EXCRETION IN BACTERIA AND PROTISTS - BIOLOGY

The excretion in bacteria and protists is performed by various cellular mechanisms. These include passive transport, made up of osmosis and diffusion, and active transport, such as exocytosis.

Likewise, there is a special type of transport for excretion in protists mediated by contractile vacuoles present in their cytoplasm. All these processes are of vital importance, since many of the substances that these organisms excrete are used by other living beings to survive.

Bacteria have various excretion mechanisms. Source: pixabay

Excretion is a natural process in any cell by means of which substances that are of no use to it and which can sometimes be toxic are released into the extracellular environment. All living things carry out the process of excretion, even the simplest ones.

Excretion in bacteria

Bacteria are organisms that have a very active metabolism. Product of this metabolism, a large number of chemicals and compounds are generated that do not fulfill any function within the bacterial cell. Because of this, they must be released outside the cell.

Various processes are carried out inside bacterial cells, such as respiration, both aerobic and anaerobic. In the first, water and carbon dioxide are obtained as products. On the other hand, anaerobic respiration, known as fermentation, generates two products: lactic acid (lactic fermentation) and acetic acid (acetic fermentation).

Likewise, as a product of the various normal metabolic processes that take place within the battery, so-called secondary metabolites are obtained, such as exotoxins, antibiotic substances and enzymes, among others.

In bacteria, the excretion of the products of their metabolism, as well as the release of the toxins that some produce, occur through two types of processes: passive transport, such as diffusion (simple and facilitated), and through secretion systems.

Passive transport in bacteria

Diffusion processes are considered passive transport because they do not require energy expenditure by the cell. This is so because they occur in favor of the concentration gradient. By concentration gradient is meant the differences in concentration between two compartments.

Bacteria carry out various very important processes, from which products are obtained that are released outside of them. As already mentioned, those products are water, carbon dioxide, lactic acid and acetic acid.

Water freely passes through the cell membrane through the osmosis process. It is important to remember that the cell membrane is semi-permeable, so it allows the free passage of certain substances, with water being between them.

On the other hand, carbon dioxide, lactic acid, and acetic acid pass through the bacterial cell membrane to the outside by diffusion. According to this process, a substance (other than water) crosses the cell membrane from a place where it is highly concentrated to another where it is poorly concentrated.

According to this, the aforementioned fermentation products cross the cell membrane to be expelled out of the bacterial cells, once they have fermented sugars such as fructose, glucose and galactose.

Secretion of toxins in bacteria

Various species of bacteria produce certain toxic substances known as exotoxins. These are highly pathogenic for other living beings such as humans and can even be fatal on many occasions.

There are a total of seven secretion systems in bacteria. These make use of various protein complexes that are anchored in the cell membrane to transport the exotocin molecules.

Although there are seven secretion systems, they transport exotocins following only two mechanisms or routes:

Sec dependent: secretion depends on the signal sequence that is located at the amino terminal end of the exotocin to be secreted. This is so because thanks to this sequence the exotoxin is recognized and therefore transported through the membrane.
Independent Sec: in this system, secretion is not conditioned by any signal sequence at the amino terminal end. The translocation of the protein (exotoxin) to be transported simply occurs.

Similarly, specialists state that the secretion mechanisms of gram negative bacteria are more complex than those of gram positive bacteria.

This is because in gram negative bacteria, the toxins that are secreted must cross the double membrane that surrounds this type of bacteria into the extracellular space. In contrast, in gram positive bacteria, exotoxins only have to cross a single membrane to be released into the extracellular space.

Excretion in protists

The Protista kingdom encompasses a great variety of organisms that have characteristics that are very similar, but at the same time others that make them totally different from each other. In general, the group of protists includes algae and protozoa.

Well, the already mentioned organisms excrete certain substances as a result of their cellular metabolism. Among those substances we can mention: water, oxygen, carbon dioxide, toxins and some nitrogenous products such as ammonia and urea.

Protozoa excrete through exocytosis and diffusion. Source: Respectively: Frank Fox, Sergey Karpov, CDC / Dr. Stan Erlandsen, Picturepest, Thierry Arnet, Dr. Tsukii Yuuji

In this type of organisms, excretion occurs through various mechanisms, some of passive transport, such as diffusion and others of active transport, such as exocytosis and the use of vacuoles.

Diffusion in protists

In protists, the diffusion process is similar to how it occurs in other organisms. Through diffusion, substances such as water and gases (oxygen and carbon dioxide) cross the cell membrane into the extracellular space.

It is a process that does not require any energy expenditure on the part of the cell. It occurs in favor of the difference in concentrations.

Exocytosis in protists

Exocytosis is a process that is included within the active transport modality because it involves a certain expenditure of energy by the cell.

Through this process, the substances to be excreted are enveloped by a vesicle in the cell cytoplasm. This vesicle moves towards the cell membrane, with the help of some structures belonging to the cell cytoskeleton.

Once it comes into contact with the cell membrane, it fuses with it and proceeds to release its contents to the outside of the cell.

This mechanism is used to release compounds that are the product of its metabolism or waste substances, such as some toxins.

Excretion through vacuoles

Many protist organisms have contractile vacuoles to remove excess water and thus keep cells stable.

These vacuoles are made up of collecting tubes, as well as a central reservoir and a conduit that connects it directly with the cell membrane, forming a pore in it.

The excess water in the cytoplasm is redirected to the collecting tubes and from there to the reservoir. When this is full, the water is propelled through the exit tube towards the pore in the membrane to finally be excreted from the cell.

References

Beckett, BS (1987). Biology: A Modern Introduction. Oxford University Press.
Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition.
Forbes BA, Sahm DF. and Weissfeld AS. editors. (2002) Bailey & Scott's. Diagnostic Microbiology. 11th. ed. St. Louis, Missouri. Mosby.
Oudega, B., Mooi, F. and De Graaf, F. (1984). Excretion of proteins by gram-negative bacteria: export of bacteriocins and fimbrial proteins by Escherichia coli. Antonie Van Leeuwenhoek. 50 (5-6). 569-584
Varela, G. and Grotiuz, G. (2008) Physiology and bacterial metabolism. Bacteriology and medical virology topics. Taken from: hygiene.edu.uy

EXCRETION IN BACTERIA AND PROTISTS - BIOLOGY - 2025