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Nutrition, Classification and Reproduction of Fungi

Nutrition.

Unlike green plants, which use carbon dioxide and light as sources of carbon and

energy, respectively, fungi meet these two requirements by assimilating preformed

organic matter; carbohydrates are the preferred nutrient source. Fungi can readily

absorb and metabolize a variety of soluble carbohydrates, such as glucose, xylose,

sucrose, and fructose, but are also characteristically well equipped to use insoluble

carbohydrates like starches, cellulose, hemicelluloses, and lignin. To do so, they must

first digest these polymers extracellularly. Saprobic fungi obtain their food from dead

organic material; parasitic fungi do so by feeding on living organisms (usually plants),

thus causing disease.

Mycology is the study of fungi.

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Mycology is the study of fungi.

Mycology is the study of fungi.

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Mycology is the study of fungi.

Mode of Nutrition: Absorption

The mode of nutrition or the matter in which fungi "eat" is called absorption.

Among eukaryotes, absorption is unique to the fungi. Fungi obtain their food

by transporting it through their cell walls. In order to eat, the spores that give

rise to fungi must be dispersed to a location where there is food and after the

spore germinates, the mycelium of the fungus must grow into its food. Another

word, usually fungi must live in their food if they are to eat. If the food is

composed of simple molecules such as glucose or sucrose, soluble food can be

immediately transported through their cell walls. However, most food that a

fungus might consume is composed of complex, organic compounds, e.g.,

cellulose, lignin, pectin, starch, etc., which is insoluble. In order for this food

to be utilized by the fungus, it must be broken down into simpler molecules

that can be transported through their cell walls. The fungus breaks down the

complex material by secreting digestive enzymes through their cell wall that

will digest the complex organic compounds and convert them into simple

molecules that can readily be transported through their cell walls. For example,

If a fungus is growing in wood, digestive enzymes would be secreted from the

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fungus, into the wood, and break down the complex compounds of wood, e.g.

cellulose and lignin into simpler materials, such as simple sugars, which then

can be transported into the mycelium. Food must enter the hyphae in solution,

and, since most fungi have no special absorbing organs, the entire mycelial

surface is capable of taking in materials dissolved in water. Some fungi,

however, produce special rootlike hyphae, called rhizoids, which anchor the

thallus to the growth surface and probably also absorb food. Many parasitic

fungi are even more specialized in this respect, producing special absorptive

organs called haustoria.

Although this process may seem very different than our own means of

obtaining food. It is not that different. The essential difference between fungi

and animal digestive systems is that fungi digest their food first and then "eat"

it, while animals eat their food before digesting it. It is important to understand

here that different kinds of fungi will secrete only a specific number of

different enzymes. This means that they can only "eat" certain materials. A

summary of absorption is illustrated in Fig. 26, below:

Figure 26: Illustration of the process of absorption, the mechanism by

which fungi consume their food.

Although yeasts are quite different in their appearance than mycelial fungi,

their means of obtaining food is identical.

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Naming and Classification of Fungi

Taxonomy: A subject which deals with classification and naming of organisms

is called taxonomy. The species name is called a binomial, which consists of

two parts. The first part is the genus (plural: genera) and the second is the

specific epithet. For example, the species name for the human animal is Homo

sapien. When written, the species name is emphasized by italicizing or

underlining the binomial. Each known species, once it has been described by

science, is given a unique species name. Another words, there is only one valid

scientific name that is unique to a species regardless of where it may be found.

Classification of Organisms

All organisms are classified into following three domains:

a. Eukarya – eukaryotic organisms, the Eukaryotes

b. Bacteria- prokaryotic organisms, the true Bacteria

c. Archaea- prokaryotic, Archaebacteria

The Domain is a relatively new level in the classification system. The Domain

eukarya is divided into five Kingdoms namely Plants, Animals, Fungi,

Stramenopila and Protista. Fungi are very distinct from the other kingdoms.

The Kingdom Protista is a “dumping ground” for organisms that don’t fit into

the first three kingdoms. The kingdom Stramenopila includes organisms

such as the former fungi Oomycota as well as brown algae and several

other types of algae.

The classification of fungi is generally based on the following criteria:

i. Morphology of reproductive structures

ii. Types of spores formed

iii. Characteristics of the life cycle

iv. Morphology of the thallus

v. Habit

vi. Habitat

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The Fungi are classified as a kingdom that is separated from plants, animals

and bacteria. One major difference is that fungal cells have cell walls that

contain chitin, unlike the cell walls of plants, which contain cellulose. These

and other differences show that the fungi form a single group of related

organisms, named the Eumycota (true fungi), that share a common ancestor (a

monophyletic group). This fungal group is distinct from the structurally similar

slime molds (myxomycetes) and water molds (oomycetes).

