Fungus Kingdom Reading Guide
Seeing fungi growing on wet leaves or on a fallen log, you might assume they are some type of plant. But based on cell structure, molecular comparisons, reproduction, and modes of nutrition, fungi are not closely related to plants. They are assigned their own kingdom. In fact, DNA comparisons show that fungi are more closely related to you than they are to plants! Fungus are eukaryotic (they have a nucleus), heterotrophic (they cannot make their own food and they must obtain nutrient molecules from a food source), have cell walls and are multicellular (with the exception of yeast). Fungus are diverse and are classified by how they reproduce?
Structure and Function of Fungi
Fungi have body structures and modes of reproduction unlike those of any other eukaryotic organisms. The bodies of most fungi are made of structures called hyphae (singular, hypha). Hyphae are tiny threads of cytoplasm surrounded by a plasma membrane and covered by a cell wall. The cell walls of fungi differ from the cellulose cell walls of plants. Most fungi build their cell walls out of chitin, a strong, flexible polysaccharide that is also found in the external The hyphae of most fungi have additional cell walls, called cross-walls, that divide the long filaments into many separate end-to-end cells. This makes most fungi multicellular. The cross-walls of many fungi have pores large enough to allow ribosomes, mitochondria, and even nuclei to flow from cell to cell. The movement of cytoplasm from one cell to another helps a fungus distribute nutrients from one part of its body to another. The hyphae of a single fungus typically branch as they grow, forming an interwoven mat called a mycelium (plural, mycelia). The mycelium functions as the feeding structure of a fungus. Its fibrous structure maximizes contact with the food source. Fungi cannot run, swim, or fly in search of food. But the mycelium makes up for the fungus's lack of mobility by its ability to grow rapidly throughout a food source. A fungal mycelium can grow as much as a kilometer of hyphae each day as it branches within its food.A mycelium is an efficient structure for the heterotrophic lifestyle of fungi. The branching mycelium enables the fungus to obtain food by absorptive nutrition, a method by which the fungus absorbs small organic molecules from its surroundings. First, the fungus digests food outside its mycelium by secreting powerful enzymes into its surroundings. These enzymes break down materials and the fungus absorbs the nutrients as it grows through the material.
Reproduction of Fungi
Fungi reproduce asexually by releasing large numbers of microscopic spores. Spores are haploid single cells with thick cell walls that function as the dispersal stage in the reproduction of fungi. These tough reproductive cells are spread by the wind and can withstand unfavorable conditions for long periods of time. When conditions are favorable again, they can germinate and grow into new fungi. Most fungi produce spores asexually by mitosis at the tips of specialized hyphae. In these situations, the spores are haploid because the hyphae from which they come are haploid. Many fungi also produce spores sexually. Haploid hyphae from different mycelia called mating strains fuse together and combine their genetic material. Eventually, diploid cells resulting from these pairings undergo meiosis. These meiotic divisions produce haploid spores.
Disease-causing Fungi
Of the 100,000 known species of fungi about 30 percent are parasites, mostly on or in plants. In some cases, fungi that infect plants have literally changed landscapes. One species, for example, causes Dutch elm disease, which has eliminated most elm trees in North America. The fungus was imported from Europe where it caused a minor disease in European elms. The fungus was accidentally introduced into the United States on logs sent from Europe after World War I. American elms had not evolved with this fungus and had no resistance to it. Fungi are also serious agricultural pests. Some species infect grain crops, ruining the harvest or causing disease in people who eat the infected grain.
Animals are much less susceptible to parasitic fungi than are plants. Only about 50 species of fungus are known to be parasitic in humans and other animals. Among these are infections of the lungs called Aspergillosis which can be fatal. Other fungal parasites produce a skin disease called ringworm, so named because it appears as circular red areas on the skin. Most commonly, these fungi attack the feet and cause intense itching and sometimes blisters. This condition, known as athlete's foot, is highly contagious, but it can be treated with various fungicides. Fungicides are substances that kill fungi without seriously harming the host organism.
Commercial Uses of Fungi
Focusing only on disease-causing fungi would give an unfair picture of the kingdom. There are many practical uses for fungi. In addition to edible mushrooms, the distinctive flavors of certain kinds of cheeses come from the fungi used to "ripen" them. As mentioned earlier, yeasts are particularly important in baking, brewing, and winemaking. And a number of antibiotics also come from fungi.
The Role of Fungi in Chemical Cycling
Fungi and bacteria are the principal decomposers that supply ecosystems with the nutrients essential for plant growth. Without decomposers, elements such as carbon and nitrogen would accumulate in organic matter. Plants and the animals they feed would starve because elements taken from the soil would not be returned. Fungi are well adapted as decomposers. Their hyphae invade the tissues and cells of nonliving organic matter and break down complex molecules such as cellulose. Fungi, in concert with bacteria and some animals, are responsible for the complete breakdown of organic material. The air is so loaded with fungal spores that as soon as a leaf falls or an insect dies, it is covered with spores that quickly grow into fungal hyphae. Some fungi can even decompose plastic
Answer the following questions on your own sheet of paper
1. What are the 4 main characteristics of the fungus kingdom?
2. How are fungus classified?
3. What does it mean to be eukaryotic?
4. What does it mean to be heterotrophic? How do fungi get their food? (explain absorptive nutrition.)
5. What are the tiny threads of cytoplasm surrounded by a plasma membrane and covered by a cell wall called?
6. Fungus cell walls are not composed of cellulose like plants. What substance do fungi use to make their cell walls?
7. Fungus bodies are made of interwoven mats of hyphae called ______?
8. What structure does fungus normally use to reproduce asexually?
9. What are the special hyphae used for sexual reproduction called?
10. How are spores well adapted for spreading fungus?
11. Name 3 fungus pathogens of man.
12. Name 5 ways man uses fungus.
13. What would happen if there were no bacteria or fungus in an ecosystem?