Fungi Structure

[Basic Body Structure][Nutrition][Classification][Reproduction]


This is a picture of an Amanita Muscaria
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Basic Body Structure

A typical fungus consists of hyphae, which form the fungal body. These hyphae are microscopic walled tubes or filaments that are lined with plasma membrane and contain cytoplasm. The hyphae branch into a complicated network known as the mycelium, which is the feeding network of the fungus. The cell walls of the hyphae are made of chitin, a nitrogen containing polysaccharide that is strong but flexible. The cytoplasm is multinucleate and the hyphae may have septa, which are cross-walls used for more strength. Fungi can grow rapidly because of the structure of their mycelium. Because materials can move very quickly through the mycelium, these materials become available to growing hyphae, making fungi grow. Several species of fungi are able to change their form in response to change in their environment and resources because of this. This ability to change is called dimorphism.


Histoplasma Capsulatum causes a severe disease in humans that can resemble tuberculosis. Normally it grows as mycelium in its natural environment, but when it invades a human, the increased temperature and available nutrients causes the fungus to grow unicellular like a yeast.

Nutrition

Fungi are heterotrophs, meaning they cannot synthesize its own food and is dependent on complex organic substances for nutrition. They acquire all nutrients by absorption and secrete hydrolytic enzymes to decompose complex molecules into simpler ones that could also be absorbed. Absorbing nutrients in its immediate environment allows fungi to act in 1 of 3 niches: saprobe, parasite, or mutualistic. Saprobic symbionts tend to decompose nonliving organic material. Parasitic symbionts obtain nutrients from their living host's cells. Mutualistic symbionts also absorb materials from a living organism; however, these fungi provide beneficial services for their host.

Classification


Fungi are divided into three main phylums based on how related they are to each other and shared reproductive habits. These phylums are: Basidiomycota, Ascomycota, and Zygomycota. Basidiomycota are characterized by the presence of basidia and dikaryotic fruiting bodies and include: mushrooms, puffballs, and shelf fungi. Ascomycota are a diverse group and are characterized by the presence of asci. They include: the cup fungi and mildews as well as the unicellular fungi, yeast. Zygomycota are characterized by the presence of a thick-coated zygospore and include bread and other molds, plant and animal parasites, as well as many plant-root symbionts.

Reproduction

Fungi reproduce asexually and sexually to produce spores. These fungal spores come in all different shapes and sizes. Under non dynamic conditions, the spores are created asexually; under changing conditions, the spores are created sexually. The different types of fungi are classified into phylums because of the way they reproduce.

Basidiomycota reproduce both asexually and sexually. To produce asexually, these fungi go through budding, which is when a small portion of the cell membrane and cytoplasm receive a nucleus and pinch off from the parent cell, or through spore formation, which takes place at the ends of specialized structures called condiophores. The septae of terminal cells become fully defined, and then divide a random number of nuclei into individual cells. The cell walls then thicken into a protective coat. The protected spores break off and are disbursed. Sexual reproduction takes place in the fruiting body, in the basidia. The basidia are formed by plasmogamy between mycelia from two different spores. Plasmogamy results in hyphae with two types of nuclei, one from each parent. In the gills of the fruiting body, some cells undergo fusion of these two nuclei. These now diploid cells are the basidia. The diploid phase is very brief because soon after fusion, meiosis takes place, resulting in four haploid nuclei. The nuclei then migrate to the terminus of the basidium and form four individual projections. These projections are then separated by cell walls to become spores.


The sexual reproductive cycle of Basidiomycota.

 

Ascomycota also asexually produce the same way the Basidiomycota does. Sexually, the Ascomycota are very different. Ascomycota have male and female gametangia in their haploid stage. These structures form the mycelia. Plasmogamy then takes place when the trichogyne fuses with the antheridium and produce the binucleate. This phase is prolonged and a series of dikaryotic cells called an ascogonius hypha is produced. At the tip of this hypha a nuclear fusion takes place resulting in the formation of diploid asci. Within this structure, the diploid nucleus undergoes meiosis, producing four haploid nuclei. These nuclei then undergo mitosis to form eight haploid ascospores, which is twice as many spores as produced in the basidium in Basidiomycota.


The sexual reproductive cycle of Ascomycota.

Zygomycota reproduce asexually, but it varies greatly among orders and species. Spores may be formed by the separation and thickening of hyphal cells. They may also be produced in specialized organs, whose structure is also widely varied. Sexually, the Zygomycota are similar to the Ascomycota in that some have two mating types, though there are individual species within the phylum that has only one mating type. When two opposite mating hyphae meet, they produce structures called progametangia. Cell walls form to separate the tips of the progametangia into gametangia. Plasmogamy occurs between the two gametangia forming a zygote. Nuclear fusion takes place within the zygote. The walls of the zygote are thin, but then thicken into a zygospore. Germination begins when the diploid nucleus undergoes meiosis and a sporangium develops at the end of a germ tube. Spores are produced within the sporangium.


The sexual reproductive cycle of Zygomycota.
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