straighten up vs straighten

straighten up and straighten both are verbs.

Word	Noun	Adjective	Verb	Adverb
straighten up	No	No	Yes	No
straighten	No	No	Yes	No

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

straighten up (verb)	straighten (verb)
straighten oneself	put (things or places) in order
	make straight or straighter
	make straight
	straighten by unrolling
	straighten up or out; make straight
	get up from a sitting or slouching position