Biology,  Botony

KINGDOM FUNGI

Position of Fungi

During warm humid days slices of bread, chapati, leather belts or shoes, etc. develop powdery layer on them. In lawns and flower beds, mushrooms come out. These are all fungi.
Fungi were earlier classified as plants without chlorophyll and without differentiation of parts into root, stem and leaves. They are now included, in a separate Kingdom Fungi.

Characteristics of Fungi

  • Fungi are multicellular eukaryotes
  • Fungi exist as the slender thread-like filaments called hyphae. Hypha has many nuclei. Yeast, however, is single-celled.
  • The cell walls are made of chitin
  • A hypha may be divided into cells by partitions called septa.
  • Septa have pores through which cytoplasm streams freely.
  • A group of hyphae forming a network is called mycelium, mycetos meaning fungus.
  • Mycelia spread out on the substrate, or on the ground and even extend up to several kilometres.
  • They do not possess chlorophyll os their nutrition is by absorption.
  • No flagellum at any stage of life cycle.
  • Reproduction in fungi is both asexuals by means of spores and sexual through conjugation.

Fungi are eukaryotic, multicellular saprotrophs having filament which grow through soil, wood and other substrates.

Four main kinds of Fungi

The fungi are of four main kinds
1. Yeasts, which are unicellular.
2. Slime moulds, which have an irregular shape.
3. Mushrooms and toadstools, which are large enough to be seen by naked eyes.
4. Lichens and mycorrhizae which exist in symbiotic associations.

1. Yeasts

Yeasts are fungi which do not produce any hyphae. These are generally in the form of single oval cells.
Fig. shows the general structure of a yeast cell. Note the following features in it:

  • ovoid cell.
  • distinct cell wall and nucleus.
  • one or more vacuoles in the cytoplasm.
  • cytoplasm is granular and has glycogen and fat (oil) globules.

Nutrition

Yeast is saprotrophic. It can directly absorb simple sugar (glucose) but for obtaining sucrose (cane sugar) it gives out the enzyme zymase which breaks down sucrose into simple sugars. The simple sugars are then simply absorbed into the cell.
Yeast respires anaerobically to yield energy as follows
C6H12O6       →    2C2H5OH + 2CO2 + 2ATP
(glucose)               (ethyl alcohol)       (energy)

Reproduction

Yeast reproduces asexually by budding

Sexual reproduction may also occur by its conjugation between two yeast cells. The fused contents divide twice to produce four individuals with a thick wall around each. These are the spores. The spores may be carried by wind and germinate under suitable conditions to produce new yeast cells.

2. Slime Mould

These consist of a naked, creeping multinucleate mass of protoplasm sometimes covering up to several square metres.

3. Mushrooms and Toadstools

Mushroom

The vegetative part of the mycelium lies concealed in the substratum (in the ground or in wood, etc.) When conditions are favourable the umbrella-like mushroom grow out with a stalk and a cap.

4. Lichens

These are a combination of certain fungi and green algae which live in a symbiotic (mutually beneficial) association: the green algae prepare food while the fungi give protection.

The economic importance of Fungi

A. Harmful Fungi

Several agricultural plants like sugar cane, maize, cereals, vegetables suffer from diseases caused by fungi.
1. Puccinia graminis (Wheat Rust)
It causes brown patches on leaf and stem of wheat. It decreases the yield of wheat and makes it unfit for human consumption.
2. Rhizopus or (Bread Mould) grows on bread.
If the bread is exposed to warm and humid conditions a cottony mass develops in few days. This white cotton mass later develops a greyish black colour.

  • The whitish network is called mycelium.
  • The mycelium contains thread-like structures called hyphae.
  • The root-like structures growing out of the hyphae to penetrate the bread, and
    secrete digestive enzymes (extracellular digestion) and absorb the digested food.
  • Greyish black colour of the mould develops due to the formation of sporangium which releases dark coloured spores. The spores scatter by wind and germinate after falling oil a suitable place. This is the asexual reproduction.
Reproduction in Rhizopus
Reproduction in Rhizopus

3. In human, skin diseases like ringworm and athlete’s foot are caused by fungi. Some ear infections are also caused by fungi.

B. Beneficial Fungi

Certain Mushrooms (such as Agaricus campestris) are edible. Yeasts are used for fermentation during manufacture of bread, beer, soya sauce, cheese and wine.

    • Mycorrhizae are fungi associated with roots of plants. Roots benefit in getting
      minerals from the environment while fungi get food from the plant in return
      through such association.
    • Neurospora has been a favourite experimental material in Genetics.
    • Various antibiotics are derived from fungi. Penicillin is obtained from Penicillium
      notatum. Its antibiotic effect was discovered by chance by Alexander Flemming in 1927.
    • They reproduce asexually as well as sexually.
    • Examples of protoctists ae Paramecium, Amoeba, malarial parasiste, Chlorella, Euglena, Chlamydomonas, Spirogyra etc.
    • Some protozoa cause diseases. Algae provide food for fish, and are rich sources of some minerals and vitamins. Blue-green algae fix atmospheric nitrogen. Walls of diatoms which have silica are used as filters and for lining furnaces.
    • Diatoms form bulk of plankton and are food for many aquatic organisms.
    • Prokaryotes lack the true nucleus. Genetic material in Prokaryotes is in the form of single circular DNA.
    • DNA is placed in the special region in a bacterial cell called nucleoid. A small ring of extra DNA is present called plasmid.
    • Bacteria exhibit four different kinds of nutrition – autotrophic, saprotrophic, symbiotic and parasitic.
    • Cyanobacteria possess chlorophyll that helps in photosynthesis.
    • Some bacteria fix atmospheric nitrogen to enrich the soil, some help in sewage treatment.
    • Certain bacteria cause diseases like cholera, typhoid, tetanus and tuberculosis, etc.
    • There are bacteria that survive in extreme environments like these with high temperature, high salinity, and presence of methane.
    • Fungi are eukaryotic, multicellular saprotrophs.
    • Fungi are of several kinds such as yeasts, slime moulds, mushrooms, lichens and mycorrhizae.
    • Yeasts are unicellular, which commonly reproduce asexually by budding. Sexual reproduction occurs by conjugation.
    • Slime moulds are naked, creeping multinucleate mass of protoplasm.
    • Lichens are symbiotic combinations of fungi and algae.
    • Rhizopus is the common bread mould that produces whitish network (mycelium) on stale bread, in warm humid weather.
    • Rhizopus reproduces asexually by spores, and sexually by producing zygospore which in turn produce spores.
    • Wheat rust (Puccinia graminis) causes brown patches on leaf and stem of wheat.
    • Ringworm and athlete’s foot are two common fungal diseases of humans.
    • Certain mushrooms are edible.
  • Yeast is used in making bread, beer, etc.
  • Neurospora is used in experiments on genetics.
  • Peniaillium notatum yields penicillin.
  • Various other fungi produce other antibiotics.

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