By Sarah Izzo
(ORS 18)
Diagnostic Characteristics of Fungi:

  1. All fungi are eukaryotic. This means fungi are made up of eukaryotic cells, which have a membrane-enclosed nucleus and membrane-enclosed organelles.
  2. Most fungi are multicellular heterotrophs. Heterotrophs are organisms that obtain organic food molecules by eating other organisms or their by-products.
  3. Fungi acquire nutrients through absorption, where small organic molecules are absorbed into the fungi from its surroundings. This absorption’s productivity is maximized through a large surface area.
  4. Fungi are decomposers. They break down the organic material they absorb from their surroundings.
  5. Fungi can be pathogenic, parasitic, or mutualistic. Parasitic fungi absorb nutrients from the cells of their hosts. Pathogenic fungi cause disease in humans or other animals. Mutualistic fungi absorb nutrients from their host organism, but also benefit their host in some way.
  6. Fungi reproduce and disperse through releasing spores, cells that divide through mitosis, eukaryotic cell division, and produce a multicellular organism.

Fungi are the main decomposers that continue to supply ecosystems with inorganic nutrients, which are essential to plant growth. Fungi have a wide range of habitats. Roughly 30 percent of fungi’s total population, act as living parasites on or in plants. These fungi can severely affect the environment by killing entire species of plants. Only 50 species of fungi have been discovered to be parasitic in humans or other animals. Fungi that are parasitic to humans can cause many illnesses, ranging from ringworm and athletes food to some deadly lung diseases.

Fungi can be found in almost every habitat on the planet. Some can be found in the woods and in meadows, while others can be found in open plains of grass, animal feces, woodlands, seawater, and in soil. (CC) (17

Major Types:
There are over 100,000 fungi species known to man; mycologists, biologists who study fungi, estimate that we have yet to discover 1.4 million species of fungi. There are four main phyla in the kingdom of fungi: chytridiomycota, zygomycota, ascomycota, and basidiomycota.
Chytrids are mainly aquatic fungi. They are the only phyla in the fungi kingdom with flagellated cells. Chytrids reproduce through moblie spores with flagella. Chytrids were originally thought to be part of the protista kingdom, because scientists believed that all fungi lacked flagella. Further molecular comparisons showed strong support for the idea that chytrids are a fungus. Chytrids are also thought to be the most primitive fungi. This means that chytrids, on an evolutionary scale, were the earliest to diverge from protists. This also shows that fungi evolved from protists that had flagella, and chytrids were the only fungi phyla to retain this trait.
Chytridiomycota, commonly known as chytrids, has a pot-like structure that contain the unreleased spores. They produce mobile zoospores for the purpose of propagation. The movement of these spores is facilitated by the single flagellum present on their body. Chytrids are quite distinct from the other divisions of fungi and composed of four main clades.(14)(ZS)


external image gymrust.jpg(MC)

Zygote fungi are mostly terrestrial, living in soil or on decaying plant an animal material. An important example of zygote fungi is the mycorrhizae, which forms a mutualistic relationship with the roots of plants. Zygote fungi include many molds, which rapidly grow and reproduce through asexual reproduction. Asexual reproduction is reproduction with only one parent. Rhizopus stolonifer, a zygote fungus, is a frequent household mold that grows on bread.

Ascomycota are also known as sac fungi, have a variety of marine, freshwater, and terrestrial habitats. They range from unicellular yeasts to cup fungi. Half of the ascomycete species live with algae, forming a mutualistic relationship that develops into lichens. The ascomycota phyla reproduces by produces spores in sacs called asci, giving ascomycota the nickname "sac fungi".
In the ascus, one round of mitosis typically follows meiosis to leave 8 nuclei and eventually 8 ascospores. Ascospores are formed within the ascus by an enveloping membrane system that packages each nucleus with its adjacent cytoplasm and provides the site for ascospore wall formation. (AK) (20)

Basidiomycta, also known as club fungi, include mushrooms, shelf fungi, puffballs, and rusts. This phyla produces sexual spores in club-shaped structures, giving it the nickname "club fungi". Basidiomycota play an important role in decomposing wood and other plant material. Club fungi reproduce sexually by producing fruiting bodies call basidiocarps. A mushroom is an example of this fruiting body. Basidiocarps also include some yeast, unicellular fungi that inhibit liquid or moist habitats. Yeast is often used to raise bread or ferment alcoholic beverages.
Yeast is a unicellular type of fungi. This is a popular ingredient in baking, used to ferment sugar to produce ethanol and carbon dioxide which causes the baked good to rise. Besides commonly living on plants, yeasts can also share a symbiotic relationship with an animal, sometimes acting as a parasite. Yeasts multiply as single cells through budding or direct division. In sexual reproduction, a single diploid cell will undergo meiosis and form 4 haploid cells which are then tightly packed together and spores form around these cells, which are referred to as asci. Yeast has also been used to make alchol from wheat, barley, and rice. (NG)
Basic Anatomy:
Most fungi are made up of tiny filaments called hyphae, with unicellular yeasts being the exception. Hyphae are composed of tubular walls that surround plasma membranes and cytoplasm. The hyphae interweave to create a mat called mycelium. This mycelium can range from inches long to miles long in diameter.
Fungi hyphae are divided by septa, or cross walls. These septa have pores large enough for ribosomes mitochondria, and even nuclei to move from cell to cell. Some fungi have mycelium that are not divided by septa. These are called aseptate. Some types of fungi have a body structure that changes between mycelium and yeast. These are said to be dimorphic, or in other words two forms. (3,4,5)(RL) Fungi build cell walls out of chitin and strong yet flexible polysaccharide.
Myceliums grow rapidly, adding up to a kilometer of hyphae per day. By increasing the amount of hyphae and the length of mycelium, the surface area increases rapidly as well. By increasing surface area, the nutrient absorption increases as well. Mycelium grow from the ends of the hyphae and are usually hidden in the ground.(KL)(12)
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More on the anatomy of a fungus (GR)(11)

external image Plate-II-Mycelium-And-Early-Vegetation-Of-A-Mushroom.jpg

The mycelium of a fungus. (NI) 19

Transport Materials:
Fungi acquire nutrients through absorption. Fungi digest food outside their body by secreting powerful hydrolytic enzymes into their food. These enzymes are called exoenzymes. The mycelium of a fungus is referred to as the “feeding network” because they absorb nutrients. "Fungi will absorb the food if the food is soluble. They will secrete the hydrolytic enzymes to break down more complex food molecules into ones that can be easily absorbed." (SD) (21)

