Re: Kryptonian Physiology in Man of Steel
People are overthinking this a bit, though it's fun to think about.
It is still a yellow (younger) sun that powers Superman. His cells absorb its energy in a different way than Krypton's red sun. But, as Jor-El says, "his cells will drink in the solar energy". The film also shows this.
The whole atmosphere thing isn't so much about Superman's powers...it's about Superman's survival, period.
He was born on Krypton, but he did not have time to acclimate to their environment. Most of his life was spent getting used to Earth's environment.
As such, as an adult who had never had to acclimate to Krypton's atmosphere, his body was rejecting the sudden change of being in Krypton's atmosphere. He was essentially dying, or very sick as a result. So he couldn't even stand, or breathe, let alone use his powers.
Think about the way the human body reacts under extreme atmospheric differences. Less air, more water, pressure changes, etc.
This is their backdoor to Kryptonite, and I think it was pretty smartly written.
Writer and Lyricist of GOTHAM'S KNIGHT: THE BATMAN MUSICAL
And if I'm right
The future's looking bright
A symbol in the skies at night