EXCRETION IN FUNGI: PROCESSES AND CHARACTERISTICS - BIOLOGY - 2025

The excretion in fungi involves a series of processes from which release extracellular space to a variety of substances, some beneficial for other living beings and other life threatening.

Excretion is the process by which cells release certain substances that are the product of their metabolism. These substances that are released are of no use to the cell, which is why they are excreted outside the cell.

Fungi produce substances that they excrete, harmful or beneficial. Source: pixabay.com

Depending on the evolutionary level of the organism, the excretion mechanisms will be different. From the simplest, such as diffusion, to more complex processes such as those carried out in higher organisms, which have specialized tissues for this purpose.

Unicellular fungi

Yeasts belong mainly to the group of unicellular fungi. These are organisms that are made up of a single cell. They are generally between 3 and 40 microns in size.

These types of organisms produce certain substances that they excrete or release into the extracellular space. Yeasts are widely recognized as organisms that carry out the alcoholic fermentation process.

This is a process by which yeasts, among other microorganisms, process some sugars such as sucrose, fructose and glucose to obtain ethanol (ethyl alcohol) and carbon dioxide (CO2) as final products. This process is carried out anaerobically, that is, in the absence of oxygen.

Saccharomyces cerevisiae seen under a microscope. Source: A doubt

Likewise, some yeasts such as Saccharomyces cerevisiae, apart from producing ethanol by fermentation, can also produce riboflavin (vitamin B2). They have even begun to be used in the pharmacological industry to synthesize this compound.

Excretion in unicellular fungi

The metabolic mechanism of single-celled fungi is quite simple. These organisms do not have a very elaborate mechanism to excrete the substances that they produce mainly through fermentation.

In this sense, the mechanism by which yeasts excrete these substances is through a form of passive transport known as diffusion.

Diffusion is the process by which a substance crosses the cell membrane down the concentration gradient. This means that it diffuses from a place where there is a high concentration of it to a place where there is little concentration. This is done in order to balance the concentrations on both sides of the membrane.

It is through the diffusion process that single-celled fungi such as yeast excrete the products of fermentation. This is thanks to one of the characteristics of the cell membrane: permeability.

It is important to remember that the cell membrane is a semi-permeable structure, which means that it allows certain molecules to pass through it, including some gases such as carbon dioxide and alcohols such as ethanol, both products of fermentation.

The mechanism through which riboflavin is excreted in yeast has not been fully elucidated, however, taking into account the process carried out in some bacteria, some specialists suggest that this vitamin crosses the membrane with the help of some of the transporter proteins What is there.

Multicellular fungi

Multicellular fungi are the largest and most varied group. They are characterized because they are made up of many cells that associate, but without forming specialized tissues. The cells make up the hyphae, which in turn constitute the mycelium of the fungus.

This group of fungi includes the basidiomycetes (the well-known mushrooms), ascomycetes, and zygomycetes.

This type of mushroom is highly appreciated at an industrial level, since it is used mainly in the gastronomic and pharmacological area, among others. Multicellular fungi have also been characterized because they synthesize certain substances, some beneficial for man, others not so much.

These include: some hallucinogenic substances, toxins (some even deadly) and penicillary substances.

Excretion in multicellular fungi

While it is true that these fungi do not have a specialized tissue in the excretion process, it is also true that their metabolic mechanisms are a little more complex than that of single-celled fungi.

In multicellular fungi, excretion occurs through a process known as exocytosis. This is defined as the process by which certain compounds are released from cells through vesicles that transport them to the outside of the cell. It is a process that requires the expenditure of energy by the cell.

Representation of exocytosis, excretion process in multicellular fungi. Source: OpenStax

The vesicles that are used to release the different compounds to the outside are made by the Golgi apparatus. Once they are ready, with the content properly packed inside, they move towards the cell membrane with the help of the cell's cytoskeleton, as well as microtubules and proteins such as actin.

When the vesicle comes into contact with the cell membrane it begins to fuse with it, which allows its contents to flow out of the cell. This process is mediated by a protein complex called SNARE, which, in some cases, even functions as a regulatory element.

Products of excretion

As already mentioned, both unicellular and multicellular fungi produce certain substances that they secrete. Some of these are harmful, some are not.

Ethanol

It is a compound whose chemical formula is C ₂ H ₅ OH. It is produced through an anaerobic fermentation process, specifically alcoholic fermentation. This process is carried out by yeast-type fungi.

It has a density of 0.789 g / cm ³ and a boiling point of 78 ° C. It is also colorless. It is used mainly in the gastronomic industry as an essential element in alcoholic beverages. It also has other uses such as a solvent, disinfectant, antifreeze and even as a fuel.

Riboflavin

Also known as vitamin B2. Structurally, it is made up of a flavin molecule (nitrogen base) and a ribitol molecule.

It has a wide variety of positive effects on the body, such as maintaining the integrity of the mucosa and the skin, as well as maintaining the good condition of the cornea.

Mycotoxins

They are toxic chemical compounds that some multicellular fungi synthesize. Many mycotoxins are synthesized inside mold-type fungi, so they can be found on the surface of food that has been invaded by these fungi.

There are several types of mycotoxins. Among the best known are:

• Ochratoxin A: it is synthesized mainly by fungi of the Aspergillus and Penicilium genera. Among the mechanisms of action it has are mentioned: alteration of cellular respiration, alteration of protein synthesis. It is also considered to be carcinogenic, teratogenic, neurotoxic, nephrotoxic, and immunosuppressive.
• Patulin: it is produced by fungi of the Aspergillus, Penicilium, Gymnoascus and Paeocilomyces genera. It has detrimental effects on the liver, kidney, and spleen, as well as the immune system.
• Aflatoxins: they are secreted by fungi of the genus Aspergillus, especially Aspergillus flavus and Aspergillus parasiticus. This mycotoxin has a particular damaging effect on the liver, such as necrosis, cirrhosis and even liver cancer.

Penicillin

It is a bactericidal substance that is secreted by fungi of the genus Penicilium mainly. Its discovery in 1928 by Alexander Fleming was a milestone in the field of medicine, as it began to be used to fight infections caused by previously life-threatening bacteria.

Although the mechanism by which they kill bacteria is not fully established, it is believed that they activate autolytic enzymes that act on the cell wall of some bacteria, destroying them.

Hallucinogenic substances

They are substances that are synthesized by various types of fungi that act on the central nervous system altering the perception of reality, causing visual and auditory hallucinations.

Among the most well-known hallucinogenic substances we can mention: psilocybin, baeocystin and ibotenic acid.

References

1. Conesa, A., Punt, P., Van Luijk, N., Van den Hondel, C. (2001) The secretion pathway in filamentous fungi: a biotechnological view. Fungal Genet Biol. 33 (3) 155-171.
2. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition.
3. Diener, S. (2005). Insight filamentous fungal secretion and evolution through genomic analysis. Retrieved from:
4. Suárez, C., Garrido, N. and Guevara, C. (2016). Saccharomyces cerevisiae yeast and alcohol production. Bibliographic review. ICIDCA on derivatives of sugar cane. 50 (1).
5. Wagner, J., Otero, M., and Guerrero I. Yeasts and their derived products as ingredients in the food industry. National University of Quilmes Editorial.