write up vs pen

write up and pen both are nouns.

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

As verbs, pen is a hypernym of write up; that is, pen is a word with a broader meaning than write up:

Other hypernyms of write up include compose, indite, write.

write up (noun)	pen (noun)
a short account of the news	female swan
	a writing implement with a point from which ink flows
	an enclosure for confining livestock
	a correctional institution for those convicted of major crimes
	a portable enclosure in which babies may be left to play

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