tense vs tense up

tense is a noun but tense up is not a noun.

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

As verbs, tense and tense up are synonyms defined as:

Other synonyms of tense include strain.

As verbs, tense and tense up are synonyms defined as:

tense (noun)	tense up (noun)
a grammatical category of verbs used to express distinctions of time

tense (adjective)	tense up (adjective)
taut or rigid; stretched tight
pronounced with relatively tense tongue muscles (e.g., the vowel sound in `beat')
in or of a state of physical or nervous tension

tense (verb)	tense up (verb)
cause to be tense and uneasy or nervous or anxious	cause to be tense and uneasy or nervous or anxious
become tense, nervous, or uneasy	become tense, nervous, or uneasy
increase the tension on
become stretched or tense or taut