Mon. Oct 21st, 2019

Fungi (Yeast and Molds)

3 min read
Fungi are eukaryotic, spore bearing organisms that obtain simple organic compounds by absorption. The organisms have no chlorophyll and reproduce by both sexual and asexual means. The filamentous fungi are called molds, while the unicellular fungi are called yeasts.

The fungi (singular, fungus) include several thousand species of eukaryotic, spore bearing organisms that obtain simple organic compounds by absorption. The organisms have no chlorophyll and reproduce by both sexual and asexual means. The fungi are usually filamentous, and their cell walls have chitin.

The filamentous fungi are called molds, while the unicellular fungi are called yeasts.

Fungi grow best where there is a rich supply of organic matter. Most fungi are saprophytic (obtaining nutrients from dead organic matter). As they lack photosynthetic pigments, fungi cannot perform photosynthesis and must obtain their nutrients from preformed organic matter. i.e. chemoheterotrophic organisms.

Most fungi grow at an acidic pH of about 5.0, although some species grow at lower and higher pH levels. Most fungi grow at about 25°C (room temperature) except for pathogens, which grow at 37°C (body temperature). Fungi store glycogen for their energy needs and use glucose and maltose for immediate energy metabolism. Most species are aerobic, except for the fermentation yeasts that grow in both aerobic and anaerobic environment.

  • Fungi are eukaryotes.
  • Have absorptive nutrition.
  • Lack chlorophyll.
  • Spore-bearing.
  • Reproduce sexually and asexually.
  • Nearly all multicellular (yeasts are unicellular).

 

Distinguished from other kingdoms by:

  1. Nutrition
  2. Structural organisation
  3. Growth
  4. Reproduction

 

Molds 

  • These are fungus with long, branching filaments of cell called hyphae(singular, hypha). A tangled mass of hyphae visible to the unaided eye is a mycelium (plural, mycelia).
  • Mycelium is complex of several hyphal filaments. Hyphae can occur in three forms:
  1. Nonseptate or coenocytic.
  2. Septate with uninucleate cells.
  3. Septate with multinucleate cells.
  • In some molds, the cytoplasm passes through and among cells of the hypha uninterrupted by cross walls. These fungi are said to be coenocytic fungi.Those fungi that have cross walls are called septate fungi, since the cross walls are called septa.
  • Mycelium can be vegetative or reproductive. Vegetative mycelium penetrate into medium to obtain nutrients. Reproductive mycelium are responsible for spore production and usually extend from medium into air.

New hyphae generally arise from spore which on germination gives rise to germ tubes or tubes. These tubes elongate and branch to form hyphae.

 

 

Yeasts 

  • These are microscopic, unicellular fungi with a single nucleus and eukaryotic organelles. They range from 1 to 5 µm in width and 5 to 30 µm in length. They are commonly egg shaped but some are spherical as well as elongated.
  • They reproduce asexually by a process of In this process, a new cell forms at the surface of the original cell, enlarges, and then breaks free to assume an independent existence.

Dimorphic Fungi

Some species of fungi have the ability to shift from the yeast form to the mold form and vice versa. These fungi are dimorphic. Many fungal pathogens exist in unicellular yeast like form in their host and while growing saprophytically in the laboratory medium or in soil have filamentous mold form. E.g. Histoplasma capsulatum, Blastomyces dermatidis and Penicillium marneffei etc.

They can exist in yeast or mold form depending upon environmental conditions, genetic characteristics and physiological characteristics etc. E.g. Some dimorphic fungi can switch yeast to mold form and vice-versa depending upon prevailing temperature.

 

Reproduction

 Reproduction in fungi occurs by both asexual and sexual means.

Reproduction in yeasts usually involves spores. Spores are produced by either sexual or asexual means. Asexual spores may be free and unprotected at the tips of hyphae, where they are called conidia. Asexual spores may also be formed within a sac, in which case they are called sporangiospores.

Asexual reproduction

  • Also termed as vegetative or somatic reproduction doesn’t involve union of nuclei, sex cells or sex organs.  It occurs in the fungi when spores form by mitosis.
  • These spores can be conidia, sporangiospores (formation of single-celled spores within sacs, sporangia), arthrospores (fragments of hyphae), blastospores and chlamydospores (spores with thick walls).

 

Sexual reproduction

  • The compatible nuclei unite within the mycelium and form sexual spores. It begins with joining of two cells and fusion of protoplasts.
  • Sexually opposite cells may unite within a single mycelium, or different mycelia may be required.
  • When the cells unite, the nuclei fuse and form a diploid nucleus. Several divisions follow, and the haploid state is reestablished.

Fungal spores are important in the identification of the fungus, since t spores are unique in shape, color, and size.

A single spore is capable of germinating and reestablishing the entire mycelium. Spores are also the method for spreading fungi in the environment. Nature of the sexual spores is used for classifying fungi into numerous divisions.

 

 

 

 

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