Unicellular Organisms – Concept, types and characteristics

We explain what unicellular organisms are, their characteristics and the first unicellular organism. Classification, importance and examples.

Unicellular organisms
The appearance of single-celled organisms is still difficult to explain.

What are single-celled organisms?

Unicellular organism is called all those forms of life whose body is composed of a single cell, and that they do not form any type of tissue, structure or joint body with others of their kind. These are microscopic beings whose body is a single cell and that are often classified as protists (if they are eukaryotic, that is, if they have a cell nucleus) or bacteria and archaea (if they are prokaryotic, that is, if they do not have it).

Single-celled organisms they are the smallest and simplest of all living things, and they tend to inhabit numerous habitats, with very diverse metabolic strategies, ranging from photosynthesis or chemosynthesis, to the decomposition of organic matter, parasitism, or the predation of other single-celled creatures. This, in part, is because they are much older than multicellular organisms, whose appearance is still difficult to fully explain.

Characteristics of single-celled organisms

Bacteria - single-celled organisms
Single-celled organisms can form colonies but not complex structures.

Single-celled organisms can be very diverse from one another and can have very different characteristics, but they generally share the following:

  • They are necessarily made up of a single cell, and can have nuclei and organelles (eukaryotes) or not (prokaryotes). The latter are the most numerous.
  • They are nourished through the plasma membrane, which allows them to exchange matter and energy with the outside of the cell. This exchange can be passive (without energy expenditure) or active (with energy expenditure), and in some cases occurs through invaginations of the cytoplasm.
  • They move (if they do) by flagella or cilia, that is, through appendages of the membrane that allow them to move
  • They can be grouped in colonies, but never in more complex tissues or structures.
  • They are microscopic, although their size can vary enormously: eukaryotes are several times larger than prokaryotes.
  • They reproduce asexually, through various processes of cell division, such as mitosis, binary fission, budding, etc. This means that they are not sexed species: there are no males and females.

First unicellular organism

Not much is known about the first unicellular organism, in part because being composed of such tiny and soft tissues it is impossible to find fossils or geological traces. Furthermore, the Earth has changed so much in the billions of years of atmospheric, geological and chemical change that it is not easy to determine the very origins of life.

Nevertheless, it is speculated that the first living being on the planet was a unicellular organism that scientists call LUCA (acronym for Last Universal Common Ancestor or the Last Universal Common Ancestor), and from which all the other kingdoms of life have descended, in a long and complex process of evolutionary diversification. It is estimated that it lived 3.5 billion years ago in the waters of the early planet.

Types of single-celled organisms

Unicellular organisms
Parasites invade the interior of a larger organism to nourish themselves.

The most common classification of unicellular organisms is the one that distinguishes between prokaryotes and eukaryotes. However, single-celled organisms can also be classified based on their nutritional mechanisms, as follows:

  • Autotrophs. They can synthesize the necessary nutrients to keep cellular metabolism going, simply by taking advantage of inorganic matter. They can do it in two different ways:
    • Photosynthetics. They photosynthesize, taking advantage of sunlight and carbon dioxide to metabolize sugars. For that, they require chloroplasts, small organelles with a pigment called chlorophyll, which reacts with the sun.
    • Chemosynthetics. Instead of taking advantage of solar energy, they take advantage of that released by chemical reactions of geological or inorganic origin, and they use these reactions to obtain chemical energy that they convert into biochemistry.
  • Heterotrophs. They cannot synthesize their own nutrients, and must take them from the organic matter of other organisms, living or dead, or from their waste. They can do it in a number of ways:
    • Saprophytes. They break down residual organic matter, and help compounds of organic origin to become simpler substances, and obtain energy in the process.
    • Parasites. They must invade the interior of larger organisms (especially metazoans) to nourish themselves within and reproduce at the expense of your body, often damaging them in the process.
    • Predators. They use their plasma membrane to capture and digest other unicellular living beings, which they assimilate into the cytoplasm itself.

Importance of single-celled organisms

Unicellular organisms are the basis of life on the planet, ancestors of all higher life forms. At some point in the history of life on the planet, the marine waters were full of these microorganisms, engaged in a blind race to multiply and spread, until at some point the possibility arose of grouping themselves, sacrificing their individuality and forming more organisms. vast, more complex, to take an irreversible step towards life as we know it.

On the other hand, the study of single-celled organisms has allowed us to understand previously ignored aspects of the field of health and biology, and paved the way for modern medicine and the study of biochemistry.

Difference between unicellular and multicellular organisms

multicellular organisms - unicellular organism
Multicellular organisms are made up of numerous cells.

The most obvious difference between unicellular and multicellular (or metazoa) is that the latter have bodies composed of tissues, that is, numerous cells that have a common origin and that form a single individual. It is a much closer relationship than a colony because they sacrifice their independent life in terms of security, stability and distribution of the functions necessary to survive.

Examples of single-celled organisms

Unicellular organisms
Amoebas feed by hunting and engulfing other microscopic organisms.

Some examples of single-celled organisms are:

  • Amoebas. They are protozoa of irregular shape, that move moving their cytoplasm as if they were “fingers” (pseudopods) and through themselves they feed, hunt and engulf other microscopic organisms.
  • Paramecia. They are another type of ciliated protists. They have a membrane covered with microflagella that allow them to move at high speed in the aquatic environments they inhabit, such as puddles and ponds.
  • Euglenas. Euglena are flagellated unicellular organisms that have chloroplasts and feed on sunlight but, in the event of a lack of light, they can nourish themselves in a heterotrophic way, ingesting other organisms as food.
  • Arches. Also called archaebacteria, they are very primitive prokaryotic unicellular organisms, which can be found in very hostile conditions of life, since they are nourished by anaerobic chemosynthesis.
  • Bacteria. They are the most predominant unicellular life forms in the world, and also the oldest, those responsible for most of the infections that we can suffer, along with viruses and other pathogenic forms. Many of them are free-living and lead an autotrophic existence, doing photosynthesis (like cyanobacteria).