We explain what unicellular organisms are, their characteristics and the first unicellular organism. Classification, importance and examples.
What are unicellular organisms?
A single-celled organism is called all those forms of life whose body is made up of a single cell and that they do not form any type of tissue, structure or body together with others of their species. These are microscopic beings whose body is a single cell and that are often classified as protists (if they are eukaryotes, that is, if they have a cell nucleus) or bacteria and archaea (if they are prokaryotes, that is, if they do not have one).
unicellular organisms are the smallest and simplest of all living beings and they usually inhabit numerous habitats, with very diverse metabolic strategies, ranging from photosynthesis or chemosynthesis, to the decomposition of organic matter, parasitism, or predation on other unicellular creatures. This, in part, is because they are much older than multicellular organisms, whose appearance is still difficult to fully explain.
See also: Kingdom Monera
Characteristics of unicellular organisms
Unicellular organisms can be very diverse from each other and can have very different characteristics, but they generally share the following:
- are necessarily made up of a single cell and they may have a nucleus and organelles (eukaryotes) or not (prokaryotes). The latter are the most numerous.
- 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 it occurs through invaginations of the cytoplasm.
- move (if they do so) by means of flagella or cilia that is, through appendages of the membrane that allow movement
- can group in colonies but never in more complex tissues or structures.
- are microscopic although their size can vary enormously: eukaryotes are several times larger than prokaryotes.
- reproduce asexually through various cell division processes, such as mitosis, binary fission, budding, etc. This means that they are not sexual species: there are no males and females.
First unicellular organism
Not much is known about the first single-celled organism, partly because it was composed of such soft and tiny tissues. it is impossible to find fossils or geological traces. Furthermore, the Earth has changed so much in billions of years of atmospheric, geological, and chemical change that it is not easy to determine the very origins of life.
However, It is speculated that the first living being on the planet was a single-celled organism which scientists call LUCA (acronym for Last Universal Common Ancestor or the Last Universal Common Ancestor), and from which all the other kingdoms of life would 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 primitive planet.
Types of unicellular organisms
The most common classification of unicellular organisms is the one that distinguishes between prokaryotes and eukaryotes. However, unicellular organisms can also be classified based on their nutrition mechanisms, as follows:
- Autotrophs. They can synthesize the necessary nutrients to keep cellular metabolism going, simply by using inorganic matter. They can do it in two different ways:
- Photosynthetic. They carry out photosynthesis, taking advantage of sunlight and carbon dioxide to metabolize sugars. To do this, 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 the energy released by chemical reactions of geological or inorganic origin, and use these reactions to obtain chemical energy that they convert into biochemical energy.
- 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 various ways:
- Saprophytes. They break down residual organic matter, helping organic compounds become simpler substances, and gaining energy in the process.
- Parasites. They must invade the interior of larger organisms (especially metazoans) to feed inside and reproduce at the expense of their body, often causing damage in the process.
- Predators. They use their plasma membrane to capture and digest other single-celled living beings, which they assimilate into their own cytoplasm.
Importance of unicellular organisms
Unicellular organisms are the basis of life on the planet, ancestors of all forms of higher life. 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 together, sacrificing their individuality and forming larger 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
The most obvious difference between unicellular and multicellular (or metazoans) is that the latter have bodies composed of tissues that is, numerous cells that have a common origin and that form a unique 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 unicellular organisms
Some examples of unicellular organisms are:
- amoebas. They are irregularly shaped protozoans that move by moving their cytoplasm as if they were “fingers” (pseudopods) and through themselves they feed, hunting and phagocytosing other microscopic organisms.
- Paramecia. They are another type of ciliated protists. They have a membrane covered in microflagella that allows them to move at high speed in the aquatic environments they inhabit, such as puddles and ponds.
- Euglenas. Euglenas are flagellated unicellular organisms, which have chloroplasts and feed on sunlight but, if there is a lack of light, they can nourish themselves heterotrophically, ingesting other organisms as food.
- Archaea. Also called archaebacteria, they are very primitive prokaryotic single-celled organisms, which can be found in very hostile living conditions, since they are nourished through anaerobic chemosynthesis.
- bacteria. They are the most predominant single-celled life forms in the world, and also the oldest, 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).
References
- “Unicellular organism” in Wikipedia.
- “Unicellular and multicellular beings” (video) in Happy Learning Spanish.
- “Unicellular organisms” in Euston96.
- “Unicellular vs. Pluricellular” (video) in FuseSchool.
- “Protist” in The Encyclopaedia Britannica.
- «Biology», Curtis H., Barnes S., Schnek A and Massarini A. (2008) 7th Edition. Panamericana Medical Editorial.
