air cell vs sac

air cell and sac both are nouns.

Word	Noun	Adjective	Verb	Adverb
air cell	Yes	No	No	No
sac	Yes	No	No	No

As nouns, sac is a hypernym of air cell; that is, sac is a word with a broader meaning than air cell:

air cell: a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways
sac: a structure resembling a bag in an animal

air cell (noun)	sac (noun)
a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways	a structure resembling a bag in an animal
	a case or sheath especially a pollen sac or moss capsule
	an enclosed space