According to Ainsworth fungi were classified into two kingdoms:

a) Kingdom: Myxomycota (Slime molds)

b) Kingdom: Eumycota (true fungi)

Eumycota were grouped into five sub-division, viz.

i. Mastigomycotina (Lower fungi)

ii. Zygomycotina (Lower fungi)

iii. Ascomycotina (Higher fungi)

iv. Basidiomycotina (Higher fungi) and

v. Deuteromycotina (Higher fungi)

Sub- Division: Mastigomycotina was classified into four classes:

i. Chytridiomycetes

ii. Hyphochytridiomycetes

iii. Oomycetes

iv. Plasmodiophoromycetes

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Kingdom

Eumycota/Mycota Kingdom

Stramenopila

Slime molds

Phylum/ Division Phylum/

Division

Phylum/

Division

Chytridiomycota

Zygomycota

Ascomycota

Basidiomycota

Deuteromycota

Oomycota

Hyphochytridio

mycota

Myxomycota

Plasmodiophorom

ycota

Dictyosteliomycota

Acrasiomycota

Labyrinthulomycota

Fungi and fungus like

organisms

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Reproduction of fungi

Fungi exhibit three major modes of reproduction - vegetative, asexual and

sexual.

Vegetative Reproduction- It is the type of reproduction which involves the

somatic portion of the fungal thallus. It occurs by the following methods.

Fragmentation: In this process, the mycelium breaks into two or more similar

fragments either accidentally or due to some external force. Each fragment

grows into a new mycelium.

Budding: The parent cell produces one or more projections called buds, which

later develop necessary structures and detach to grow into new individuals.

Budding is common in unicellular forms like yeast.

Fission: In this process, the parent cell splits into two equal halves, each of

which develop into a new individual. Fission is also common in yeast.

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Sclerotia: In some cases, the hyphae become interwoven to form a compact

mass and get surrounded by a hard covering or rind. Such structures are called

sclerotia, remain dormant under unfavourable conditions and germinate into

new mycelia on the return of favourable conditions.

Rhizomorphs: In some higher fungi, several hyphae may become interwoven

to form rope-like structures called rhizomorphs. Under favourable conditions,

they resume growth to give rise to new mycelia.

Asexual Reproduction: The asexual (imperfect, mitotic) state of fungi is

termed the anamorph. It is the types of reproduction in which special

reproductive structures called spores or propagates are formed. The fungal

spores always result from mitosis and hence are described as mitospores.

Following are the types of spores produced in different groups of fungi:

a. Zoospores: They are flagellated, motile spores produced inside

structures called zoosporangia. These spores do not have a cell wall.

Such spores are produced in lower fungi.

b. Sporagiospores: These are non-motile spores produced inside structures

called sporangia in fungi such as Zygomycota.

c. Chlamydospores: These are thick walled resting spores which arise directly

from hyphal cells. They store reserve food.

d. Oidia: These are spore like structures formed by the breaking up of hypha

cells. They do not store reserve food and hence cannot survive under

unfavourable conditions. Such spores are produced in Erysiphe spp.

e. Conidia: These are non-motile spores produced singly or in chains at the tip

of the hypha branches that are called conidiophores. Such spores are

produced in fungi like Aspergillus and Penicillium.

Sexual Reproduction

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Sexual reproduction is known to occur in all groups of fungi except the Fungi

imperfecti or Dueteromycetes. It may involve fusion of gametes, gametangia or

hyphae. The process may involve only fusion of cytoplasm (plasmogamy) or

fusion of nuclei (karyogamy) or production of meiotic spores (meiospores) In

most of the lower fungi plasmogamy is immediately followed by karyogamy

and meiosis. In higher fungi karyogamy is often delayed so that the hyphae

remain dikaryotic. This phase of fungal life cycle is called dikaryophase. Such

fungi complete their life cycle in three phases a haplophase, a dikaryophase and

a diplophase.

Types of Plasmogamy: Plasmogamy in fungi is of different types, as shown

below:

Planogametic copulation

Planogametic Copulation: Here

motile gametes called

planogametes undergo fusion.

When both the gametes are motile

and morphologically similar, the

fusion process is called isogamy.

Haplophase

Dikaryophase Dplophase

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Gametangial contact

Gametangial Contact: Here, gamete

bearing structures called

gametangia come closer to each

other and develop a fertilization

tube through which the male

gamete migrates into the female

gametangium.

Gametangial copulation

Gametangial Copulation: Here, the

gametangia fuse with each other,

lose their identity and develop into

a zygospore

Spermatization and Somatogamy

Spermatisation and Somato gamy:

In some fungi like Puccinia, tiny

unicellular spore like structures

called spermatia are formed. They

get transferred to female

gametangia through various

agencies. In case of somatogamy,

two cells of different hyphae of

opposite mating type exchange

their nuclei through a fertilization

tube.

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Figure: (a) Aspergillus niger with conidiophores

Figure: (b) Conidiophores of Penicillium sp.