Modern classification, otherwise known as taxonomy, was invented so as to have a single system used throughout the entire world in understanding different types of beings in existence. This way you avoid having different names for same things.
There are a few vital characteristics to mention here. First we have to know about which ions we're talking about as the cellular membrane is semi-permeable (lets some flow freely through the membrane and some not), it depends on the size of the compounds, and also the charge that a molecule has.
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They live in fresh water, marine or soil. They feed off of bacteria and other protozoa.
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