Everything about Multicellular totally explained
Multicellular organisms are
organisms consisting of more than one
cell, and having
differentiated cells that perform specialized functions. Most life that can be seen with the naked eye is multicellular, as are all members of the
kingdoms
Plantae and
Animalia (except for specialized organisms such as
Myxozoans in the case of the latter).
Organizational levels
Multicellular organisms exhibit orgat (currently living) multicellular organisms,
sponges, consist of multiple specialized cellular types cooperating together for a common goal. These cell types include
Choanocytes, digestive cells;
Sclerocytes, support-structure-secreting cells;
Porocytes, tubular pore cells; and
Pinacocytes, epidermal cells. Though the different cell types create an organized, macroscopic multicellular structure—the visible sponge—they are not organized into true interconnected
tissues. This is illustrated by the fact that a sponge broken up using cheese cloth and a very specific ion cocktail (the classical blender experiment does NOT work) will reaggregate from the surviving cells. If individually separated, however, the particular cell types can't survive alone. Simpler
colonial organisms, such as
Volvox, differ in that their individual cells are free-living and can survive on their own if separated from the colony.
Tissues
More complex organisms such as
jellyfish,
coral and
sea anemones possess a tissue level of organization, in which differentiated, interconnected cells perform specialized functions as a group. For instance, jellyfish tissues include an
epidermis and
nerve net that perform protective and sensory functions, along with an inner
gastrodermis that performs digestive functions. The overall spatial organization of differentiated cells is a topic of study in
anatomy.
Organs and organ systems
Even more complex organisms, while also possessing differentiated cells and tissues, possess an
organ level of development, wherein multiple tissues group to form organs with a specific function or functions. Organs can be as primitive as the brain of a
flatworm (merely a grouping of ganglion cells), as large as the stem of a
sequoia (up to 90 meters (300 feet) in height), or as complex and multifunctional as a
vertebrate liver.
The most complex organisms (such as mammals, trees, and flowers) have
organ systems wherein groups of organs act together to perform complex related functions, with each organ focusing on a subset of the task. An example would be a vertebrate
digestive system, in which the
mouth and
esophagus ingest food, the
stomach crushes and liquifies it, the
pancreas and
gall bladder synthesize and release digestive
enzymes, and the
intestines absorb nutrients into the
blood.
Evolutionary history
The oldest known taxonomically resolved multicellular organism is a
red algae,
Bangiomorpha pubescens, found fossilized in 1.2 billion year old rock from the
Ectasian period of the
Mesoproterozoic era.
In order to reproduce, true multicellular organisms must solve the problem of regenerating a whole organism from
germ cells (for example
sperm and
egg cells), an issue that's studied in
developmental biology. Therefore, the development of
sexual reproduction in unicellular organisms during the Ectasian period is thought to have precipitated the development and rise of multicellular life.
Multicellular organisms also face the challenge of
cancer, which occurs when cells fail to regulate their growth within the normal program of development.
Hypotheses for origin
There are various mechanisms which are disputed as being the first responsible for the emergence of multicellularity, but it's difficult to say which is correct. This is due to the fact that all the suggested mechanisms are viable, but establishing which was responsible for the first multicellular life requires mostly speculation.
One hypothesis is that a group of function-specific cells aggregated into a slug-like mass called a
grex, which moved as a multicellular unit. Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a
syncytium. A membrane would then form around each neucleus (and the cellular space and organelles occupied in the space), thereby resulting in a group of connected and specialised cells in one organism (this mechanism is observable in
Drosophila). A third theory is that, as a unicellular organism divided, the daughter cells failed to separate, thereby resulting in a conglomeration of identical cells in one organism which could each then specialize.
Further Information
Get more info on 'Multicellular'.
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