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 called all those life forms whose bodies are composed of a variety of cells organized, hierarchical and specialized, whose joint functioning 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 subdivides rapidly throughout gestation, forming the entire body of the creature that, from birth, begins to grow. However, There are also multicellular organisms capable of asexual reproduction like plants and fungi.
In general terms, The kingdoms of life that have 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.
See also: Biological kingdoms
Origin of multicellular organisms
It is unknown how exactly the first multicellular organisms arose from primitive unicellular life, but there are three theories in this regard:
- The symbiotic theory Multicellular organisms would be the evolutionary product of a new type of cooperative relationship between two or more cells from different species whose treatment was so close that they ended up forming the same individual.
- The theory of cellularization According to this theory, the first multicellular organisms would have emerged when a unicellular organism developed various nuclei, subsequently 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 can range from a tiny algae 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 manner, and are called “systems.” The life of the individual depends on the correct functioning of these systems, so once death occurs, for whatever cause, all the cells in the body will gradually die (because they are interdependent).
The cells of these organisms can be broadly of two different types: those endowed with the complete DNA of the individual (somatic cells) and others that have only half of it and are created for sexual reproductive purposes (germ cells or gametes). However, Each cell in the body of the organism has a function a set of biological instructions and can become sacrificial for the good of others. This involves high levels of cellular communication, cooperation, and specialization.
Multicellular organisms can be autotrophs (like plants) if they can synthesize the organism's nutrients from inorganic matter and natural energy (such as sunlight), and heterotrophs if they require the consumption of organic matter from other living beings for oxidation and thus obtain chemical energy to sustain the metabolism.
Vital functions of multicellular organisms
Multicellular organisms fulfill, like unicellular organisms, basic vital functions, but they do so through 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 sustain all the cells of the body.
- Growth 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 the 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, a certain type of reproductive cells are generated and are responsible for building a complete new individual (unlike the bipartition processes of unicellular life in which two individuals emerge from one 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 animals Terrestrial, marine, flying, vertebrate or not, from insects to reptiles and including mammals and birds.
- All mushrooms Both saprophytes and parasites, in all habitats, with the notable exception of yeasts.
- All plants Except for some cases of unicellular algae, the plant kingdom is made up of multicellular organisms, regardless of the habitat or structural complexity they present.
unicellular organisms
A unicellular organism is a form of life whose body is composed of a single cell. These cells do not form any type of tissue, structure or body with others of their species, although they can live together as a colony. These are, then, microscopic organisms whose body is a single cell. They are often classified as protists (when they are eukaryotes, that is, they have a cell nucleus) or bacteria and archaea (when they are prokaryotes, that is, they lack a cell nucleus).
Continue in: Unicellular organisms
References
- “Multicellular organism” in Wikipedia.
- “Unicellular and multicellular beings” (video) in Happy Learning Spanish.
- “Multicellular beings” in the Ministry of Education, Culture and Sports of Spain.
- “Unicellular vs. Pluricellular” (video) at FuseSchool.
- “Multicellular organism” in The Encyclopaedia Britannica.
