The Gunners went into the game trailing 2-1 from the first leg in Portugal and shorn of the services of influential skipper Cesc Fabregas due to a hamstring injury.
The home side started brightly and after eight minutes Bacary Sagna drifted over a superb cross from the right and Helton made a fine save to block Andrey Arshavin's header at the far post.
Two minutes later Arsenal took the lead as Arshavin drove through the middle and when Helton came out to block, Bendtner was on hand to fire the loose ball home from 12 yards out.
With the aggregate scores now level, the Gunners began to take control, with Arshavin a box of tricks on the left flank.
With 25 minutes played Arsenal took the lead on aggregate as the visitors gave the ball away and Arshavin moved clear down the left, before playing in a pass across the area which left the unmarked Bendtner with the easy task of turning the ball home from six yards out.
Eleven minutes before the break Arshavin should have added a third goal, but he sidefooted over after Samir Nasri had found space down the right.
Porto keeper Helton then kept out efforts from Bendtner, Abou Diaby and Nasri as the home side ended the first half well on top.
However, the home side started the second period slowly and almost paid the price as just past the hour mark as Cristian Rodriguez met a corner from the right and his header was blocked on the line by the alert Nasri.
Within a couple of minutes Nasri struck a decisive blow at the other end of the pitch, as he drifted away from three markers on the right and fired a low finish across Helton and just inside the far post to make it 3-0 on the night.
Arsenal quickly made sure of their safe passage by adding a fourth, with Arshavin leading a quick counter-attack before feeding sub Emmanuel Eboue, who went around the keeper and finished coolly into the unguarded net.
Porto were by now well beaten and they were to suffer one more blow in stoppage time as Jorge Fucile was penalised for bringing down Eboue in the area.
Bendtner strode up to complete his first senior hat-trick by burying the spot-kick into the corner of the net beyond the despairing dive of Helton.