Ion packs and cells will essentially be akin to the stars in Mario 64 or the orbs in Spyro 2. You collect them to unlock new levels and easier ways to reach old levels. Note that they won't be of crucial importance until later in the game -- that is to say, you won't need very many of them to progress past early levels.
In short, there are two things that determine whether you can play a level. The first is whether you have completed the prerequisite levels, and the second is whether you have enough Ion Packs. The Ion Pack thresholds won't become the limiting factor until most of the levels have already been unlocked via prerequisites.
I might also add some sort of bonuses (powerups, perhaps, but more likely cosmetic upgrades) that can be unlocked via obtaining Ion Packs, to give players incentive to gain further Ion Packs after having enough to complete the final stage of the game. And of course you know there will be bonuses for 100% completion.