CIRCULATION IN FUNGI: NUTRIENTS, SUBSTANCES, OSMOREGULATION - BIOLOGY - 2024

The circulation fungi is the system by which the transport of substances occurs from the outside towards the inside of fungi and vice versa. This includes the absorption of nutrients to distribute them throughout its structure, as well as the transport of enzymes and excretion of substances, in addition to other functions that require fluid exchange.

These organisms do not contain chlorophyll like plants, and neither do they contain a blood vessel system like animals do. In contrast, fungi do not have any specialized tissue for this function.

Graphic representation of fluid circulation in hyphae and yeasts. Source image on the left Flickr, image on the right Wikipedia.com

However, fungi, like all living beings, behave as dynamic systems in which there is transport of substances and nutrients. In this case they are carried out through the movement of the cytoplasm, or with the help of transporter vesicles.

Fluid circulation in fungi can be observed in the process of digestion and absorption of nutrients, in the morphogenesis of fungal structures, in the osmotic balance and in the expulsion of waste substances.

There are mechanisms in these microorganisms that regulate the entry and exit of substances, as well as specific mechanisms for their transport.

The circulation of fluids in these organisms is very important for their survival. Therefore, the substances used for the treatment of fungal infections are aimed at altering the permeability of the cytoplasmic membrane, generating an imbalance in the cell that ends in cell death.

Nutrient circulation

The feeding of the fungi is carried out by a process called direct absorption. This nutrient assimilation system requires a previous step in which the fungi secrete enzymes into the environment to degrade organic matter, and thus be able to absorb their nutrients in smaller molecules.

Thus, they perform a kind of external digestion (outside the cell structure). Later, the dissolved nutrients cross the cell wall (which is composed of chitin) to finally be distributed evenly towards the protoplasm by a process called simple diffusion or osmosis, in which there is no energy expenditure.

This form of feeding is known by the name of osmotrophy. In addition, due to the way in which fungi feed, they are said to be heterotrophic, since they cannot produce their own organic compounds as occurs in autotrophic organisms.

That is, the energy they need is obtained through the assimilation and metabolism of organic compounds dissolved by exoenzymes.

The structures in charge of distributing nutrients in filamentous or multicellular fungi are the hyphae. These participate in the exchange of nutrients and water between the different parts of the fungus.

Circulation of substances in the morphogenesis of fungal structures

The formation of the structures of the fungus also requires the circulation of substances. This is done in a slightly different way.

Hyphal elongation

The elongation of hyphae in fungi is possible thanks to the directional transport of vesicles containing precursor substances from the hyphal wall together with synthetases. These vesicles are directed towards the apical dome of the hypha, where the liberation of the vesicular contents will occur.

The generation of the new hyphal wall for the formation and polymerization of microfibrils requires the enzyme chitin synthetase. This enzyme is transported to the hyphal tip in microvesicles called chitosomes in the form of zymogens (inactive enzyme).

Chitosomes form in the cytoplasm in a free form or within larger vesicles similar to those generated by the Golgi apparatus.

Subsequently, the activation of chitin synthetase occurs by the fusion of the chitosome to the plasmalemma, allowing the interaction of a protease bound to the membrane with the inactive enzyme (zymogen). This is how chitin microfibrilogenesis begins at the hyphal tip.

Budding yeast

In the case of yeasts there is also transport of substances. In this case, it is necessary for the biosynthesis of the yeast cytoskeleton. It requires a protease synthetase that is uniformly distributed in the cytoplasm and that binds to the cell membrane.

This enzyme is active at yeast growth sites, and is inactive when there is no division.

It is believed that the activating substances of the enzyme can be transported through microvesicles to the plasmalemma at sites where cell wall biosynthesis (budding and septal separation) is active.

Balance between the synthesis of the elongation of the hypha or yeast wall and the modification of the matrix

In the processes of formation and insertion of new structures and the modification of the pre-existing matrix, both in the case of filamentous fungi and in yeast buds, there must be a balance.

In this sense, the presence of lytic enzymes that are transported in macrovesicles to target the hyphal tip or the yeast bud has been discovered.

These enzymes are β1-3-glucanase, N-acetyl-β-D-glucosaminase and chitinase. The enzymes act when the macrovesicle fuses with the plasma membrane, being released in the appropriate place to exert their action (exocytosis).

Osmoregulation

This process involves the movement of substances through various mechanisms, such as passive transport, active transport, and exocytosis.

Yeasts and some molds are characterized by being osmophilic or xerotolerant microorganisms. This means that they can grow in non-ionic environments with high osmolarity. This allows them to grow on substrates with a high concentration of organic compounds, such as glucose.

Much research has been done to understand this mechanism, revealing that yeast contain highly hydrophilic proteins within them that protect the cell from dehydration.

It has also been discovered that substances such as glycerol can act as osmoregulatory substances that protect cells from fungi, giving them the ability to adapt more quickly to osmotic changes.

Substance transport mechanisms

Inside fungi, three different types of substance transport can occur: passive transport, active transport, and exocytosis.

Passive transport is that which occurs without energy expenditure, since it occurs by simple diffusion (exit or entry of substances through any part of the membrane). In this case, the substance passes to the other side of the membrane, where the concentration of that metabolite is lower. Thus, a substance can pass from the inside of the fungus to the outside, or vice versa.

It can also be given by facilitated diffusion, which works by the same principle as the previous process, with the exception that it uses transporter proteins found in the plasma membrane.

On the other hand, active transport is one that requires energy expenditure, because it occurs against a concentration gradient.

Finally, exocytosis is the excretion of substances to the outside that are released through vesicles when they fuse with the plasma membrane.

Disposal of waste substances

Fungi, as a result of metabolism, expel waste substances that are eliminated through cell membranes. This process is known as excretion, and it occurs through exocytosis.

The substances released by fungi can be used later by other organisms or by themselves.

Effect of antifungals on the circulation of fungi

Antifungals are substances used to eliminate pathogenic or opportunistic fungi that are producing a specific pathology in humans and animals.

What these drugs do is alter the movements of certain substances (such as potassium or sodium), generally causing their exit from cells. On the other hand, others induce the entry of calcium ions into the body, causing cell death.

Two of the most common examples of antifungals are amphotericin B and triazoles. Amphotericin B binds to the fungal sterols and destabilizes cell permeability, allowing cytoplasmic material to escape, causing death.

On the other hand, triazoles prevent the synthesis of ergosterol. This causes the loss of the integrity of the fungal membrane.

Ref.

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2. Robinow C, Marak J. On the Plasma Membrane of Some Bacteria and Fungi. Circulation. 1962; 26: 1092-1104. Available in: ahajournals.org
3. Osmoregulation. Wikipedia, The Free Encyclopedia. 21 Apr 2019, 00:20 UTC. 11 May 2019, 01:13 en.wikipedia.org
4. Moreno L. Response of plants to stress due to water deficit. A review. Colombian Agronomy, 2009; 27 (2): 179-191. Available at: magazines.unal.edu.co
5. Thompson L. Antifungals. Rev. chil. infectol.. 2002; 19 (Suppl 1): S22-S25. Available at: https: // scielo.