Introduction to Fungi

Fungi are eukaryotic organisms, containing complex polysaccharides such as chitin and glucans giving them strong, rigid cell walls. However, there are some fungi that resemble bacterial plasmids that have rings of DNA as opposed to the typical membrane bound nucleus with histone proteins wrapped in DNA (Madigan et al., 2015). Fungi also have complex internal membranes e.g. Golgi apparatus and endoplasmic reticulum, they also have cell bound mitochondria (Gow, Latge and Munro, 2017). Fungi, like other eukaryotes contain plasma membranes, however most fungi stabilise their structures using ergosterol (a steroid molecule) rather than cholesterol which is used in animal cell membranes. Some fungi such as zoosporic, express structural diversity in that they have a mixture of cholesterol and 24-ethyl sterols (Weete, Abril and Blackwell, 2010). Moreover, fungal cells unlike plant cells do not have chloroplasts or chlorophyll, meaning they cannot photosynthesize.

 

Recent high throughput DNA sequencing predict there could be as many as 5.1 million different species of fungus (Blackwell, 2011), however, to date approximately 144,000 species of fungus have been identified (Willis, 2018). Funguses like baker’s yeast (Saccharomyces’s cerevisiae) are unicellular, whereas bread mould (Rhizopus stolonifera) are multicellular. Most fungi are multicellular and have 2 distinct morphological stages, vegetative and reproductive. The vegetative stage typically consists of slender thread like structures called hyphae, the hyphae are divided into separate cells by end walls known as septa. Most fungi have holes in their septa which allow nutrients and small molecules to be transferred from cell to cell throughout the hyphae. The pores in the septa open and close depending on the current environmental conditions (van Peer et al., 2009).

 

Fungi are heterotrophs, meaning they use complex organic compounds as a source of carbon, unlike plants that use photosynthesis. Like animals, fungi need to ingest their food, however, rather than ingesting, then digesting food, fungi work in the reverse order. They first decompose organic matter such as leaf litter, using exoenzymes which get transported out from the hyphae (Yuan and Chen, 2014), then the molecules produced get absorbed by the mycelia.

 

Fungi can reproduce sexually and/or asexually, in both forms’ fungi produce spores. In asexual reproduction fungi can reproduce by fragmentation, such as Fusarium in the image below, the fungal mycelium separates into smaller pieces with each piece capable of growing and forming new mycelium. Fungi can also reproduce by sexual reproduction which consists of 3 main stages, Plasmogamy where 2 haploid cells fuse, Karyogamy where the haploid nuclei fuse to form a diploid zygote, and finally meiosis where gametes are generated, and spores can then be released (Madigan et al., 2015).