Adding tenten to this row changes from a "t" sound to a "d" sound.
There are no "ti," "tu," "di," or "du" sounds.
ぢ and づ are the same sound as their third row equivalents and, in my experience, aren't really used.
Additionally there is the small つ (っ) that always comes before a consonant and is written in roomaji (roman alphabet) by doubling the consonant that follows it.
I.E. かった -> katta
It is pronounced as a short pause that elongates the consonant.