ch31 (2)

Chapter 31

FUNGI

Fungi are eukaryotic organisms with a characteristic nutritional mode, structural organization, growth and reproduction. They play an important role in nature as decomposers of the dead bodies of plants and animals, and in the recycling of nutrients. Plants depend on mutualistic fungi that help their roots absorb minerals and water from the soil. Fungi are also the source of antibiotics and drugs used in medicine. Fungi acuire their nutrients by absorption. Fungi digest food outside their bodies by secreting hydrolytic enzymes called exoenzymes , and breaking down comple! molecules into simpler compounds that the fungus can absorb and use. "aprobic fungi also known as decomposers, absorb nutrients from dead organisms or their fallen parts, e. g. bark, fallen lea#es, dead animals, fallen fruits, etc. Parasitic fungi absorb their nutrients from the tissues of their li#ing hosts. Parasitic fungi are #ery harmful to agriculture. $utualistic fungi absorb nutrients from their li#ing partners but pro#ide their partners with some beneficial nutrient or other function.

CHARACTERISTICS OF THE KINGDOM 1. $ultic $ulticell ellula ularr or unicel unicellul lular ar eukaryo eukaryotes tes.. • • •

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The filaments forming the fungal body are called hyphae %hypha&. The network of hyphae is called the mycelim! 'yphae may or may not be di#ided by cross(walls called sep"a. "epta usually ha#e pores that allow the flow of organelles from one cell to another. Coenocytic fungi lack septa and are multinucleated. Parasitic fungi ha#e hyphae modified into haustoria that penetrate the cells of the host to absorb nutrients.

). Cell Cell wall wall includin including g that that of spore sporess is made made of chi"in, a polymer of a nitrogen(containing sugar. • •

Chitin is also found in the e!oskeleton of arthropods. Chitin is #ery resistant to microbial degradation.

3. $ost reprodu reproduce ce se!ually se!ually and ase!uall ase!uallyy by means means of of spores. spores. • • •

"pores are produced by mitosis or zygotic meiosis. "pores may be produced se!ually or ase!ually. "pores are nonmotile e!cept in chytrids.

*. 'aploid 'aploid organisms organisms with the diploid diploid phase phase of the life cycle cycle represented represented only by the the zygote. zygote. • •

'yphae may be mono#a$yo"ic , %i#a$yo"ic  or coenocy"ic . 'yphae from different fungi may fuse and their mycelia may ha#e genetically different nuclei. These mycelia are called he"e$o#a$yons .

+. ack chlorophy chlorophyll, ll, are heterotrop heterotrophs hs that absorb absorb nutrient nutrientss from the the en#ironment. en#ironment. • • •

-!tracellular digestion by secreting hydrolytic enzymes and acids on the food source. eleased molecules are absorbed mostly near the growing hypha tip. "aprophytes, parasites and symbionts.

/. "tore "tore food food in the the form form of lipid lipids s and glycog glycogen. en. •

0lycogen is a glucose polysaccharide found also in animals.

&IFE C'C&E "ome fungi reproduce only ase!ually, while others are entirely se!ual. $any ha#e a se!ual and an ase!ual mode of reproduction.  se!ual spores are produced in spo$an(ia  or from coni%io(enos cells . •

Coni%ia may be produced singly or in chains.

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The stalk on which the conidia are produced is called coni%iopho$e .

"e!ual spores are produced in three stages2 plasmogamy, karyogamy and meiosis. • •

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)lasmo(amy  refers to the fusion of two hyphae %conugation&. Ka$yo(amy  is the fusion of nuclei. 4t may occur immediately after conugation or may be delayed producing a %i#a$yon , a cell with two nuclei.

5ikaryotic cells may e!ist for months and years, and multiply producing more dikaryotic cells. Meiosis  follows karyogamy sooner or later reestablishing the haploid condition. $eiosis results in the formation of specialized spores2 zy(ospo$es , ascospo$es* +asi%iospo$es . Fungi are often classified according to the types of se!ual spores that are produced

"pore 6 germination 6 mycelium 6 plasmogamy 6 heterokaryotic stage 6 karyogamy 6 zygote 6 meiosis 6 spore producing structures 6 spore... 7see fig. 31.3, page /189

DI,ERSIT' OF FUNGI  bout 1::, ::: species are known known and about 1,;:: new species are described e#ery year. year. The total number of species is estimated to be about 1.+ million.

potato wart disease? is caused by a chytrid. =ne class is unicellular@ other three classes form mycelium. =nly fungal group that produces pr oduces motile reproducti#e cells2 zoospo$es  %ase!ual& and gametes. Protein and nucleic acid seuences support their inclusion in the kingdom Fungi in spite of ha#ing motile spores, zoospores. $ostly coenocytic with a few septa at maturity that separates the reproducti#e organs. 'yphae of some chytrids ha#e pseudosepta2 incomplete partitions made of substances different from cell wall material. Contain chitin in their cell wall, some contain cellulose as well. 0ametes could be similar %isogametes& or different, one at least motile.  bout 1::: species. A=T-2 Some taxonomists consider the chytrids to be protists due to the production of motile cells. These scientists consider members of the Fungi those species that do not produce motile spores.

)hylm -y(omyco"a  %zygomycetes& •

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$ulticellular land fungi li#ing in the soil or decaying organic matter. $ostly $ostly saprophytic some parasitic on plants and insects. 5i#ided into three to se#en classes according to different schemes of classification. =ne group forms myco$$hizae  with the roots of plants. Coenocytic mycelium with septa separating the reproducti#e structures.  se!ual spores formed in sporangia. "e!ual spores formed the fusion of hypha endings called gametangia. The fused tips de#elop into a zy(ospo$an(im , which in turn will produce zy(ospo$es . $ating types or strains reuired in some for conugation to occur2 he"e$o"hallic  species. Chitin in cell wall@ no cellulose present.  bout /:: species described. The common bread mold is a zygomycete. zygomycete.

)hylm Ascomyco"a  %sac fungi& • • • •

$ulticellular or unicellular fungi. Terrestrial fungi Four classes recognized by many mycologists. 'yphae are narrower than the Bygomycota and are generally septate and multinucleated.

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Chitin present in cell wall@ cellulose absent.  se!ual spores are conidia produced at a hypha hypha tip, the coni%iopho$e . This phylum has a dikaryon in the ascogenous hyphae. This phylum is characterized by a se!ual state composed of ascospo$es  within sac(like asci %sing. ascus&. 'aploid mycelia form an"he$i%ia %& and asco(onia  %D&. Plasmogamy occurs between them. Earyogamy is delayed. The ascogonium gi#es rise to dikaryotic hyphae that are incorporated into the ascocarp. The asci %sing. ascus& are contained within %or on& a #ariety of ascoca$ps! Earyogamy occurs within the asci. 4mportant parasites and saprophytes. easts easts are unicellular ascomycetes. $any edible species. ichens are a mutualistic association of an algae and a fungus. bout half of the ascomycetes are in#ol#ed in a lichen association or in mycorrhizae.  bout /:,::: species described.

)hylm .asi%iomyco"a  %club fungi& • • • • • • •

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$ulticellular fungi. Terrestrial fungi. Two Two or three classes are recognized.