Fungi – Characteristics, Occurrence, Cell Structure, Nutrition, Thallus Organisation, Mycelium.

In this article we will learn about Fungi Characteristics, Occurrence, Cell Structure, Nutrition, Thallus Organisation, Mycelium.

Fungi Definition

Fungi (singular: fungus)  is a heterotrophic multicellular eukaryotic organism that plays an important role in the nutrient cycling of the ecosystem. The biology where we study fungi is known as mycology.

Fungi follow both sexually and asexually methods for reproduction sometimes they appear in symbiotic associations with plants and bacteria. 

The term fungus adopted from the Latin word fungus which means mushroom. The word mycology comes from the Greek word mykes which means”mushroom” and logos means “discourse”.

Example: Some examples of fungi are yeasts, rusts, smuts, mildews, molds, and mushrooms.

General Characteristic of Fungi/ Fungi Characteristics

• These are eukaryotic organisms.
• It is estimated that there are about 1.5 million fungi on earth.
• There are about 300 species of fungi which are infectious to humans.
• They contain a true nucleus, which means the nucleus is membrane-bounded.
• They are heterotrophs in nature means they don’t produce their food by themselves.
• They lack the vascular system (Xylem and Phloem absent) means fungi are non-vascular organisms.
• Chlorophyll pigment is absent in fungi because they are achlorophyllous.
• They contain a cell wall that is made up of chitin.
• Fungi lack the embryonic stage.
• They reproduce by the formation of both asexual and sexual spores. Some examples of sexual spores are Oospores, Zygospores, Ascospores, Basidiospores, etc.and some examples of asexual spores are Sporangiospores, Aplanospores, Zoospores, Conidia, etc.
• Fungi conduct biosynthesis of chitin.
• Some fungi contain small nuclei with repetitive DNA.
• Fungi are non-motile in nature, they can’t move.
• In fungi, the nuclear envelope is not dissolved during the mitosis.
• They show alternation of generation, containing both haploid and diploid stages.
• The vegetative body of the fungi is made of microscopic threads known as hyphae. Yeas don’t produce any hyphae, they are unicellular.
• Fungi form a hyphal network known as mycelium.
• The physical structure of both the plant and fungi is similar but chemically they are different. The fungal cell wall is made up of chitin (C8H13O5N)n.
• The fungal cell membrane is made up of unique sterol and ergosterol.
• Most fungi grow in acidic environments (tolerate acidic pH).
• Fungi contain several exoenzymes such as Hydrolases, Lyases, Oxidoreductase, Transferase, etc. which helps in food digestion.
• They store their food in the form of starch.
• The fungi can be saprophytes (gets energy from dead and decaying matters), or parasites (lives in a host, attack and kill) or symbionts (mutually beneficial).
• The saprophytic fungi has an optimum temperature of 20-30°C while 30-37°C for parasitic fungi.
• As compared to bacteria the fungi have a slower growth rate.
• They follow both sexual(Teliomorph) and asexual reproduction (Axamorph). The Asexual reproduction is accomplished by the fragmentation, fsomatic budding, fission, asexual spore formation. The sexual reproduction is accomplished by gametic copulation, gamate-gametangium opulation, gametangium copulation, somatic copulation and Spermatization.
• Mold or mushrooms are macroscopic fungi, they can be seen in naked eye.

Occurrence of Fungi

They are cosmopolitan means they can be found in air, water soil, and on plants and animals. The warm and humid places are preferred as the optimum environment for their growth.

Cell structure of Fungi

• The cell wall of a fungal cell is made up of chitin (fungal cellulose, C22H54N4O21).
• In case of primitive fungi, the true cellulose with or without chitin can be found.
• Occasional Plasma-lemma appears, coiled ingrowths called lomasomes which lie below the cell wall. 
• The cytoplasm of fungal cells contain different organelles such as Endoplasmic reticulum, mitochondria, ribosome, Golgi bodies etc.and inclusions such as stored foods, pigments and secretory granules.
• At the hyphal tip, the cytoplasm contains Golgi vesicles known as the chitosomes which filled with cell wall materials.
• Nucleus and mitochondria are found to connect with ER.
• During the karyochoresis, the nucleus divides. The nuclear envelope remain intact during nuclear division and a new internal spindle formed.
• They store their food in the form of glycogen and oil.

Heterothallism and Homothallism

In 1904 A. F. Blakeslee discovered mating types or genetically distinct strains in Mucor. He named those fungi with different mating types are known as heterothallic, and those without mating types are known as homothallic. 

Nowadays, if both male and female gametes produce in the same individual can fertilize each other homothallic. On the other hand if the gametes can only be fertilized by gametes from another individual of the same species are known as heterothallic. Heterothallism is responsible for the variations in the species.

