Yes, APOPTOSIS is a Scrabble word!

Letter values in Scrabble US: A 1, P 3, O 1, P 3, T 1, O 1, S 1, I 1, S 1.

Is it a valid word?

APOPTOSIS is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

a type of cell death in which the cell uses specialized cellular machinery to kill itself; a cell suicide mechanism that enables metazoans to control cell number and eliminate cells that threaten the animal's survival

Also: programmed cell death, caspase-mediated cell death.