that suits the absorptive nutrition of fungi.

·        The fungal mycelium grows rapidly, adding as much as a kilometer of hyphae each day.

·        The fungus concentrates its energy and resources on adding hyphal length 
  and absorptive surface area.    

Reproduction:

·        Fungi reproduce by releasing spores that are produced either sexually or asexually.

·        The output of spores from one reproductive structure is enormous, with 
  the number reaching into the trillions.

·        Dispersed widely by wind or water, spores germinate to produce mycelia
  if they land in a moist place where there is food

 Diversity of Fungi:

·        More than 100,000 species of fungi are known and mycologists estimate that 
there are actually about 1.5 million species worldwide.

·        Molecular analysis supports the division of the fungi into four phyla.

  Phylum Chytridiomycota: 

·        The chytrids are mainly aquatic.

·        Some are saprobes, while others parasitize protists, plants, and animals.

·        The presence of flagellated zoospores had been used as evidence 
 for excluding chytrids from kingdom Fungi which lack flagellated cells.

·        However, recent molecular evidence supports the hypothesis that 
 chytrids are the most primitive fungi.

·        Like other fungi, chytrids use an absorptive mode of nutrition and
  have chitinous cell walls.

·        While there are a few unicellular chytrids, most form coenocytic hyphae.

·        Some key enzymes and metabolic pathways found in chytrids are shared 
 with other fungal groups, but not with the so-called funguslike protists.

Phylum Zygomycota: 

·        Most of the 600 zygomycete, or zygote fungi, are terrestrial, living in
 soil or on decaying plant and animal material.

·        One zygomycete group form mycorrhizae, mutualistic associations 
 with the roots of plants.

·        Zygomycete hyphae are coenocytic, with septa found only in 
 reproductive structures.

·        The life cycle and biology of Rhizopus stolonifer, black bread mold, is typical of zygomycetes.

·        Horizontal hyphae spread out over food, penetrate it, and digest nutrients.

·        In the asexual phase, hundreds of haploid spores develop in sporangia at the tips of upright hyphae.

·        If environmental conditions deteriorate,  this species of Rhizopus reproduces sexually.

·        Plasmogamy of opposite mating types  produces a zygosporangium.

·        Inside this multinucleate structure, the heterokaryotic nuclei fuse to form diploid nuclei that undergo meiosis.

·        The zygomycete Rhizopus can reproduce either asexually or sexually.

·        The zygosporangia are resistant to freezing and drying.

·        When conditions improve, the zygosporangia release haploid spores that colonize new substrates.

  Phylum Ascomycota: (Sac fungi) 

·        Mycologists have described over 60,000 species of ascomycetes, or sac fungi.

·        They range in size and complexity from unicellular yeasts to elaborate cup fungi and morels.

·        Ascomycetes live in a variety of marine, freshwater, and terrestrial habitats.

·        Some are devastating plant pathogens.

·        Many are important saprobes, particularly of plant material.

·        About half the ascomycete species live with algae in mutualistic associations called lichens.

·        Some ascomycetes form mycorrhizae with plants or live between mesophyll cells in
  leaves where they may help protect the plant tissue from insects by releasing toxins.

·        The defining feature of the Ascomycota is
 the production of sexual spores in saclike asci.

·        In many species, the spore-forming asci are collected 
 into macroscopic fruiting bodies, the ascocarp.

·        Examples of ascocarps include the edible parts 
 of truffles and morels.

·        Ascomycetes reproduce asexually by producing 
  enormous numbers of asexual spores, which are
   usually dispersed by the wind.

·        These naked spores, or conidia, develop in long chains 
or clusters at the tips of specialized hyphae called conidiophores.

·        Ascomycetes are characterized by an 
  extensive heterokaryotic stage during 
  the formation of ascocarps.  

·        1) The sexual phase of the ascomycete lifestyle begins when haploid mycelia of opposite mating types become intertwined and form an antheridium and ascogonium.

·        2) Plasmogamy occurs via a cytoplasmic bridge and haploid nuclei migrate from the antheridium to the ascogonium, creating a heterokaryon.

·        3) The ascogonium produces dikaryotic hyphae that develop into an ascocarp.

·        4) The tips of the ascocarp hyphae are partitioned into asci.

·        5) Karyogamy occurs within these asci and the diploid nuclei divide by meiosis, (6) yielding four haploid nuclei.

·        7) Each haploid nuclei divides once by mitosis to produce eight nuclei, often in a row, and cell walls develop around each nucleus to form ascospores.

·        8) When mature, all the ascospores in an ascus are dispersed at once, often leading to a chain reaction of release, from other asci.

·        9) Germinating ascospores give rise to new haploid mycelia.

·        10) Asexual reproduction occurs via conidia.

Phylum Basidiomycota: (Club fungi)

·        Approximately 25,000 fungi, including mushrooms, shelf fungi, puffballs, and rusts, are 
 classified in the phylum Basidiomycota.

·        The name of the phylum is derived from the basidium, 
 a transient diploid stage.

·         Of all fungi, these are the best at decomposing the complex polymer lignin, abundant in wood.

·        Two groups of basidiomycetes, the rusts and smuts, include particularly destructive plant parasites.

·        The life cycle of a club fungus usually includes a long-lived dikaryotic mycelium.

