The suffix "ize" is added to the word memory to change its' meaning to one of "to commit to memory". Thus the word memory becomes memorize which is a verb. In the English language the "ize" suffix is often added to nouns and adjectives to change them into verbs so that they then become an action being performed rather than a "thing" or a "modifier for a thing". Other examples would be "apology to apologize" or "agony to agonize.