The most salient distinction between multicellular organisms (plants and animals) and unicellular ones (protists) is that protists do not differentiate or, as far as we know, undergo apoptosis. Protists exchange signals, but they respond to these signals only by migration, altered cell division rates and in some cases colony formation. On the other hand, many single-celled eukaryotes have extremely elaborate internal architectures. Compared to animal or plant cells, some protists are of a size and structural complexity that can be quite startling when we are accustomed to studying animal cells such as our own.
In all protists, as in plants and animals, there is an internal state (as defined in chapter 6) locked in reciprocal dependence with gene expression and stimulus processing. Our characterisation of "livingness" applies to amoebae, yeasts and other single-celled eukaryotes just as it does to animal and plant cells.
Some protists produce spores or spore-like ("encysted") forms when the environment becomes hostile. Like plant spores, these are instances of "suspended animation"; temporarily at least, they are not living. Their "livingness" is restored when the environment becomes friendly again. Encystment can serve as a method of reproduction among protists, but it is often used simply to cope with hostile conditions.
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