We explain what multicellular organisms are, how they originated and their characteristics. Also, its vital functions and examples.
What are multicellular organisms?
Multicellular organisms are all those life forms whose bodies are made up of a variety of cells organized, hierarchical and specialized, whose joint operation guarantees the stability of life. These cells make up tissues, organs and systems, which cannot be separated from the whole and exist independently.
Many multicellular organisms always arise from a single cell called a zygote, the result of the sexual union of two gametes (female and male). The zygote is subdivided in an accelerated way throughout the gestation, forming the totality of the body of the creature that, from its birth, begins to grow. Nevertheless, there are also multicellular organisms capable of reproducing asexually, like plants and fungi.
In general terms, the kingdoms of life that possess multicellular organisms are three: animalia, plantae and fungi. In the cells of these living beings there is a cell nucleus with the complete DNA of the individual, that is, they are eukaryotic organisms.
Origin of multicellular organisms
How exactly the first multicellular organisms arose from primitive single-celled life is unknown, but there are three theories:
- The symbiotic theory. Multicellular organisms would be the evolutionary product of a new type of cooperative relationship between two or more cells of different species whose treatment was so close that they ended up forming the same individual.
- The cellularization theory. According to this theory, the first multicellular organisms would have arisen when a unicellular organism developed several nuclei, later dividing its cytoplasm through new plasma membranes to give each nucleus an independence that ended up being a cell within another cell.
- The colonial theory. In this case, it is assumed that the joint colonial life of cells of the same type and species produced an increasingly complex symbiosis mechanism, which led to a distribution of reproductive tasks, allowing the rest of the colony to dedicate themselves to forming tissues with another. type of functions.
Characteristics of multicellular organisms
Multicellular organisms have varying degrees of complexity since they can range from a tiny alga to an elephant or a giant sequoia. Their bodies are made up of millions of cells integrated into organs and tissues that function in a coordinated and independent way, and are called “systems.” The life of the individual depends on the correct functioning of these systems, so that once death occurs, for whatever reason, all the cells of the body will gradually die (because they are interdependent).
The cells of these organisms can broadly be of two different types: those endowed with the complete DNA of the individual (somatic cells) and others that have only half and are created for sexual reproductive purposes (germ cells or gametes). Nevertheless, each cell in the body of the organism has a function, a set of biological instructions and can become expendable for the good of others. This implies high levels of cellular communication, cooperation and specialization.
Multicellular organisms can be autotrophic (like plants) if they can synthesize the body’s nutrients from inorganic matter and natural energy (like sunlight), and heterotrophic if they require the consumption of organic matter from other living beings for their oxidation. and thus obtain chemical energy to sustain the metabolism.
Vital functions of multicellular organisms
The multicellular organisms fulfill, like the unicellular ones, with the vital basic functions but they do it from much more complex processes that involve the replacement of old cells with new cells and the generation of new tissues.
- Nutrition. Multicellular organisms have a digestive system that consists of a set of organs and tissues responsible for preparing, dissolving and digesting food, so that its nutrients enter the metabolism and are translated into energy to support all the cells of the body.
- Increase. If the growth of unicellular organisms is limited by the size of the cell, in the case of multicellular organisms it is limited by the number of them according to the joint pattern established in DNA. Multicellular organisms grow according to an established genetic plan and according to the availability of nutrients so that new additional cells are generated and incorporated into the system.
- Reproduction. The reproduction of the cells of the multicellular organism is constant since replacements are being generated for damaged tissues, for old and dying cells, or to fulfill specific tasks such as defensive cells. On the other hand, the organism as a whole reproduces once it reaches maturity, through various asexual or sexual mechanisms, depending on the species. Whatever the case, certain types of reproductive cells are generated and are responsible for building a complete new individual (as opposed to the bipartition processes of unicellular life in which two emerge from an individual).
Examples of multicellular organisms
There are millions of examples of multicellular organisms in the world around us: we ourselves are a perfect case. Some examples are:
- All the animals. Terrestrial, marine, flying, vertebrates or not, from insects to reptiles and through mammals and birds.
- All mushrooms. Both saprophytes and parasites, in all habitats, with the notable exception of yeasts.
- All the plants. Except for some cases of unicellular algae, the plant kingdom is composed of multicellular organisms, regardless of their habitat or structural complexity.
Single-celled organisms
A single-celled organism is a life form whose body is made up of a single cell. These cells do not form any type of tissue, structure or joint body with others of their species, although they can live together as a colony. They are, then, microscopic organisms whose body is a single cell. They are often classified as protists (when they are eukaryotic, that is, they have a cell nucleus) or bacteria and archaea (when they are prokaryotic, that is, they lack a cell nucleus).
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