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24.1B: Fungi Cell Structure and Function - Biology

24.1B: Fungi Cell Structure and Function - Biology



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Fungi are unicellular or multicellular thick-cell-walled heterotroph decomposers that eat decaying matter and make tangles of filaments.

Learning Objectives

  • Describe the physical structures associated with fungi

Key Points

  • Fungal cell walls are rigid and contain complex polysaccharides called chitin (adds structural strength) and glucans.
  • Ergosterol is the steroid molecule in the cell membranes that replaces the cholesterol found in animal cell membranes.
  • Fungi can be unicellular, multicellular, or dimorphic, which is when the fungi is unicellular or multicellular depending on environmental conditions.
  • Fungi in the morphological vegetative stage consist of a tangle of slender, thread-like hyphae, whereas the reproductive stage is usually more obvious.
  • Fungi like to be in a moist and slightly acidic environment; they can grow with or without light or oxygen.
  • Fungi are saprophyte heterotrophs in that they use dead or decomposing organic matter as a source of carbon.

Key Terms

  • glucan: any polysaccharide that is a polymer of glucose
  • ergosterol: the functional equivalent of cholesterol found in cell membranes of fungi and some protists, as well as, the steroid precursor of vitamin D2
  • mycelium: the vegetative part of any fungus, consisting of a mass of branching, threadlike hyphae, often underground
  • hypha: a long, branching, filamentous structure of a fungus that is the main mode of vegetative growth
  • septum: cell wall division between hyphae of a fungus
  • thallus: vegetative body of a fungus
  • saprophyte: any organism that lives on dead organic matter, as certain fungi and bacteria
  • chitin: a complex polysaccharide, a polymer of N-acetylglucosamine, found in the exoskeletons of arthropods and in the cell walls of fungi; thought to be responsible for some forms of asthma in humans

Cell Structure and Function

Fungi are eukaryotes and have a complex cellular organization. As eukaryotes, fungal cells contain a membrane-bound nucleus where the DNA is wrapped around histone proteins. A few types of fungi have structures comparable to bacterial plasmids (loops of DNA). Fungal cells also contain mitochondria and a complex system of internal membranes, including the endoplasmic reticulum and Golgi apparatus.

Unlike plant cells, fungal cells do not have chloroplasts or chlorophyll. Many fungi display bright colors arising from other cellular pigments, ranging from red to green to black. The poisonous Amanita muscaria (fly agaric) is recognizable by its bright red cap with white patches. Pigments in fungi are associated with the cell wall. They play a protective role against ultraviolet radiation and can be toxic.

The rigid layers of fungal cell walls contain complex polysaccharides called chitin and glucans. Chitin, also found in the exoskeleton of insects, gives structural strength to the cell walls of fungi. The wall protects the cell from desiccation and predators. Fungi have plasma membranes similar to other eukaryotes, except that the structure is stabilized by ergosterol: a steroid molecule that replaces the cholesterol found in animal cell membranes. Most members of the kingdom Fungi are nonmotile.

Growth

The vegetative body of a fungus is a unicellular or multicellular thallus. Dimorphic fungi can change from the unicellular to multicellular state depending on environmental conditions. Unicellular fungi are generally referred to as yeasts. Saccharomyces cerevisiae (baker’s yeast) and Candida species (the agents of thrush, a common fungal infection) are examples of unicellular fungi.

Most fungi are multicellular organisms. They display two distinct morphological stages: the vegetative and reproductive. The vegetative stage consists of a tangle of slender thread-like structures called hyphae (singular, hypha ), whereas the reproductive stage can be more conspicuous. The mass of hyphae is a mycelium. It can grow on a surface, in soil or decaying material, in a liquid, or even on living tissue. Although individual hyphae must be observed under a microscope, the mycelium of a fungus can be very large, with some species truly being “the fungus humongous.” The giant Armillaria solidipes (honey mushroom) is considered the largest organism on Earth, spreading across more than 2,000 acres of underground soil in eastern Oregon; it is estimated to be at least 2,400 years old.

