Biology noob here, trying to expand his basic scientific knowledge.


A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells.

But to me, this is ambiguous. Is a unicellular organism one that contains one cell and one cell only? Or is a unicellular organism one that contains one kind of cell, but lots of copies of that cell? So for example, are plants not unicellular because they contains groups of different types of cell, or are plants not unicellular simply because plants contain more than one cell?

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    $\begingroup$ Why the downvote? $\endgroup$ Aug 1 '21 at 23:41
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    $\begingroup$ The definition in the question seems to clearly (to me) refer to unicellular as a single cell only. The second half of the definition states "unlike a multicellular organism that consists of multiple cells", which seems to exclude any organism composed of two or more cells (regardless of what "type" the cells are) $\endgroup$
    – Luigi
    Aug 2 '21 at 13:33
  • $\begingroup$ Ok, so a unicellular organism is a single cell that can survive on it's own (under the right conditions). Is this what the definition means? $\endgroup$ Aug 2 '21 at 13:44

A unicellular organism is one where a single individual can survive all by itself under the right conditions. Some unicellular organisms live in colonies with many others of the same type or other types, and survive better in cooperation with the others (e.g. in biofilms) but each individual there has the potential to be independent.

Multicellular organisms need more cells to survive (how many I don't know, likley organism specific).

  • $\begingroup$ Another way to think about it is if you take one cell, is that a recognizable organism, or is it just a part of an organism? Just to confuse things, there are certain organisms that seem to be a bit of both, like the slime molds. From wiki, "Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic organisms that can live freely as single cells, but can aggregate together to form multicellular reproductive structures." $\endgroup$
    – Armand
    Aug 2 '21 at 3:11

a unicellular organism is a single-celled organism that can conduct all the functions needed to survive on its own, in one cell. whereas multicellular organisms have different kinds of cells for different functions.

in the plant example, the plant is multicellular because it has different cells performing different functions, for example palisade cells are mainly responsible for photosynthesis while spongy cells are mainly for gas exchange.

keeping that in mind, there can always be exceptions in multicellular organisms where some are able to survive as single-celled organisms

hope this cleared stuff up.


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