writing vs write up

writing and write up both are nouns.

Word	Noun	Adjective	Verb	Adverb
writing	Yes	No	No	No
write up	Yes	No	Yes	No

writing (noun)	write up (noun)
the activity of putting something in written form	a short account of the news
the act of creating written works
the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)
letters or symbols that are written or imprinted on a surface to represent the sounds or words of a language
(usually plural) the collected work of an author

writing (verb)	write up (verb)
	bring to public notice by writing, with praise or condemnation
	put into writing; write in complete form