round up vs roundup

round up is not a noun while roundup is a noun.

Word	Noun	Adjective	Verb	Adverb
round up	No	No	Yes	No
roundup	Yes	No	No	No

round up (noun)	roundup (noun)
	the systematic gathering up of suspects by the police
	the activity of gathering livestock together so that they can be counted or branded or sold
	a summary list; as in e.g. "a news roundup"

round up (verb)	roundup (verb)
seek out and bring together