close up vs back up

close up and back up both are verbs.

Word	Noun	Adjective	Verb	Adverb
close up	No	No	Yes	Yes
back up	No	No	Yes	No

As verbs, back up is a hyponym of close up; that is, back up is a word with a more specific, narrower meaning than close up:

close up (verb)	back up (verb)
refuse to talk or stop talking; fall silent	establish as valid or genuine
block passage through	become or cause to become obstructed
unite or bring into contact or bring together the edges of	make a copy of (a computer file) especially for storage in another place as a security copy
cease to operate or cause to cease operating	move backwards from a certain position
	give moral or psychological support, aid, or courage to

close up (adverb)	back up (adverb)
very close