What is Fungi?
Fungi are eukaryotic organisms that include microorganisms such as yeasts, moulds and mushrooms. These organisms are classified under Kingdom Fungi. The organisms found in Kingdom Fungi contain a cell wall and are omnipresent. They are classified as heterotrophs among the living organisms.
Fungi are found mostly in humus-rich soil. But in the presence of moisture, these can grow on leather, wood, pickle, and bread. Some fungi live parasitically in plants, animals, and the human body. The chloroplast is absent in fungi, so fungi are heterotrophs. Fungi obtain their own food from dead organic matter or living organisms.
Structure of Fungi
The structure of fungi can be explained in the following points:
- Almost all the fungi have a filamentous structure except the yeast cells.
- They can be either single-celled or multicellular organism.
- Fungi consist of long thread-like structures known as hyphae. These hyphae together form a mesh-like structure called mycelium.
- Fungi possess a cell wall which is made up of chitin and polysaccharides.
- The cell wall comprises protoplast which is differentiated into other cell parts such as cell membrane, cytoplasm, cell organelles and nuclei.
- The nucleus is dense, clear, with chromatin threads. The nucleus is surrounded by a nuclear membrane.
Characteristics of Fungi
Following are the important characteristics of fungi:
- Fungi are eukaryotic, non-vascular, non-motile and heterotrophic organisms.
- They may be unicellular or filamentous.
- They reproduce by means of spores.
- Fungi exhibit the phenomenon of alternation of generation.
- Fungi lack chlorophyll and hence cannot perform photosynthesis.
- Fungi store their food in the form of starch.
- Biosynthesis of chitin occurs in fungi.
- The nuclei of the fungi are very small.
- The fungi have no embryonic stage. They develop from the spores.
- The mode of reproduction is sexual or asexual.
- Some fungi are parasitic and can infect the host.
- Fungi produce a chemical called pheromone which leads to sexual reproduction in fungi.
- Examples include mushrooms, moulds, yeast.
Classification of Fungi
Kingdom Fungi are classified based on different modes. The different classification of fungi is as follows:
Based on Mode of nutrition
On the basis of the source of food, fungi are of two types:
1. Saprophytic: These fungi obtain their own food from dead organic matter such as bread, rotten fruit, vegetable, and dung. Nutrition is an absorptive type in saprophytic fungi.
2. Parasitic: These obtain their own food from living organisms such as plants, animals, and human beings. They obtain nutrition with the help of haustoria.
- Some fungi are found symbiotically associated with algae and form lichens. Some fungi are found symbiotically in the roots of higher plants and form mycorrhiza.
- The body of fungi is called mycelium. Mycelium is composed of filaments called hypha. (Hypha – plural → Hyphae)
- Cell wall is present around fungi, which is made up of chitin or fungal cellulose. Some quantity of proteins, lipids & cellulose is also present within chitin.
Note: Cell wall of the members of class-oomycetes is mainly made up of cellulose. In fungi the stored food remains in the form of glycogen and oil.
3. Symbiotic: These fungi live by having an interdependent relationship association with other species in which both are mutually benefited. Examples: Lichens and mycorrhiza. Lichens are the symbiotic association between algae and fungi. Here both algae and fungi are mutually benefited as fungi provide shelter for algae and in reverse algae synthesis carbohydrates for fungi.
Try yourself:Which one of the following is true for fungi?
Fungi lack chlorophyll, hence, they do not prepare their food by photosynthesis. They can grow where organic material is available. So, they are heterotrophs that acquire their nutrient by absorption and store in the form of glycogen.
Based on Spore Formation
Based on Spore Formation, the fungi are classified as follows:
- We find these in moist surfaces, decaying wood or aquatic habitats.
- The mycelium is septate and coenocytic.
- Some common examples include Mucor, Rhizopus and Albugo.
- We commonly call these as sacfungi.
- They are mostly found in multicellular form and rarely in unicellular form.
- In ascomycetes, the mycelium is in a branched and separate form.
- They are saprophytic, decomposers, or parasitic.
- Some examples include Aspergillus, Claviceps and Neurospora.
- They grow in soil, tree stumps or even logs.
- Their mycelium is in a separate and branched form.
- The sex organs are absent.
- Mushrooms are the most common form of basidiomycetes.
- Some common examples include Agaricus, Ustilago and Puccinia.
- They are also known as imperfect fungi.
- We only know the vegetative phase of these fungi.
- They reproduce by conidia, which are the asexual spores.
- Their mycelium is also in a separate and branched form.
- Some common examples include Alternaria, Colletotrichum and Trichoderma.
In fungi, reproduction is of three types:
- Vegetative Reproduction
- Asexual Reproduction
- Sexual Reproduction
1. Vegetative Reproduction
- Fragmentation: Some-times the fungal filament (mycelium) breaks into small pieces due to some reason. Now, these pieces form a new fungal filament and start working as a normal filament.
Fragmentation in Spirogyra
- Budding: Sometimes, a bud-like protuberance is formed in non–mycelial fungus. Now, this bud separates from the mother fungi and functions as young fungi. At the time of bud formation, the nucleus of the mother cell divides mitotically (or amitotically - in yeast) into two daughter nuclei. Out of these two nuclei, one remains within the mother cell while the other migrates to the bud.
Budding in Yeast
- Fission: Sometimes, the fungal cell divides into two parts along with the nucleus. Now the nuclei go to both cells and each cell starts working as a new cell.
Process of Fission
Note: Reproduction through bud formation and fission takes place only in non-mycelial form.
2. Asexual Reproduction
Spores are of the following types:
- They are formed in sporangia.
- Sporangia are formed at the tip of the fungal filament.
- Those fungal filaments on which sporangia are formed are called as sporangiophore. Numerous spores (sporangiospores) are present in the sporangia, which comes out by rupturing of sporangia and germinate to form fungal filaments. The formation of sporangiospores takes place endogenously.
Sporangiospores are of two types:
- Zoospore: When the sporangiospores formed in sporangia are flagellated and motile, then they are called zoospores. In this condition, the sporangia are called zoosporangia.
- Aplanospore: When sporangiospores are non-flagellated and non-motile, then they are called Aplanospores.
- The formation of conidia takes place exogenously. These conidia are formed at the tip of conidiophores.
- Conidiophore: Straight fungal filament on which conidia are formed are called conidiophore. Conidiophore may be unbranched, branched, septate, or aseptate.
- Conidia: Conidia are formed singly or in a chain. Each conidium forms a fungal filament (mycelium) by germination.
Try yourself:Fungal spores produced asexually at the tips of hyphae are called
Conidia are non-motile spores produced exogenously by constrictions at the tip of special hyphae called conidiophores.
3. Sexual Reproduction
- The structure in which gametes are formed is called gametangia.
- Sexual reproduction in fungi completes in three steps:
(i) Plasmogamy: This is the first stage of sexual reproduction. In this stage, two sex cells fuse with each other, but their nuclei do not fuse, due to which a single cell has two nuclei. This binucleate or dikaryotic stage is called dikaryon.
(ii) Karyogamy: In this stage, the nuclei present in the cell fuse with each other, to form a diploid nucleus. This process of formation of a nucleus by the fusion of two pre-existing nuclei is known as synkaryon.
(iii) Meiosis: In this stage, meiosis takes place in the diploid nucleus due to which again haploid nuclei or haploid cells are formed.