UNICELLULAR ORGANISMS: CHARACTERISTICS, REPRODUCTION, NUTRITION - BIOLOGY - 2025

The unicellular organisms are beings whose genetic material, enzymatic machinery, proteins and other molecules necessary for life are confined to a single cell. Thanks to this, they are extremely complex biological entities, often of very small size.

Of the three domains of life, two of them - archaea and bacteria - are made up of single-celled organisms. In addition to being single-celled, these prokaryotic organisms lack a nucleus and are extremely diverse and abundant.

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In the remaining domain, the eukaryotes, we find both unicellular and multicellular organisms. Within the unicellular we have protozoa, some fungi and some algae.

Main features

About 200 years ago, biologists of the time considered organisms made up of a single cell to be relatively simple. This conclusion was due to the little information they received from the lenses they used for viewing.

Today, thanks to technological advances related to microscopy, we can visualize the complex network of structures that unicellular beings possess and the great diversity that these lineages exhibit. Next we will discuss the most relevant structures in unicellular organisms, both in eukaryotes and prokaryotes.

Components of a prokaryotic cell

Genetic material

The most outstanding characteristic of a prokaryotic cell is the lack of a membrane that delimits the genetic material. That is, the absence of a true nucleus.

In contrast, DNA is located as a prominent structure: the chromosome. In most bacteria and archaea, DNA is organized into a large circular protein-associated chromosome.

In a model bacterium, such as Escherichia coli (more on its biology in the following sections), the chromosome reaches a linear length of up to 1 mm, almost 500 times the size of the cell.

In order to store all this material, DNA must take on a super-coiled conformation. This example can be extrapolated to most of the members of bacteria. The physical region where this compact structure of genetic material is located is called the nucleoid.

In addition to the chromosome, prokaryotic organisms can possess hundreds of additional small DNA molecules, called plasmids.

These, like the chromosome, code for specific genes, but are physically isolated from it. As they are useful in very specific circumstances, they make up a kind of auxiliary genetic elements.

Ribosomes

For the manufacture of proteins, prokaryotic cells have a complex enzymatic machinery called ribosomes, which are distributed throughout the cell interior. Each cell can contain about 10,000 ribosomes.

Photosynthetic machinery

Bacteria that carry out photosynthesis have additional machinery that allows them to capture sunlight and later convert it into chemical energy. The membranes of photosynthetic bacteria have invaginations where the enzymes and pigments necessary for the complex reactions they carry out are stored.

These photosynthetic vesicles can remain attached to the plasma membrane or they can be detached and located inside the cell.

Cytoskeleton

As the name implies, the cytoskeleton is the skeleton of the cell. The basis of this structure is composed of fibers of a protein nature, essential for the process of cell division and for the maintenance of cell shape.

Recent research has shown that the cytoskeleton in prokaryotes is made up of a complex network of filaments, and is not as simple as previously thought.

Organelles in prokaryotes

Historically, one of the most striking characteristics of a prokaryotic organism was its lack of internal compartments or organelles.

Today it is accepted that bacteria possess specific types of organelles (compartments surrounded by membranes) related to the storage of calcium ions, mineral crystals that participate in cell orientation, and enzymes.

Components of a single-celled eukaryotic cell

Within the lineage of eukaryotes we also have unicellular organisms. These are characterized by having the genetic material confined in an organelle surrounded by a dynamic and complex membrane.

The machinery for making proteins is also made up of ribosomes in these organisms. However, in eukaryotes these are larger. In fact, the difference in size in the ribosomes is one of the main differences between the two groups.

Eukaryotic cells are more complex than the prokaryotic cells described in the previous section, since they have subcompartments surrounded by one or more membranes called organelles. Among them we have the mitochondria, endoplasmic reticulum, Golgi apparatus, vacuoles and lysosomes, among others.

In the case of organisms capable of photosynthesis, they have the enzymatic machinery and pigments stored in structures called plasts. The best known are chloroplasts, although there are also amyloplasts, chromoplasts, etioplasts, among others.

Some unicellular eukaryotes have cell walls, such as algae and fungi (although they vary in their chemical nature).

Differences between bacteria and archaea

As we mentioned, the domains of archaea and bacteria are made up of unicellular individuals. However, the fact of sharing this characteristic does not mean that the lineages are the same.

If we thoroughly compare both groups we will realize that they differ in the same way that we - or any other mammal - differ from a fish. The fundamental differences are as follows.

Cellular membrane

Starting from the cell boundaries, the molecules that make up the wall and the membrane of both lineages differ profoundly. In bacteria, phospholipids consist of fatty acids attached to a glycerol. In contrast, archaea present highly branched phospholipids (isoprenoids) attached to glycerol.

In addition, the bonds that form phospholipids also differ, resulting in a more stable membrane in archaea. For this reason, archaea can live in environments where temperature, pH and other conditions are extreme.

Cellular wall

The cell wall is a structure that protects the cell organism from the osmotic stress generated by the difference in concentrations between the cell interior and the environment, forming a kind of exoskeleton.