Most fungal hyphae are divided into separate cells by endwalls called septa (singular, septum) ( a, c). In most phyla of fungi, tiny holes in the septa allow for the rapid flow of nutrients and small molecules from cell to cell along the hypha. They are described as perforated septa. The hyphae in bread molds (which belong to the Phylum Zygomycota) are not separated by septa. Instead, they are formed by large cells containing many nuclei, an arrangement described as coenocytic hyphae ( b). Fungi thrive in environments that are moist and slightly acidic; they can grow with or without light.

Nutrition

Like animals, fungi are heterotrophs: they use complex organic compounds as a source of carbon, rather than fix carbon dioxide from the atmosphere as do some bacteria and most plants. In addition, fungi do not fix nitrogen from the atmosphere. Like animals, they must obtain it from their diet. However, unlike most animals, which ingest food and then digest it internally in specialized organs, fungi perform these steps in the reverse order: digestion precedes ingestion. First, exoenzymes are transported out of the hyphae, where they process nutrients in the environment. Then, the smaller molecules produced by this external digestion are absorbed through the large surface area of the mycelium. As with animal cells, the polysaccharide of storage is glycogen rather than the starch found in plants.

Fungi are mostly saprobes (saprophyte is an equivalent term): organisms that derive nutrients from decaying organic matter. They obtain their nutrients from dead or decomposing organic matter, mainly plant material. Fungal exoenzymes are able to break down insoluble polysaccharides, such as the cellulose and lignin of dead wood, into readily-absorbable glucose molecules. The carbon, nitrogen, and other elements are thus released into the environment. Because of their varied metabolic pathways, fungi fulfill an important ecological role and are being investigated as potential tools in bioremediation.

Some fungi are parasitic, infecting either plants or animals. Smut and Dutch elm disease affect plants, whereas athlete’s foot and candidiasis (thrush) are medically important fungal infections in humans.


Fungal Structure

Hypha is characterized as a tube-like structure with a rigid wall that contains a moving slug of protoplasm. The length of the hypha varies in different fungal species however, the diameter ranges from 2 to 30 micrometers and depends on the species and growth stage of the organism. The growth occurs at the tip of the hypha and the tip of the hypha contains a tapered region called extension zone. Apart from the growing tip region, the rest of the part of hypha ages accordingly and breaks down due to autolytic enzymes. The presence of cross walls is an important feature in most of the fungi. These cross walls are called septa (singular septum). However, members of the oomycete and Zygomycota lack the septa in their hypha. The hypha is also surrounded by a complex wall-like structure which thin at the apex region but the thickness increases later on. The plasma membrane is closely attached to the cell wall which made the hypha safe from plasmolyze.


Cell Structure of Yeast (With Diagram) | Fungi

The yeasts are unicellular fungi. Cells may remain attached in short chains forming a pseudomycelium, but they do not produce true mycelium. The cells are extremely variable in shape being globose, oval, elongated, or rectangular.

The yeast cells are very polymorphic and are capable of assuming different forms depending upon the medium in which they grow and their age. Individually yeast cells are hyaline but in colonies they appear white, cream-coloured or slightly brownish.

The structure of yeast cell has been very thoroughly worked out by a large number of investigators who differ in their interpretations. Each yeast cell has a disti­nct cell wall enclosing granular cytoplasm, within which can be seen a large vacoule and a nucleus (Fig. 214). The vacuole varies much in size according to the state of activity of the cell.

It may at times become much contracted, but it does not disap­pear completely except during spore formation. The wall is thin delicate and is com­posed of chitin in combination with other compounds. Reserve materials are present in the cytoplasm in the form of oil globules, glycogen, and volutin.

Glycogen accumula­tion increases with the decrease in fermentation. The volutin content is also very much linked with the metabolic conditions of the yeast cells. Difference of opinion exists whether the vacuole is a part of the nucleus or the vacuole and the nucleus are separate entities.

Despite the fact that Saccharomyces cerevisiae has been subject of cytological study for over 50 years, investigators at the present time are still not in agreement. According to old idea of Wager and Peniston the nucleus has a large vacuole and that the body to one side of it is the nucleolus. But Wager during later time described the nucleus as a homogeneous body having its nucleolus at one end.