Thallus organization of Fungi

There are present two types of thallus in fungi such as 1. Unicellular Thallus 2. Filamentous Thallus.

Unicellular Thallus 

• The thallus of some lower fungi for example chytrids is more or less a spherical, single-celled structure (A).
• In holocarpic fungi the thallus becomes a reproductive unit during the time of reproduction, latter it develops the asexual or sexual cells.
• The vegetative and reproductive stages do not occur in thallus of holocarpic fungi.
• The vegetative form of Plasmodiophora is made of a naked, multinucleate, amoeboid mass of protoplasm (D), is known as Plasmodium.
• In diploid Plasmodium the protoplast is differentiating to form the resting spores.
• The filamentous forms of yeast contain a unicellular thallus (B).
• The mycelium is absent in unicellular holocarpic forms.

 Filamentous Thallus

• The Fungal cell also contains Filamentous Thallus, which is developed through the germination of spores. When spores land on a suitable substratum where other favorable conditions of life are present, they start to germinate.
• The fungal spores give rise to a fluffy thallus consisting of a cottony mass of fine, branched filaments.
• The long, fine filaments are known as hyphae. Later these hyphae extend into the air and bear the reproductive bodies. And the remainings are continued the normal activities by spreading over or within the substratum.
• The interwoven mass of thread-like hyphae is known as mycelium, hence hyphae is the structural unit of the mycelium.
• The mycelium grows upon a medium known as substratum.
• In the life cycle, mycelium functions as the food procuring structure. It carries on the general activities of a plant cell for example absorption, digestion, respiration, excretion and growth but not photosynthesis. 
• The mycelium branch consists of hyphae. It forms a loose and ramifying network by spreading in all directions within or over the substratum.
• Those hyphae embedded within the substratum are usually colorless, but in some fungi, the aerial hyphae is colored. Black, orange, yellow, red, blue and brown are the usual tints. 

Mycelium of Fungi

There are present three types of Mycelium in fungi such as;

1. Plectenchyma (fungal tissue)

The hyphae in fungal cells are organized loosely or compactly woven to form a tissue called plectenchyma. There are two types of plectenchyma  such as;

• Prosenchyma or Prosoplectenchyma

In this type the hyphae are loosely interwoven lying more or less parallel to each other.

• Pseudoparenchyma or paraplectenchyma

In this type the hyphae are compactly interwoven looking like a parenchyma in cross-section.

2. Sclerotia (Gr. Skleros=haid)

Sclerotia is a hard dormant body made of compact hyphae which is protected by external thickened hyphae. Under favourable conditions they germinate within a mycelium e.g., Penicillium.

3. Rhizomorphs

Rhizomorphs are the root-like compactly interwoven hyphae which contains distinct growing tips. These are helpful in absorption and perennation (to tide over the unfavourable periods), e.g., Armillaria mellea.

Nutrition of Fungi

Fungi don’t produce their food by own because they lack chlorophyll. Based on the type of source fungi are classified into different groups such as;

a. Saprotrophs (= saprobes)

Saprotrophs get their nutrition from dead and decaying organic matter by releasing digestive enzymes which digest the substratum and then absorb nutrients.

Example: Mucor, Agarious. Rhizopus (bread mould) etc.

b. Parasitic

• Parasitic fungi get their nutrients from living cells.
• They can be facultative or obligate.
• The facultative parasites are found in a variety of tissues and often cause ‘soft rot’ of the tissue, e.g., Ustilago.
• There are present another type of parasite such as ectoparasites or ectophytic parasites. They grow on the host cell surface and absorb nutrients through haustoria. Example: Mucor, Erisphae.
• Some parasites grow inside the host cell, they are known as the endoparasites or endophytic. Such as Pythium, Puccinia.

c. Predacious

Predacious is a soil fungi, it forms a ring-like nose to trap annelids, nematodes etc. Example: Arthrobotrys, Zoophagus, Dactylella etc.

d. Symbiotic

• Fungi can be found in a mutualistic relationship with another organism, where both organisms are benefited. Example: lichens and mycorrhiza.
• The symbiotic association of fungi and algae is known as the Lichens. In this symbiotic relation fungi(ascomycetes or basidiomycetes) provides water and nutrients whereas the algae (green algae or cyanobacteria) produces food by photosynthesis.
• The symbiotic relationship between the soil fungi and plant roots is known as the mycorrhizae. In this relationship, fungi provide minerals, nutrients, water, vitamins to the plants, Whereas the plant provides food to fungi. Example: ectomycorrhizas (ECM) and endomycorrhizas.