What are single-celled organisms? Unicellular organisms

What are single-celled organisms? Unicellular organisms

It is called a unicellular organism for all life forms whose body is made up of a single cell and which do not form any kind of tissue, structure, or joint body with others of their kind. In other words, they are microscopic beings whose body is a single cell and who are often classified as protists (if they are eukaryotic, that is, if they have a nucleus) or bacteria and archaea (if they are prokaryotic, that is,) . if you don’t have it).
Single-celled organisms are the smallest and simplest of all living things. They usually inhabit numerous habitats and perform very different metabolic functions, ranging from photosynthesis or chemosynthesis to the decomposition of organic substances, parasitism or predators. from other unicellular creatures. This is partly because they are much older than multicellular organisms, the appearance of which is still difficult to fully explain.

Characteristics of single-celled organisms

Unicellular organisms can be very different and have very different properties, but generally have the following in common:

  • They are necessarily made up of a single cell.  Whether it has a nucleus and organelles ( eukaryotes ) or not ( prokaryotes ). The latter are the most numerous.
  • They feed through the plasma membrane.  That allows them to exchange matter and energy with the outside of the cell. This exchange can be passive or active, 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 free movement.
  • They can group 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 it is made up of such tiny and soft tissues that it is impossible to find fossils or geological traces. In addition, the earth has changed so much over the billions of years of atmospheric, geological, and chemical change that it is not easy to determine the origins of life.
However, it is speculated that the first living thing on the planet was a unicellular organism that scientists call LUCA (short for Last Universal Common Ancestor or Last Universal Common Ancestor), and from which they would descend in a long and complex process of evolutionary diversification, all other kingdoms of life. It is estimated that it lived in the waters of the early planet 3.5 billion years ago.

Types of single-celled organisms

The most common classification of unicellular organisms is that which, as we have already seen, distinguishes between prokaryotes and eukaryotes. However, based on their nutritional mechanisms, they can also be classified as follows:

  • Autotrophs. Those that 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:
    • Photosynthetic. Those that photosynthesize, taking advantage of sunlight and carbon dioxide to metabolize sugars. For this they require chloroplasts, small deposits of a pigment called chlorophyll that reacts with the sun.
    • Chemosynthetic. Those who instead of taking advantage of solar energy, take advantage of that released by chemical reactions of geological or inorganic origin, using these reactions to obtain chemical energy to convert into biochemistry.
  • Heterotrophs. Those who cannot synthesize their nutrients, and must take them from the organic matter of other organisms, living or dead, or their waste. They can do it in several ways:
    • Saprophytes. Those that decompose residual organic matter, helping organic compounds to become simpler substances, and feeding in the process.
    • Parasites Those that must invade the interior of larger organisms (especially metazoans) to nourish themselves within and reproduce at the expense of your body, often damaging it in the process.
    • Predators. Those that use their plasma membrane to capture and digest, assimilating them to the cytoplasm itself, to other unicellular living beings.

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, the oceans were full of these microorganisms, engaged in a blind race to reproduce and spread, until at some point the opportunity arose to group, sacrifice their individuality, and form organisms. more extensive, more complex and thus an irreversible step towards life as we know it.
On the other hand, studying unicellular organisms has allowed us to understand previously ignored aspects of health and biology, paving 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 metazoa) is that the latter have bodies made of tissues, that is, made up of numerous cells that have come together to form a single individual in a much closer relationship than a colony in order to sacrifice their independent life for the benefit of them of security, stability and distribution of the functions necessary for survival.
In any case, the transition from unicellular (more primitive) to multicellular (more complex) is still somewhat of a mystery to biology. The truth is that it represented a great step forward in evolutionary matters, to the point that our body is made up of millions of cells, all of which are committed to one common existence: that of our body.

Examples of single-celled organisms

Some examples of single-celled organisms are:

  • Amoebas. 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.
What are single-celled organisms? Unicellular organisms
What are single-celled organisms? Unicellular organisms
  • Paramecia. 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 is flagellated unicellular organisms that have chloroplasts and feed on sunlight, but if they lack it, they can nourish themselves in a heterotrophic way, ingesting other organisms as food.
  • You arch. Also called archaebacteria, they are a very primitive type of prokaryotic unicellular organism, which can be found in very hostile living conditions since they are nourished by anaerobic chemosynthesis.
  • Bacteria. The most predominant unicellular life forms in the world are also the oldest and are 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).

What are single-celled organisms? Unicellular organisms


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