The PK (Penalty Kick), did not give Barca a man advantage.
Demichaelis foul (that just so happened to have occured inside the penalty box according to the referees) was clearly red card worthy, and that is why Barca had the man advantage.
I know a good bit about soccer/football and have been watching it for years. Man City is good now, but they are not on Barcelona's level.
I am a Real Madrid fan by the way and I hate Barcelona, but real recognize real.
Even with 10 men, Man City still had plenty of time to tie the game but they couldn't do it and then their goalkeeper got nutmegged for Barcelona's second goal.