His explanation was supported by Lindegren, who stated that the central area of the yeast nucleus is not a vacuole but a true nucleus which is traversed by chromatin threads and the body which Wager described as nucleolus, is centrosome. But Guillermond is of opinion, there is a central vacuole which does not belong to the nucleus.

Apposed to the vacuole is the nucleus which according to Wager is the nucleolus and to Lindegren is centrosome. But great majority of investigators have concluded that the vacuole is a cytoplasmic constituent and that the distinctive chromatic body referred to by Wager and Peniston is actually the nucleus.

The small size of the yeast cell renders observation of its contents under the light microscope difficult, and there has previously been controversy about the interpreta­tion of its structure. Studies of thin sections of yeast cells (Saccharomyces cerevisiae) under the electron microscope have clarified our knowledge.

The cell wall contains protein, lipid, and at least two polysaccharides (a mannan and a glucan). Chitin has also been reported. Immediately below the cell wall is the cytoplasmic membrane. The mem­brane is made up of particles penetrated by fibrils which probably correspond to the glucan fibrils of the cell wall. Within the cytoplasm are mitochondria, endoplasmic reticulum and reserves of fat and glycogen (Fig. 215).

A large vacuole surrounded by a single unit membrane contains strands and granules of dense material sometimes linked into a network. The larger granules are probably volutin, i.e., polymetaphos- phate. The nucleus is surrounded by a double unit membrane (nuclear membrane) perforated by pores. The nuclear membrane is distinct from the vacuolar membrane which surrounds the vacuole.

The problem in Saccharomyces has been both to identify the nucleus and to describe its division. Nagel (1946), in a valuable and painstaking comparative study, employed a wide range of stains and fixatives as well as living cells and decided that the nucleus is Feulgen-positive, small body lying to the side of a central vacuole.

This view was confirmed much later by electron microscopy. Moor and Muhlethaler made it clear that there is a large, more or less central vacuole with which is closely associated a much smaller nucleus. There does not seem to be a structure comparable to the ‘nucleolus’ or ‘central body’ of other fungi. The nucleus is bipartite.

The larger part is Feulgen-positive and during division becomes elongated, constricted in the centre and eventually separates into two halves (Fig. 216). Individual chromosomes are not usually recognizable. A second, crescentic, smaller part is strongly basiphilic but Feulgen- and Giemsa-negative (Fig. 216 A-D).

This body becomes attenuate, sometimes breaking into small, irregular pieces and eventually is more or less evenly divided between the daughter nuclei (Fig. 216 E-H).

Robinow and Marak (1966) provided evidence that there is a single, intranuclear fibre. Opposite the crescentic, basiphilic body within the Feulgen- positive material is a small bead-like structure (Fig. 21bA).

This bead of fibre initial, expands into a long, straight fibre which becomes anchored at each end to the nuclear membrane (Fig. 216 C-F). It is, perhaps, not unreasonable to apply the terms nucleolus and centriole to the basiphilic crescent and fibre initial respectively. Other yeasts, notably Lipomyces, differ from Saccharomyces in possessing distinct, countable chromosomes.

Recent investigators, regardless of their diverse opinions on the mechanics of nuclear division and the number of chromosomes present, have agreed that the nucleus is the relatively small body and is distinct from the large vacuole in the yeast cell. The nucleus is surrounded by a membrane.

The nuclear membrane remains intact during mitosis and the spindle is intra-nuclear as it is in all fungi. Mitotic division of the yeast nucleus was for the first time demonstrated by Guillermond (1901) and was subsequently persued by many other investigators. He also pointed out that the yeast cells have typical nuclei similar to other fungi.


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Growth

Although dimorphic fungi can change from the unicellular to multicellular state (depending on environmental conditions), most fungi are actually multicellular organisms. They display two distinct morphological stages: the vegetative and reproductive. The vegetative stage consists of a tangle of slender thread-like structures called hyphae (singular, hypha), whereas the reproductive stage can be more conspicuous.

Fungal hyphae, although microscopic, allow for the rapid flow of nutrients and small molecules across the fungal body. Many fungi create networks of these hyphae into a mass called a mycelium. The mycellium can grow on a surface, in soil or decaying material, in a liquid, or even on living tissue. Although individual hyphae are tiny, the overall mycelium of a fungus can be very large, with some species truly being “the fungus humongous”. The giant Armillaria solidipes (honey mushroom) is considered the largest organism on Earth, spreading across more than 2,000 acres of underground soil in eastern Oregon it is estimated to be at least 2,400 years old!