Generally, the cell exhibits a high concentration of solutes. According to the principles of osmosis and diffusion, the water would enter the cell, expanding its volume.

The wall protects the cell from rupture, thanks to its firm and fibrous structure. In bacteria, the main structural component is peptidoglycan, although certain molecules, such as glycolipids, may be present.

In the case of archaea, the nature of the cell wall is quite variable and in some cases unknown. However, peptidoglycan has been absent in studies to date.

Genome organization

In terms of the structural organization of genetic material, archaea are more similar to eukaryotic organisms, since the genes are interrupted by regions that will not be translated, called introns - the term used for the regions that will be translated is "exon ».

On the contrary, the organization of the bacterial genome is carried out mainly in operons, where the genes are in functional units located one after another, without interruptions.

Differences with multicellular organisms

The crucial difference between a multicellular organism and a unicellular one is the number of cells that make up the organism.

Multicellular organisms are made up of more than one cell, and generally each one is specialized in a particular task, the division of tasks being one of its most outstanding characteristics.

In other words, since the cell no longer has to perform all the activities necessary to keep an organism alive, a division of tasks arises.

For example, neuronal cells perform completely different tasks than kidney or muscle cells.

This difference in the tasks performed is expressed in morphological differences. That is, not all the cells that make up a multicellular organism are the same in shape - neurons are tree-shaped, muscle cells are elongated, and so on.

The specialized cells of multicellular organisms are grouped in tissues and these in turn in organs. Organs that perform similar or complementary functions are grouped into systems. Thus, we have a structural hierarchical organization that does not appear in unicellular entities.

Reproduction

Asexual reproduction

Single-celled organisms reproduce asexually. Note that in these organisms there are no special structures involved in reproduction, as occurs in different species of multicellular beings.

In this type of asexual reproduction, a father gives rise to offspring without the need for a sexual partner, or for the fusion of gametes.

Asexual reproduction is classified in different ways, generally using as a reference the plane or form of division that the organism uses to divide.

A common type is binary fission, where an individual gives rise to two organisms, identical to the parent. Some have the ability to perform fission by generating more than two offspring, which is known as multiple fission.

Another type is budding, where an organism gives rise to a smaller one. In these cases, the parental organism sprouts a prolongation that continues to grow to an adequate size and is subsequently detached from its parent. Other single-celled organisms can reproduce by forming spores.

Although asexual reproduction is typical of single-celled organisms, it is not unique to this lineage. Certain multicellular organisms, such as algae, sponges, echinoderms, among others, can reproduce through this modality.

Horizontal gene transfer

Although there is no sexual reproduction in prokaryotic organisms, they can exchange genetic material with other individuals through an event called horizontal gene transfer. This exchange does not involve passing the material from parents to children, but occurs between individuals of the same generation.

This occurs by three fundamental mechanisms: conjugation, transformation and transduction. In the first type, long pieces of DNA can be exchanged through physical connections between two individuals by means of a sexual pili.

In both mechanisms, the size of the exchanged DNA is smaller. Transformation is the taking of naked DNA by a bacterium and transduction is the reception of foreign DNA as a consequence of a viral infection.

Abundance

Life can be divided into three main domains: archaea, bacteria, and eukaryotes. The first two are prokaryotic, because their nucleus is not surrounded by a membrane and they are all unicellular organisms.

According to current estimates, there are more than 3.10 ³⁰ individuals of bacteria and archaea on earth, most of them unnamed and without description. In fact, our own body is made up of dynamic populations of these organisms, which establish symbiotic relationships with us.

Nutrition

Nutrition in single-celled organisms is extremely varied. There are both heterotrophic and autotrophic organisms.

The former have to consume their food from the environment, generally engulfing nutritional particles. Autotrophic variants have all the machinery necessary for the conversion of light energy into chemistry, stored in sugars.

Like any living organism, unicellular plants require certain nutrients such as water, a source of carbon, mineral ions, among others, for their optimal growth and reproduction. However, some also require specific nutrients.

Examples of single-celled organisms

Due to the great diversity of unicellular organisms, it is difficult to list examples. However, we will mention model organisms in biology and organisms with medical and industrial relevance:

Escherichia coli

The best-studied organism is, without a doubt, the Escherichia coli bacteria. Although some strains can have negative health consequences, E. coli is a normal and abundant component of the human microbiota.

It is beneficial from different perspectives. In our digestive tract, the bacteria help the production of certain vitamins and competitively exclude pathogenic microorganisms that could enter our body.

In addition, in biology laboratories it is one of the most used model organisms, being very useful for discoveries in science.

Trypanosoma cruzi

It is a protozoan parasite that lives inside cells and causes Chagas disease. This is considered an important public health problem in more than 17 countries located in the tropics.

One of the most outstanding characteristics of this parasite is the presence of a flagellum for locomotion and a single mitochondrion. They are transmitted to their mammalian host by insects belonging to the Hemiptera family, called triatomines.

Other examples of microorganisms are Giardia, Euglena, Plasmodium, Paramecium, Saccharomyces cerevisiae, among others.

References

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