(1) Two haploid mycelia of opposite mating type undergo plasmogamy, 
(2) creating a dikaryotic mycelium that ultimately crowds out the haploid parents.
(3) Environmental cues, such as rain or temperature change, induce the 
      dikaryotic mycelium to form compact masses that develop into basidiocarps.
(4) The surface of the basidiocarp’s gills is lined with terminal dikaryotic cells called basidia.
(5) Karyogamy produces diploid nuclei which then undergo meiosis, 
(6) each yielding four haploid nuclei.
(7) When mature, the basidiospores are propelled slightly by electrostatic 
      forces into the spaces between the gills and then dispersed by the wind.
(8) The basidiospores germinate in a suitable habitat and grow into a short-lived haploid mycelia.

* A billion sexually produced basidiospores may be produced by a single store-bought mushroom.

·        The cap of the mushroom supports a huge surface area of basidia on gills.

·        These spores drop beneath the cap and are blown away.  

Deuteromycetes: (imperfect fungi)

mold:

·        The mycelia of these fungi grow as saprobes or parasites on a variety of substrates.

·        Early in life, a mold, a term that applies properly only to the asexual stage, produces asexual spores.

·        Later, the same fungus may reproduce sexually, producing zygosporangia, ascocarps, or basidiocarps.

        ·        Most  molds cannot be classified as zygomycetes, ascomycetes, or basidiomycetes because
         they have no known sexual stages.

Yeasts:

·        Yeasts reproduce asexually by simple cell division or budding off a parent cell.

·        Some yeast reproduce sexually, forming asci (Ascomycota) or basidia (Basidiomycota), but others have no known sexual stage (imperfect fungi).

·        Humans have used yeasts to raise bread or ferment alcoholic beverages for thousands of years.

·        Various strains of the yeast Saccharomyces cerevisiae, an ascomycete, have been developed as baker’s yeast and brewer’s yeast.

·        Baker’s yeast releases small bubbles of CO₂ that leaven dough.

·        Brewer’s yeast ferment sugars into alcohol.

·        Some yeasts cause problems for humans.

·        A pink yeast, Rhodotorula, grows on shower curtains and other moist surfaces in our homes.

·        Another yeast, Candida, is a normal inhabitant of moist human epithelial surfaces, such as the vaginal lining.

·        An environmental change, such as a change in pH or compromise to the human immune system, can cause Candida to become pathogenic by growing too rapidly and releasing harmful substances.

  lichens:.

·        The fungal component is commonly an ascomycete, but several basidiomycete lichens are known.

·        The photosynthetic partners are usually unicellular or filamentous green algae or cyanobacteria.

·        The merger of fungus and algae is so complete that they are actually given genus and species names, as though they were single organisms.

·        Over 25,000 species have been described.

·        The fungal hyphae provides most of the lichen’s mass and gives it its overall shape and structure.

·        The algal component usually occupies an inner layer below the lichen surface.

·        Lichen algae reproduce independently by asexual cell division.

·        Lichens are important pioneers on newly cleared rock and soil surfaces, such as burned forests and volcanic flows.

·        The lichen acids penetrate the outer crystals of rocks and help break down the rock.

·        This allows soil-trapping lichens to establish and starts the process of succession.

·        Nitrogen-fixing lichens also add organic nitrogen to some ecosystems.

 Some fungi are pathogens

·        About 30% of the 100,000 known species of fungi are parasites, mostly on or in plants.

·        Invasive ascomycetes have had drastic effects on forest trees, such as American elms and American chestnut, in the northeastern United States.

·        Other fungi, such as rusts and ergots, infect grain crops, causing tremendous economic losses each year.

·        Some fungi that attack food crops produce compounds that are harmful to humans.

·        For example, the mold Aspergillus can contaminate improperly stored grains and peanuts with aflatoxins, which are carcinogenic.

·        Poisons produced by the ascomycete Claviceps purpurea can cause gangrene, nervous spasms, burning sensations, hallucinations, and temporary insanity when infected rye is milled into flour and consumed.

·        On the other hand, some toxins extracted from fungi have medicinal uses when administered at weak doses.

·        Animals are much less susceptible to parasitic fungi than are plants.

·        Only about 50 fungal species are known to parasitize humans and other animals, but their damage can be disproportionate to their taxonomic diversity.

·        The general term for a fungal infection is mycosis.

·        Infections of ascomycetes produce the disease ringworm, known as athlete's foot when they grow on the feet.

·        Inhaled infections of other species can cause tuberculosis-like symptoms.

·        Candida albicans is a normal inhabitant of the human body, but it can become an opportunistic pathogen.

  Fungi are commercially important

·        In addition to the benefits that we receive from fungi in their roles as decomposers and recyclers of organic matter, we use fungi in a number of ways.

·        Most people have eaten mushrooms, the fruiting bodies (basidiocarps) of subterranean fungi.

·        The fruiting bodies of certain mycorrhizal ascomycetes, truffles, are prized by gourmets for their complex flavors.

·        The distinctive flavors of certain cheeses come from the fungi used to ripen them.

·        The ascomycete mold Aspergillus is used to produce citric acid for colas.

·        Yeasts are even more important in food production.

·        Yeasts are used in baking, brewing, and winemaking.

·        Contributing to medicine, some fungi produce antibiotics used to treat bacterial diseases.

·        In fact, the first antibiotic discovered was penicillin, made by the common mold Penicillium.