• What are single-celled organisms?
• Characteristics of single-celled organisms
• First unicellular organism
• Types of single-celled organisms
• Importance of single-celled organisms
• Difference between unicellular and multicellular organisms
• Examples of single-celled organisms

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

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

What are single-celled organisms?

It is called a unicellular organism to all those forms of life whose body is composed of a single cell, and that do not form any type of tissue, structure or joint body with others of its species. They are microscopic beings whose body is a single cell and which are often classified as protists (if they are eukaryotes, that is, if they have cell nucleus) or bacteria and archaea (if they are prokaryotic, that is, if they do not have it).

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

Characteristics of 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 may have core and organelleseukaryotes) or not (prokaryotes). The latter are the most numerous.
• They are nurtured through plasma membrane, which allows them to exchange matter Y Energy with the outside of the cell. This exchange can be passive (without energy expenditure) or active (with energy expenditure), and in some cases it 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 movement
• They can be grouped in colonies, but never in more complex tissues or structures.
• They are microscopic, although their size can vary greatly: eukaryotes are several times larger than prokaryotes.
• They reproduce asexually, through various processes of cell division, such as mitosis, the Binary fission, the gemmation, 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. Besides, 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 the life.

However, 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 they would have descended, in a long and complex process of evolutionary diversification, all others kingdoms of life. It is estimated that it lived 3.5 billion years ago in the waters of the primitive planet.

Types of single-celled 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 on the basis of their mechanisms of nutrition, as follows:

• Autotrophs. They can synthesize the nutrients necessary to maintain the metabolism cell phone walking, simply by taking advantage of inorganic material. They can do it in two different ways:
  • Photosynthetics. They photosynthesize, taking advantage of sunlight and carbon dioxide to metabolize sugars. For this, they require chloroplasts, small organelles with a pigment called chlorophyll, which reacts with the sun.
  • Chemosynthetics. Instead of taking advantage of solar energy, take advantage of the release chemical reactions of geological or inorganic origin, and use these reactions to obtain chemical energy that turn into biochemistry.
• Heterotrophs. They cannot synthesize their own nutrients, and must take them from organic material of other organisms, living or dead, or 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 your body's expense, 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

Single-celled 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, 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 larger, more complex organisms, to take an irreversible step towards life as we know it.

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

Difference between unicellular and multicellular organisms

Multicellular organisms are made up of many cells.

The most obvious difference between unicellular and multicellular (or metazoans) is that the latter have bodies made up 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

Amoebas feed by hunting and engulfing other microscopic organisms.

Some examples of single-celled organisms are:

• Amoebas. Are protozoa irregularly shaped, they move moving their cytoplasm as if they were "fingers" (pseudopods) and through themselves 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 possess chloroplasts and feed on the sunlight but that, in case of lack of light, they can be nourished in a heterotrophic way, ingesting other organisms such as food.
• You arch. 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 the virus and other pathogenic forms. Many of them are free-living and lead an existence autotrophic, doing photosynthesis (like cyanobacteria).