Fungi are widely distributed organisms on Earth. The study of fungi is known as mycology. They have great environmental and medical importance like penicillium as an antibiotic but some fungi cause diseases in plants and animals (like wheat rust causes puccinia). Many fungi are free-living in soil and water. Some other fungi form parasitic or symbiotic relationships with plants or animals because they lack chlorophyll. They are eukaryotic organisms. They can grow anywhere but preferably in warm and humid places. Their cells contain membrane-bound organelles and well-defined clear nuclei. They are unicellular as well as multicellular. They reproduce by both sexual and asexual methods. Many fungi are harmful; they cause infections of plants known as rots, rusts, blights, wilts, and smuts.
Examples: Yeasts (Unicellular), Molds
(Multicellular), Mushrooms (Multicellular), etc.
Importance of Fungi:
• They are decomposers and recyclers of nutrients.
• Plants live in a symbiotic relationship with mycorrhizae
fungi.
• Yeasts are used to make beer and bread.
• They produce penicillin antibiotics.
• Mushroom fungi are eaten as food.
• Aspergillus is used to make soy sauce.
• Yeasts ferment sugar to produce alcohol.
• Trichoderma is used to produce cellulose which is used to
make fruit juice.
• Genetically, engineered yeast strains are used to make
proteins (Hepatitis B vaccine).
Morphology of Fungi:
Fungi are divided into two morphological forms viz. yeasts
and hyphae (mold). Yeasts are unicellular nucleated and rounded fungi that
reproduce asexually by budding formation (blastoconidia) or fission. Hyphae are
multi-cellular and filamentous fungi that reproduce asexually or sexually.
Dimorphism is another condition whereby a fungus can exhibit either the yeast
form or the hyphal form, depending on growth conditions.
Generally, Yeast cells are 1-5 micrometers in width and 5-30
micrometers in length which are larger in size than bacteria. Commonly they are
oval or egg-shaped but sometimes they are elongated and spherical. Yeast cells
do not have flagella (Fig.1).
Fig.1: General structure of Yeast cell
They have granular cytoplasm enclosed with a cell wall that
contains reserved materials like oil globules, glycogen, and volutin but the
content of glycogen accumulation is increased with the decrease in
fermentation. They have large nuclear vacuoles with prominent nuclei and
nucleolus. The cell wall is composed of chitin (1-10%) along with protein,
lipid, mannan (15-23%), and glucan (50-60%). Glucan is the main structural
component of the cell wall which is the branched polymer of glucose i.e. beta
1, 6-glucan, beta 1, 3-glucan and beta 1, 3-beta 1, 6-complexed with chitin.
Glucan and chitin are the important components that regulate cell division as
well as provide rigidity to the cell wall. The mode of nutrition of fungi is organotropic
and heterotrophs and mostly are saprophytic and some are parasitic. They grow
best in acidic environments. They can tolerate high sugar concentrations and
dry conditions. Most of the fungi are obligate aerobes (molds) and few are
facultative anaerobes (yeasts). The optimum temperature of growth for most
saprophytic fungi is 20-30°C while parasitic fungi grow in 30-37°C.
Mold is a type of fungus that has two parts mycelium and
spores. Mycelium is made up of several filaments known as hyphae. The hypha is
generated from spores. Each hypha is about 5-10 micrometers wide. Hyphae are
composed of another tuber wall surrounded by a cavity known as a lumen. This
lumen is filled with protoplasm. Between the protoplasm and wall, there is a
plasmalemma. Hyphae are of three types.
(i) In some molds like coenocytic fungi, the cytoplasm
passes through and among cells of the hypha uninterrupted by a cross wall which
is known as non-septate fungi.
(ii) Septate with unicellular cells and
(iii) Septate with multicellular cells which contain more
than one nucleus (Figs.2 and 3).
Fig.2: Structure of Mold
Fig.3: Various types of Hyphae
Dimorphic Fungi:
Dimorphic fungi are fungi that can exist in the form of both
mold and yeast. Dimorphic fungi are fungi that can reproduce in either a
mycelial or a yeast-like state. Generally, the mycelial saprotrophic form grows
at 25°C, and the yeast-like pathogenic form at 37°C.
The dimorphism is important in the identification of
mycoses. These fungal pathogens generally overcome the physiological and
cellular defenses of the normal human host by changing their morphological
form. An example: is Penicillium marneffei, a human pathogen that grows as a
mold at room temperature, and as yeast at human body temperature.
Some diseases caused by dimorphic fungi are blastomycosis,
histoplasmosis, sporotrichosis, etc. Candida albicans cause oral and genital
infections.