Unicellular and multicellular organisms: examples and differences

The living unicellular organisms or , as its name suggests we are those which are formed only by a single cell in which all vital functions necessary for life are produced. For this reason, they are mostly microscopic organisms , but there are some unicellular living beings that can reach large sizes of up to 20 centimeters, such as xenophiophores, a type of foraminifera, commonly called “living sand”, of marine life mainly with a shell formed by one or more cameras.

Most unicellular beings are prokaryotic cells , that is, cells that do not have a nucleus. Their genetic material is not enveloped and “enclosed” by a membrane, but rather is dispersed throughout the cytoplasm, like bacteria. However, there are other unicellular organisms, such as protozoa, that do have a nucleus. These types of cells are called eukaryotic cells , which have a more complex structural organization and reach larger sizes than prokaryotic cells. Here you can learn more about eukaryotic and prokaryotic cells .

Unicellular living beings can reproduce both sexually (through conjugation) and asexually depending on the organism to which we refer. There are several asexual reproduction strategies , such as:

• The bipartition or binary fission. The nucleus and cytoplasm of the cell divide, giving rise to two identical daughter cells.
• Budding in yeasts. The nucleus divides and separates from the mother cell with a portion of the cytoplasm that generates the daughter cell.
• Sporulation . The nucleus divides several times and each one of them will generate a spore that will be released when the membrane of the mother cell breaks.

These beings are considered the most primitive beings , since their system is simpler than that of multicellular beings, which we will comment on later. In addition, currently contrary to what they may seem, they represent the majority of living beings that inhabit our planet and live in very remote places where other forms of life cannot develop.

Unicellular organisms have representatives of some kingdoms of living beings such as:

• The Monera kingdom , formed by unicellular bacteria .
• Some organisms of the Protista kingdom .
• The yeasts of the Fungi kingdom .

Some more specific examples of unicellular organisms , within the great variety that exist, are:

• Yeasts such as Saccharomyces cerevisae , the yeast used in beer.
• Escherichia coli , bacteria.
• Toxoplasma gondii , protozoan responsible for Toxoplasmosis.
• Trypanosoma cruzi, a protozoan that causes Chagas disease.
• Trichomonas vaginalis.
• Candida albicans , the fungus responsible for candidiasis.
• Mycobacterium tuberulosis, the bacterium responsible for tuberculosis.
• Neisseria gonorrhea , the bacterium that causes gonorrhea.
• Mycoplasma pneumoniae , bacteria responsible for pneumonia.
• Clostridium botulinum , a bacterium that causes botulism.
• Pneumococci (bacteria).
• Staphylococci (bacteria).
• Dinoflagellates (protista).
• Some single-celled algae such as diatoms .
• Paramecia (protist).

You may be interested in these other articles on Bacteria, good allies of ecology and Types of bacteria . In addition, we recommend that you watch this video about protists, to learn more about these single-celled organisms.

The celled living things , as its name and as opposed to living unicellular, are those which are formed by two or more eukaryotic cells . They present, therefore, a greater complexity than unicellular organisms in terms of the functions they develop.

These multicellular organisms arose from a primitive single-celled organism. Although the process by which this jump to multicellularity occurred is one of the great unknowns and there are several hypotheses in this regard, the truth is that it is known that it occurred several times simultaneously in different evolutionary groups of various organisms, such as animals, land plants, algae and fungi.

By itself, all current multicellular organisms come from a single cell, that is, they begin their life being unicellular, such as the zygote, which is the cell resulting from the union of male and female sex gametes in fertilization. This cell divides and multiplies giving rise to a multicellular organism whose cells will undergo differentiation processes and will work independently and together, forming tissues , organs and systems that will make up the individual’s body. In some simpler groups of species, such as sponges , true tissues do not form and their cells function more independently.

Cells of multicellular organisms reproduce asexually through two processes: mitosis , originating daughter cells identical to the mother cell and with the same number of chromosomes; and meiosis, typical of reproductive cells whose purpose is to originate the gametes, the sex cells, with half of the genetic endowment. The reproduction of multicellular organisms is also very varied and can be sexual reproduction or asexual reproduction .

There is a great diversity of multicellular organisms , although we must remember that unicellular living beings widely exceed this multicellular diversity. Even so, if we want to give some examples of multicellular living beings, we find a wide range that ranges from fungi to animals and plants:

• All vertebrate animals , which belong to the Animal Kingdom , such as mammals , birds , reptiles , amphibians , fish , and all invertebrate organisms , such as sponges, annelids , arthropods , etc.
• Within plants and algae we can find green algae, red algae, brown algae and all the variety of terrestrial plants , such as mosses , liver plants , angiosperms or gymnosperms , among many other groups.
• All fungi , except single-celled yeasts.

After knowing what they are and several examples of unicellular and multicellular organisms, we will discuss in a little more detail how they differ. These are the main differences between unicellular and multicellular organisms:

• The main difference, as we have already commented from the beginning, is the number of cells that these living beings have. Unicellular have only one and multicellular have more.
• Unicellular organisms can be part of the group of prokaryotes and eukaryotes, while multicellular organisms are only part of eukaryotic living beings.
• There is a difference in the size of unicellular and multicellular organisms, the former being mostly smaller than the latter.
• Unicellular living beings have more resistance to extreme environmental conditions than multicellular ones.
• Unicellular living beings do not make up complex structures, such as tissues, organs and systems, whereas multicellular beings do.