Fungi have an intimate contact with its surroundings because of its filamentous growth. The filamentous growth causes the surface area of fungi to be much larger than the volume of the fungi. This causes nutrients to diffuse easier into the hyphae. However, it also causes the fungi to be more vulnerable to ion imbalance as well as dessication. This requires the majority of fungi to grow within a moist substrate. (8)(MF)

Fungi obtains energy for metabolism and biosynthesis from carbon by the way of symbiosis, parasitism, or saprophytism, the growing on dead organisms. Soluble carbohydrates must enter hyphae by diffusionand then travel through the fungal membrane. In such saprophytic fungi most the carbon in the environment is present as a complex polymer like cellulose, or lignin. Before these materials can be used, they must be broken down by enzymes. For this reason, fungi releases degradative enzymes into their environments such as cellulases, chitinases, and multi-component lignin degrading enzymes. (Regulation of these enzymes is by substrate-induction and end-product inhibition.) Most fungal carbohydrate metabolism is completed through glycolysis and the tricarboxylic acid cycle, also known as the fermentative and anaerobic pathways. Although Fungi cannot fix gaseous nitrogen, they are able to use nitrate, ammonia and some amino acids by direct uptake across the hyphal membrane. More complex nitrogen sources, such as proteins and peptides, can be used as well. (YA)(13)

Fungi reproduce through the process of releasing spores that are produced both sexually and asexually. A single fungus tends to produce a large number of spores. Spores can be carried by wind or water. Spores grow into a fungus through a process called germination. The spore produces mycelium once they land in a moist place that is abundant with food.
Many fungi that have sexual life cycles have two distinct stages in the union of partners: plasmogamy and karykogamy. Plasmogamy, which occurs first, is the fusion of two parents’ cytoplasm when their mycelia come together. The second stage of the process, karykogamy, is the fusion of the haploid nuclei contributed by the two parents. Haploid nuclei are nuclei that only contain one set of chromosomes.

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The image above demonstrates fungal reproduction. Fungi reproduce both sexually and asexually. The top portion of the image demonstrates how fungi release spores that eventually germinate and become news organisms. The bottom half demonstrates the formation of spores through fertilization, which grow to become new organisms. (LW) (6)

Dormancy, a temporary break in the life cycle, occurs when spores do not germinate following formation. Endogenous and exogenous are two types of dormancy that prevent the spores from germinating. Endogenous dormancy is caused by an internal barrier. For example, a physiological barrier may prevent the spore from receiving vital nutrients or water. Endogenous dormancy is caused by an external barrier, which inhibits germination until it is removed. (MT)

Environmental Adaptations:
Because they are immobile, fungi have adapted by creating many ways to extend their living area and spread their species to new locations. Myceliums grow by synthesizing proteins and other materials that are streamed to the tips of hyphae. Since fungi cannot move to search for food or mates, the rapidly growing mycelium allow fungi to explore more territory.
Fungi absorb nutrients. This means that fungi must have a large surface area to absorb a sufficient amount of nutrients. Extending hyphae allow fungi to frequently increase surface area, subsequently increasing nutrient absorption.
Fungi’s inability to move poses a problem when it comes to reproduction. Fungi have adapted to this problem in two ways. Mycelium that extend are able to make contact with other fungi, enabling reproduction. Fungi also produce spores, which are dispersible through many modes, such as wind and water. This allows fungi to inhibit new areas.

Mycorrhizae are fungi that colonize plant roots and develop mutualistic relationships with these roots. The fungi grow deep into the ground and absorb nutrients for the plant, acting as roots. In return, the fungi are given sugars produced when the plant photosynthesizes. (CSR, 9, 10)

Review Questions:
1. Explain how the production of spores have impacted the population of different fungi. How might fungi have adapted differently if they did not produce spores. Explain. (CW)
2. Explain the material transport system and how this is related to locomotion. (LJ)
3. What are the two stages of the sexual reproductive cycles in fungi that involve the union of partners? (KS)
4. Explain the structure of hyphae and mycelium and how they relate to function in expansion and material absorption. (JS)
5. Compare and contrast two different types of fungi. How do the differences benefit each? Why may they have developed this way?

6. What is the use of the flagella/why have so many types of ferns adapted to get rid of them? (ZXU)

Amanita phalloides (SR) (Poisonous Mushroom) (15)

Original Source: Campbell, N.C., Reece, J.R. (2002). Biology. (Sixth Edition). San Francisco: Benjamin Cummings. (NG) (NG)
3. (RL)
4. Gobat, Jean-Michel, Michel Aragno, and Willy Matthey. The Living Soil: Fundamentals of Soil Science and Soil Biology. Enfield, NH: Science, 2004. Print. (RL)
5. (RL)
6. (LW)
7. (MT) (7)(MF)
9. (CSR)
10. (CSR)
11. (GR)
14. (ZS)
15. Personal Mushroom Hunting Photos (SR)
16. (MC)
17. (CC)
18. (ORS)
19 (NI)
20. (AK)