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Chapter 20: Fungi

Chapter Worksheet

Ch. 20.1 Introduction to Fungi

Fungi are unicellular or multicellular eukaryotic heterotrophs that are decomposers.

Characteristics of Fungi

There are more than 100,000 species of fungi that have been identified and they are some of the largest and oldest organism on Earth.

  • Multicellular Fungi- most fungi are multicellular and were once thought to be plants.
  • Unicellular Fungi- yeasts are unicellular fungi. Candida albicans causes yeast infection in humans.

Major Features in Fungi

Features that distinguish fungi: cell walls, hyphae, and cross walls.

Cell Walls

Fungi cell walls are composed of chitin. Chitin- a strong, flexible polysaccharide that is found in the cell walls of fungi and in the exoskeletons of insects and crustaceans.


Hyphae- multicellular filaments that are the basic structural units that make up the body of a fungus.
Mycelium- a netlike mass of branching hyphae.
In a mushroom, the hyphae are tightly packed.
Fruiting body- the reproductive structure of fungi.
Hyphae form all parts of a fungus. This network of hyphae provide a large surface area for nutrient absorption.

Cross Walls

Septa- cross walls that divide hyphae into cells.
Septa have large pores that allow nutrients, cytoplasm, organelles, and nuclei to flow between cells.
Some fungi are aseptate, they have no septa. Cytoplasm flows freely through the hyphe. Many nuclei, nutrients, and other materials flow quickly through aseptate hyphae.

Nutrition in Fungi

All fungi are heterotrophs. Many fungi produce enzymes that break down organic material and then absorb them through their cell walls.

Three different methods to obtain nutrients:

  • Saprophytic Fungi- an organism that feeds on dead organisms or organic waste. Serve as decomposers and recycle nutrients back into food webs.
  • Parasitic Fungi- absorb nutrients from the living cells of another organism, called a host.
    Many produce specialized hyphae called haustoria- which grow into the host's tissues and absorb their nutrients.
  • Mutualistic Fungi- some live with another organism, such as a plant or an alga.
    In some the mycelia cover the root of a soybean plant. Fungus receives sugar from plant, plant receives extra water and mineral absorption.

Reproduction in Fungi

Fungi are classified by their structure and patterns of reproduction. Reproduction can occur asexually by: mitosis, fragmentation, budding, and spores. Some fungi reproduce asexually and sexually. Sexually produce spores by the process of meiosis.

Budding- unicellular yeast cells reproduce asexually by budding. A new cell develops while attached to the parent cell. The plasma membrane pinches off to partially separate the new individual from the parent cell.

Fragmentation- reproduction occurs when mycelium is physically broken apart, or fragmented.

Spore Production- most fungi are able to reproduce sexually and asexually by producing spores. Spore- a haploid reproductive cell with a hard outer coat.
Spores develop into a new organism without the fusion of gametes. A spore generates haploid hyphae that can fuse with the hyphae of other compatible fungi to become diploid.
Diploid hyphae can then produce fruiting bodies that will produce zygotes. The zygotes go through meiosis to produce haploid spores.

Adaptations for Survival

Most fungi produce trillions of spores, this ensures that at least a small percentage will survive.

Spores are very small, and lightweight and can be carried great distances by wind.
Spores have a cell wall that is tough and waterproof allowing it to withstand extreme conditions.


The fruiting body of a spore-forming fungus is called a sporophore.
Classification of a fungus is based primarily on the type of sporophore that it produces.
Black bread mold have sporangiophores with a spore structure called a sporangium which protects the spores.
Sporangium- a sac or case in which spores are produced.

The common names of some fungi that are descriptive of the type of sporophores the fungi produce.

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Ch. 20.2 Diversity of Fungi

Fungi are classified into four major phyla.

Classification of Fungi

Fungal structure and reproduction are used to classify fungi: Chytridiomycota, Zygomycota, Ascomycota, and Basidiomycota.
Molecular evidence supports the view that fungi are more closely related to animals than to plants.


Phylum Chytridiomycota are referred to as "chytrids". Some are saprophytes and others are parasites.
Aquatic with flagellated spores; originally thought to be related to protists.

Reclassified due to: chitin in cell walls, similar protein and DNA sequence.
May have been first fungi.

Common Molds: Phylum Zygomycota

Grows on bread and other foods; Rhizopus stolonifer.
Mostly terrestrial, some mutualistic with plants.
Stolons- hyphae that spread across the surface of food.
Rhizoids- hyphae that penetrates the food and absorbs nutrients. Also anchors the mycelium and produces digestive enzymes.

Life Cycle

Reproduce both asexually and sexually.
Asexual when sporangia form on upright hyphae called sporangiophores. The sporanguim will produce thousands of spores.

