Fungi are achlorophyllous, heterotrophic thallophytes, live as saprophytes or parasites. Most fungi are mycelia in nature, mycelium being formed by an msaa of loosely interwoven hyphae. Some primitive fungi are unicellular. Fungi absorb food in form of solution. They may be obligate parasites (in no case thrive as saprophyte), facultative saprophytes (parasites but can thrive as saprophyte in absence of their living host), obligate saprophytes (purely saprophytes), facultative parasites (saprophytes but with a capacity to thrive as parasite) or symboints (as mycorrhizal in roots of higher plants or mycobionts in lichen with algal partner).
Lower fungi are holocarpic (entire thallus involved for formation of reproductive body) or eucarpic. Reproduction by vegetative (budding and fragmentation), asexual (by zoospores, aplanospores, conidia, oidia, chlamydospores) and sexual methods. Sexual reproduction involves plasmogamy (fusion of protoplasm), karyogamy (fusion of two nuclei) and haplodization (meiosis). It may be by planogametic copulation, gametangial contact, gametangial copulation, spermitization or somatogamy.
Mushrooms, trufles, morels, yeasts, etc are very good food due to taste and high protein and mineral content. They are used in industries for large-scale production of alcoholic beverages, enzymes, organic acids, plant hormones, vitamins etc. more than 700 species are known to produce antibiotics.
Alexander Flemming (1928) obtained the first antibiotic penicillin from Penicillium notatum. On the other hand, many fungi are responsible for food spoilage, deterioration of household articles, destruction of timber and wood. Some moulds like Aspergillus produce aflatoxins, which are highly toxic to plants and animals. Some also produce string hallucinogenic substances. Most fungi are pathogenic causing diseases.
Yeasts (sugar fungi) are unicellular eukaryotes, but sometimes form suedomycelium by repeated budding. They are saprophytes and hydrolyse complex sugar into glucose and fructose by secreting invertase enzyme. Simple sugar enters into cell and further decomposes into alcohol and CO2 by enzyme zymase. Yeasts reproduce asexually by budding or fission and sexually by conjugation between two haploid vegetative cells, between a haploid vegetative cell and a haploid ascospore or between two ascospores.
Different species show three types of cycles- (i) Haplobiontic (Schizosaccharomyces octosporus), (ii) Diplobiontic (Saccharomyces ludwigii) and (iii) Haplo-diplobiontic (Saccharomyces cerevisiae). Yeasts are very important to humankind. Different species are used to produce large-scale alcoholic beverages like wine, brandy, rum, whisky, and scotch from sugar of different sources. Released CO2 is also used in baking industry to make bread light and dough due to thermal expansion. Torulopsis utilis contains high protein and vitamins and is used as a supplement to protein deficient diet. On the other hand, they spols fruits and some cause human diseases like Torulopsis, Cryptococcosis, etc.