sitter vs organism

sitter and organism both are nouns.

Word	Noun	Adjective	Verb	Adverb
sitter	Yes	No	No	No
organism	Yes	No	No	No

As nouns, organism is a hypernym of sitter; that is, organism is a word with a broader meaning than sitter:

sitter: an organism (person or animal) that sits
organism: a living thing that has (or can develop) the ability to act or function independently

Other hypernyms of sitter include being.

sitter (noun)	organism (noun)
a domestic hen ready to brood	a living thing that has (or can develop) the ability to act or function independently
a person engaged to care for children when the parents are not home	a system considered analogous in structure or function to a living body
an organism (person or animal) that sits
a person who poses for a painter or sculptor