Sexual when conditions are unfavorable; plus (+) and minus (-) strains will form a gametangium and fuse. Gametangium- a reproductive structure that contains a haploid nucleus.
Haploid nuclei fuse to form a diploid zygote that develops into a dormant, thick-walled zygospore.

When conditions become favorable again zygospore will germinate and undergo meiosis to produce hyphae with a sporangium. Each haploid spore produced can form a new mycelium.
Creates genetic diversity.

Sac Fungi: Phylum Ascomycota

More than 60,000 species making it the largest phylum. Ascomycetes are called "sac fungi".
Most are multicellular but yeasts are unicellular.

Life Cycle

Reproduce both asexually and sexually.
Asexual by conidiophores- spore producing hyphae. Produce spores called conidia externally at the tips.

Sexual when hyphae from opposite mating types fuse and eventually a reproductive structure called an ascocarp. In the ascocarp haploid nuclei fuse to form a zygote which divides by meiosis to produce four haploid nuclei. These undergo mitosis and develop into spores in the ascus.
a saclike structure. Ascospores- spores produced by ascus.
Ascospores can develop into a haploid mycelium.

Club Fungi: Phylum Basidiomycota

Contains mushrooms and are called "club fungi". Can be saprophytic, parasistic, or mutualistic.
Produce enzymes that can break down lignin in wood so they function as decomposers.

Life Cycle

Rarely produce asexual spores.
Have dikaryotic mycelia (each cell has two nuclei).
Periodically reproduce sexually by forming basidiocarps. Basidiocarps- fruiting body of basidiomycetes.

Can grow quickly by cell enlargement not cell division. Underside of cap contains basidia. Basidia- club shaped hyphae that produce four haploid basidiospores.
Basidiospores- haploid spores released by basidia during reproduction.

Other Fungi

Phylum Deutormycota appear to lack a sexual reproduction so are referred to as "imperfect fungi".

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Ch. 20.3 Ecology of Fungi

Fungi and Photosynthesizers

Lichens and mycorrhizae are two examples of mutualistic relationships between fungi and other organisms.


Lichens- a symbiotic relationship between a fungus and an alga or a photosynthetic partner.
The fungus usually is an ascomycete, but lichens also may contain basidiomycetes.
The photosynthetic partner is either a green alga or cyanobacterium, which provides food for both organisms. The fungus provides a dense web of hyphae to grow in.

Diversity of Lichens

Lichens need only light, air, and minerals to grow. Lichens are found in some of the harshest environments.
Lichens absorb needed minerals from rainwater or dust in the air.

The majority of lichens grow in temperate or arctic areas, forming primary ground cover on the tundra. Caribou utilize lichenase to help digest the lichen.

Lichens survive sever drought and can dry out, break off, blow away, and start growing in another location.

Lichens are a pioneer species. Growing in areas with little soil or on rocks. Acids produced by the fungal portion help penetrate and break down rocks to form soil. As lichens become established, they trap soil and fix nitrogen, which helps plants colonize the new soil.

Lichens as Bioindicators

Lichens are found in rural areas where there is little or no air pollution. When air pollution levels rise, lichens die since they absorb much of their water and minerals from the air and rain.

Bioindicator- a living organism that is sensitive to changes in environmental conditions and is one of the first organisms to respond to changing conditions.


Mycorrhizae- a symbiotic relationship between a specialized fungus and plant roots.
Plants with mycorrhizae are healthier and more vigorous than similar plants that lack mycorrhizae.
The fungus absorbs and concentrates various minerals for the plant; additionally the hyphae increase the plant's root surface area for water and mineral absorption.
In return, the fungus receives carbohydrates and amino acids from the plant.

80-90% of plants, have mycorrhizae.

Fungi and Humans

Fungi are important as decomposers, recycling nutrients in dead organisms as well as preventing dead organisms and their wastes from littering the Earth.

Beneficial Fungi

Fungi have many medical uses: Penicillium notatum is a source of penicillin; Claviceps purpurea used for migraines, high blood pressure, and more; Tolypocladium inflatum for immune suppressant drug given to organ transplant patients to keep their bodies from rejecting new organs.


Many of the foods we eat are made from fungi or fungal products: mushrooms, truffles, cheeses (Brie, Camembert, Roquefort).
Yeast makes bread rise by releasing carbon dioxide gas during fermentation.


A process in which fungi are mixed with water or soil, where they decompose organic materials in pollutants. The pollutants are broken down into harmless substances.

Harmful Fungi

Some fungi can be harmful to other organisms. Some can destroy crops while others can parasitize humans and other animals. Cordyceps militaris can infect the larvae and pupae of butterflies and moths.
Athlete's foot, ringworm, yeast infections, and oral thrush are infections in humans that are caused by fungi.

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Page last updated April 3, 2017.