Fungi - part 2 Dimorphic fungi The fungal cell wall Reproduction in fungi Sustenance of fungi

Fungi - part 2

Dimorphic fungi

While it is advantageous to segregate fungi into two categories - moulds and yeasts, there are several types of fungi that can adapt their arrangements in reaction to the modifications in their respective surroundings. They have the capability to develop as mycelia or in a yeast form to a large extent based on the prevailing development conditions. Such fungi that are able to adapt themselves in accordance with the environmental changes are known as dimorphic fungi. It may be noted here that the mycelia created by dimorphic fungi are genuine mycelia and very dissimilar to the pseudomycelia formed by some other varieties of yeasts. In fact, several of the dimorphic fungi are responsible for causing disease in the humans. For instance, the dimorphic fungi called Candida albicans is responsible for causing the disease thrush (an ailment, especially occurring in children and characterized by whitish spots and ulcers on the membranes of the mouth, etc). The Candida albicans fungus contaminates the mucous membranes, usually those present in the mouth and the genital tracts. The most common and noticeable symptoms of thrush in the mouth include white plaques in the mouth, while the signs of vaginal thrush include prickly white vaginal ejections.

The fungal cell wall

Fungi are often depicted similar to plants primarily because they are generally immobile and also because, like the plants, their cells are also enclosed by properly delineated and multi-layered cell walls. Notwithstanding these two similarities, the arrangement of the cell walls of the plants and fungi are significantly different. While the plant cell walls are made up of celluloses and hemi-celluloses, the cell walls of fungi are primarily made up of different polysaccharides, such as chitin - a polymer of N-acetylglucosamine. In addition to being one of the main elements of the cell walls of the fungi, chitin is also a morphological polymer that is present in the exoskeleton (a hard outer structure or covering like a shell) of the arthropod invertebrates.

Polysaccharides form a major element of the cell walls of the fungi - both moulds and yeasts. In fact, as much as 80 per cent of the materials comprising the cell walls are crystalline micro fibrils in a shapeless matrix substance. The remaining 20 per cent of the fungi cell wall components comprise of approximately equal extent of proteins and lipids. The nature of the polysaccharides composing the cell wall of the fungi largely depends on the type of the fungi. Chitin is the major fibrillar element in moulds, while polymers of glucose called glucans comprise the amorphous of shapeless matrix substance in this case. On the other hand, the yeast cell walls in the main comprise mannans - polymers of mannose. In addition, glucans are also present in the yeast cell walls. In Saccharomyces cerevisiae, commonly known as the bakers' yeast, chitin is present below 1 per cent in the cell walls. On the other hand, this polymer is mainly related to bud scars wherein it develops a mass of materials. Cellulose comprises the main structural element in the cell wall materials of Oomycetes, a special group of fungi contained by the Phycomycetes.

Generally, the cell wall of the fungi comprises five strata. This is best demonstrated by the structural design of the cell wall of the full-grown hyphae of the mould called Neurospora crassa. The foundation or base of the fungi cell wall is composed of plasmalemma. On top of the plasmalemma, there is a layer of chitin micro fibrils that is present in an amorphous or shapeless milieu of proteins, glucans and mannans. This layer is approximately 20 nanometers thick or wide. Outside the chitin micro fibrils, there is a separate layer of protein that has a width of around 10 nanometers. The protein layer sustains a network of glycoprotein set in protein. The layer of glycoprotein in the fungi cell is approximately 50 nanometers thick. However, the thickest layer of the fungi cell wall is the outer most stratums, which is composed of amorphous glucans and is around 90 nanometers thick, particularly in the fungus Neurospora crassa.

Reproduction in fungi

As discussed earlier, the reproduction or replication process in fungi may be both - asexual as well as sexual. However, in either instance, the spores are the formations that are basically responsible for scattering the new offspring to develop colonies in fresh areas. Some spores in the fungi are intended to endure or survive even in conditions not suitable for the microorganism's growth or even offer them a suitable environment for some time of dormancy. In addition, the mycelia present in the moulds may also break up into pieces and each of these fragments may afterward grow into separate thallus by means of the vegetative reproduction method. It may be mentioned here that the expression vegetative reproduction denotes a procedure where, apart from the spores, particular replication formations are not developed. When a fungus reproduces by means of asexual or vegetative procedures it is known as anamorph, while fungus reproducing sexually is called teleomorph.

Sustenance of fungi

Owing to their resemblance, quite often people mistake fungi to be a type of plants. Nevertheless, plants are able to produce intricate organic amalgams from simple inorganic substances like water and carbon dioxide by means of a process known as photosynthesis. On the other hand, fungi require complete organic compounds for energy generation as well as their development. Hence, fungi are expressed as heterotrophic organisms derived from two Greek terms 'heteros' and 'trophikos' denoting nourishment. Therefore, it is evident that heterotrophs are nurtured from some other place, instead of themselves being able to nourish themselves in the manner of the autotrophic plants. It may be mentioned here that majority of the fungi are usually in dark and moist environments, but are commonly present where organic substances are generally found.

As fungi may reproduce asexually as well as sexually, they may also be saprophytes (organisms surviving on dead organic matter) or parasites (organisms surviving on an organism of another species, known as the host, from the body of which it obtains nutriment). The term saprophyte has been derived from the Greek terms 'sapros' and 'phuton' that denote dead plants. Similarly, word parasite has also been derived from the Greek expressions 'para' denoting other or beyond and 'sitos' meaning food. Hence, parasites obtain their sustenance from other living organisms - plants or animals, and usually cause or pass on diseases on to their hosts. In fact, most of the fungi are present as saprophytes in the soil. They survive on decomposing plant materials wherein they have a crucial role in the reprocessing organic substances. Usually, fungi obtain their nourishment by exuding hydrolytic enzymes in their surroundings and these enzymes assimilate different polymers to manufacture soluble substances that the fungus is able to take up.

Fungi are also able to grow on a mineral salts agent enclosing a source of nitrogen salts in synthetic or stimulated culture. However, in this instance it is necessary that the mineral salt medium possesses glucose as a source of carbon. The presence of glucose enables the fungi to produce all the intricate organic molecules they require for growth by the metabolism of glucose. On the other hand, other types of fungi need an external supply of vitamins or other developmental aspects which they themselves cannot produce with the purpose of growing in synthetic or stimulated conditions/ culture. Provided the vegetative cells in an artificial culture are required to initiate production of spores, the fungi essentially require additional growth aspects. It may be noted here that the fungi have a particular need for trace elements, such as iron, calcium, copper, magnesium, manganese and zinc, to grow in artificial culture and otherwise. However, thus far scientists have not identified any fungus that is able to fix nitrogen from the atmosphere.

The most important polymer used by fungi for storage purpose is glycogen. However, they also use oil globules for storage of nutrients. Akin to other eukaryotes, fungi also require air or oxygen for free breathing and they acquire energy from the aerobic respiration of glucose. Nevertheless, a marginal species of yeasts are equipped to survive under different sets of conditions and do not require air for respiration. In other words, they are able to survive in anaerobic conditions and acquire their energy from fermentation. However, this procedure is not as effectual as aerobic respiration. In artificial culture, approximately 1 per cent of the transformed baker's yeast or Saccharomyces cerevisiae is found having no mitochondria. As a result, these fungi cells are unable to carry out respiration as they are incapable of pulling mitochondria together. Therefore, when they develop on any glucose-based firm medium, they are able to get the requisite energy only by means of fermentation. In fact, compared to the wild kind of cells that enclose mitochondria, such transformed or altered cells usually develop quite small colonies. Hence, these types of cells are also known as petite mutants.


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