Fungi thrive in environments that are moist and slightly acidic, and can grow with or without light and oxygen. Most fungi are obligate aerobes, requiring oxygen to survive, however some species, such as the Chytridiomycota that reside in the rumen of cattle, are obligate anaerobes for these species, anaerobic respiration is used because oxygen will disrupt their metabolism or kill them. Yeasts, like those used in wine or beer-making, are intermediates: facultative anaerobes. They grow best in the presence of oxygen using aerobic respiration, but can survive using anaerobic respiration when oxygen is not available.


The nitrilase superfamily: classification, structure and function

The nitrilase superfamily consists of thiol enzymes involved in natural product biosynthesis and post-translational modification in plants, animals, fungi and certain prokaryotes. On the basis of sequence similarity and the presence of additional domains, the superfamily can be classified into 13 branches, nine of which have known or deduced specificity for specific nitrile- or amide-hydrolysis or amide-condensation reactions. Genetic and biochemical analysis of the family members and their associated domains assists in predicting the localization, specificity and cell biology of hundreds of uncharacterized protein sequences.

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How Do We Get Rid Of Candida Yeasts & Fungi?

Since fungi are decomposers, the common speculation that it eats only sugar is a misnomer in my honest opinion. Yeast eats sugar and produces ethanol and carbon dioxide yeast is the unicellular form. Candida is the multicellular form that sends out hyphae that feed by releasing exoenzymes into whatever it is attached to, or comes in contact with. This external digestion allows Candida to absorb the nutrients through the hyphae. Therefore, everything is a possible food source though it is mostly looking for glucose.

Generally, the highly acidic environment of the stomach will not allow Candida to live so it is found in the small and large intestine mostly. Some kind of colon cleansing product is in order so a person stays regular, removes excess mucous from the top of the biofilm while helping to break down the biofilm itself, flushing it out and dislodging it from the intestinal wall.

Once you get through the biofilm, the cell wall of the fungi must be broken down to expose the soft lipoprotein membrane to destruction. Since the cell wall is composed of mostly polysaccharides, mannoproteins and chitin, i.e. proteins and carbohydrates, if we introduce enzymes that specifically eat these substances we can break through more of the biofilm and cell wall.

Once the lipoprotein membrane is exposed we can use medically proven natural anti-fungals to attack the membrane and explode the nucleus.

Here is a testimonial for a fungal infection-treated satisfied person:

I have been battling Candida for 9 months, tried many products and even many trips to doctors.਋iofase and Profase are the first products I have taken where I am noticing a change. WHY don't they educate you on the biofilms. Thank you for opening my eyes to that fact. I am going to be ordering more soon.

Most people that are infected with Candida suffer from immune system dysfunction whether they are aware of it or not. For instance, in vaginal yeast infections there have been studies suggesting a deficiency in T-lymphocyte responses that permit the Candida to take hold and allow its overgrowth.

So it wouod be best if we boost or stimulate immune system function, raise antioxidant levels in the intestine, and in general provide the body with good nutrition through supplementation and diet. The immune system should also come into play and the macrophages, lymphocytes, and neutrophils can begin to clean up the mess as they help destroy the fungi. IgA levels can also be increased  through supplement usage to increase protection and heal the walls and other mucous linings of the intestine.

In cases of chronic vaginal infections the hormones may have to be balanced since the yeast becomes hormone dependent as evidenced by the re-occurrence at particular times of the menstrual cycle. We know that it must have excess estrogen to survive and estrogen influences vaginal glucose levels.

Excess estrogen not only causes cancer, mostly breast, but also raises the glycogen levels in the vagina and glycogen is the food supply for yeast. If you suffer from bad pms symptoms the chances are you have high estrogen levels that need to be corrected. Your menstrual cycle should sneak up on with you with very little discomfort.

High cholesterol levels, plaque, scar tissue as in Fibromyalgia, etc, that are possibly created by the fungi, can be resolved with another special enzyme designed to digest these substances from within the body.

