サ	シ	ス	セ	ソ
sa	shi	su	se	so

Adding " (tenten) to this row changes from an "s" sound to a "z" sound.
There are no "si" or "zi" sounds, as you can see.

ザ	ジ	ズ	ゼ	ゾ
za	zi	zu	ze	zo