General Biology/Classification of Living Things/Eukaryotes/Fungi

Introduction
Although you may not recognise fungi, they are just as prevalent as plants and animals. Their spores are in the air which we breathe, fungi allow us to make bread, and mushrooms (a type of fungi) are eaten by us. A few types of fungi are unicellular. For example, yeasts live as individual oval or cylindrical cells. However, the majority of fungi are multicellular. Their bodies are composed of hyphae, a network of fine filaments. In a mushroom, the hyphae are densely packed so it is difficult to see the individual structures when a mushroom is eaten. However, a mushroom is only a specialised reproductive part of the whole fungus. The main part of the fungi is underground in a whole web of hyphae, called a mycelium.

In the mycelium, each fungal cell is separated from each other by a septum. Each fungal cell may have one or more nuclei and remains connected to the mycelium because the septa are porous, allowing cytoplasm to flow through the hyphae and fungal cell walls, made of a hard material called chitin. Some fungi do not have septa, and they appear to be large, branching, multinucleate cells.

Nutrition
Fungi are saprophytes. When they find a source of food (e.g. dead wood, orange peel), they decompose it and digest it. The enzymes break down larger organic molecules in the substrate into smaller molecules. These smaller molecules diffuse into the fungus, where they are used to allow growth and repair.

Fungi which feed on living cells are parasitic. For example, athlete's foot grows on the human foot. These kinds of fungi produce hyphae called haustoria, which can penetrate host cells without immediately killing them.

However, they are friendlier species of fungi. Many fungi live symbiotically with plants or animals. For example, most trees have fungi living in close contact with their roots. In this relationship, known as a mycorrhiza, there are many benefits:


 * Growing around the plant roots and often entering plant cells, the hyphae absorb minerals from the soil and release them in the roots. The fungi gets its source of food (organic nutrients) while delivering food to the plant.


 * The mycelium here would increase the surface area, thus the absorptive surface, of the plant roots.


 * The fungal cells help to maintain air and water flow in the soil around the roots.


 * The fungi may prevent other potentially pathogenic fungi to attack the tree.

Fungal Reproduction
Fungi can reproduce in two ways. Firstly, they may asexually produce through fragmentation. This occurs when pieces of hyphae are broken off, which then grow into new mycelia.

The second method is by spores. Spores are lightweight structures and windblown designed to be transported over long distances and by many mediums, such as on the bodies of insects and birds. They are additionally light enough to be blown away for hundreds of kilometers. Spores may be asexual and sexual. Their sexual properties can be analysed to classify the four phylla of fungi.

Zygospore Fungi (Zygomycetes)
This phylum includes bread molds and other saprotrophs. Comparable to bacteria, this phylum prefers asexual reproduction over sexual reproduction.

1. Two haploid hyphae of opposite types, also known as mating strain + and mating strain -, combine and fuse together.

2. Plasmogamy, the union of the two parent hyphae, occurs and results in the creation of a heterokaryotic (n + n) zygosporangium or zygospore. Note that the zygospore is NOT diploid yet; the haploid nuclei are simply clumped together.

3. Immediately, a thick wall develops around the zygospore to protect it from drying and other hazards. The zygospore becomes dormant.

4. When conditions are favourable, the zygospore absorbs water and undergoes karyogamy (n + n = 2n), where the haploid nuclei contributed by the two parents fuse to produce diploid zygosporangia.

5. The now diploid zygosporangium then undergoes meiosis to form haploid sporangia.

6. Through asexual reproduction of fungi (See above for more information), the spores from the sporangia germinate and grow into new mycelia.

7. Back to step #1.

Club Fungi (Basidiomycetes)
This phylum increases mushrooms and shelf fungi. In many ways, the reproduction stages of this phylum is similar to that of zygomycetes.

1. Two haploid hyphae of opposite types, also known as mating strain + and mating strain -, combine and fuse together.

2. Plasmogamy takes place, and a dikaryotic mycelium forms. The dikaryotic mycelium grows faster then the haploid parental mycelia.

3. Environmental factors cause the dikaryotic mycelium to form compact masses which develop into basidiocarps, short-lived reproductive structures. An example is the mushroom.

4. The basidiocarp gills are lined with terminal dikaryotic cells called basidia, which then undergo karyogamy.

5. The basidia are now diploid. They undergo meiosis to develop haploid basidiospores, a term referring to a basidiomycete's spores.

6. Still remaining on the basidiocarp, the haploid basidiospores eject, fall from the basidiocarp, and are dispersed by the wind when mature.

7. In a favourable environment, the basidiospores germinate and grow into short-lived haploid mycelia.

8. Back to Step #1.