The Treatment Plan I suggest for fungi, Candida, and yeast addresses all these problems and the Candida has no natural defense against some of these products. They can't adapt and shape shifting doesn't help them either, they are totally defenseless, it is only a matter of time before you have won your battle with this beast!

Does any of this make sense to you?


Function of Microtubules

Cell Movement

Microtubules give structures like cilia and flagella their structure. Cilia are small protuberances of a cell. In humans, they are found on cells lining the trachea, where they prevent materials like mucus and dirt from entering the lungs. They are also found in the fallopian tubes of the female reproductive system, where they help move the egg that is released from the ovary to the uterus. Flagella are tail-like appendages that allow cells to move. They are found in some bacteria, and human sperm also move via flagella. Microtubules also allow whole cells to “crawl” or migrate from one place to another by contracting at one end of the cell and expanding at another.

Cell Division

Microtubules play a key role in forming the mitotic spindle, also called the spindle apparatus. This is a structure that is formed during mitosis (cell division) in eukaryotic cells. The mitotic spindle organizes and separates chromosomes during cell division so that the chromosomes can be partitioned into two separate daughter cells. Its components include microtubules, the MTOC, and microtubule-associated proteins (MAPs).

Three subgroups of microtubules aid in the process of mitosis: astral, polar, and kinetochore microtubules. Astral microtubules radiate from the MTOCs of a cell to the cell membrane, keeping the mitotic spindle in place. Polar microtubules intertwine between two MTOCs and help separate chromosomes. (All microtubules are polar these ones are just specifically called polar microtubules.) Kinetochore microtubules attach to chromosomes to help pull them apart the chromosomes are attached to the microtubules by a complex of proteins called a kinetochore.

Cell Transport

As part of the cytoskeleton, microtubules help move organelles inside a cell’s cytoplasm, which is all of the cell’s contents except for its nucleus. They also help various areas of the cell communicate with each other. However, even though microtubules help components of the cell to move, they also provide the cell with shape and structure.


Distribution and abundance

Fungi are either terrestrial or aquatic, the latter living in freshwater or marine environments. Freshwater species are usually found in clean, cool water because they do not tolerate high degrees of salinity. However, some species are found in slightly brackish water, and a few thrive in highly polluted streams. Soil that is rich in organic matter furnishes an ideal habitat for a large number of species only a small number of species are found in drier areas or in habitats with little or no organic matter. Fungi are found in all temperate and tropical regions of the world where there is sufficient moisture to enable them to grow. A few species of fungi live in the Arctic and Antarctic regions, although they are rare and are more often found living in symbiosis with algae in the form of lichens (see below Lichens). About 144,000 species of fungi have been identified and described, but mycologists estimate that there may be between 2.2 million and 3.8 million total species.


Parasitic Fungi

Parasitic fungi are the principal disease-causing organisms in plants. Fungal attacks can result in devastating agricultural losses.

Phytophthora infestans is not strictly a fungus although it was classified as a fungus for many years. It is, in fact, a colourless, filamentous alga and its walls contain some cellulose, unlike the true fungi. It is described here because its parasitic life-style closely resembles that of the pathogenic fungi which infest plants and also because it causes serious plant diseases such as tomato blight and the potato blight which caused the devastating Irish potato famine in 1845.

The hyphae of this parasite spread internally through the leaves. Short branches from the hyphae penetrate the cell walls, with the aid of enzymes, and absorb nutriment from the cell contents. The cells are eventually killed and then the leaves and finally the whole shoot die.

Before this happens, branching hyphae grow out of the stomata and produce sporangia at their tips. The tips of the hyphae constrict to cut off individual sporangia which are blown away in air currents. If a sporangium lands on a leaf of a healthy potato plant in warm moist conditions, a new hypha grows out from it and penetrates the leaf.

When sporangia fall on the ground, they may be washed into the soil by rain, so reaching and infecting the potato tubers, causing them to rot. The close proximity of the plants in the potato field allows very rapid spread of the disease from one individual to the next.

Agricultural research is constantly trying to find varieties of food plants which are resistant to this infection and to other types of fungus disease. Researchers also aim to develop sprays which destroy the fungus without causing harmful side effects on the crop or on the other organisms in the area.