PepsiCo’s Indra Nooyi may be struggling lately. Yes, investors are impatient with her healthy-products strategy, the stock is down, and the Pepsi boss dropped to No. 2 behind Kraft Foods CEO Irene Rosenfeld on the 2011 Most Powerful Women list.

But no one can deny that Nooyi, who became PepsiCo ‘s chief in 2006, has injected the company with a clear long-term vision and a reputation for being a good citizen of the world.

Nooyi’s vision, which she labels “Performance with Purpose,” harks back to her days as a girl, growing up in southern India, as she explained in an interview with Fortune executive editor Stephanie Mehta at our Most Powerful Women Summit. In a city with little water, young Indra had to watch her mother go to collect three containers of water everyday. The family used that meager amount for drinking, cooking, and washing, while nearby corporations had more than enough water for their own needs.

You can see Nooyi tonight at 9 p.m. in a CNBC documentary, Pepsi’s Challenge. From the MPW Summit, here’s Nooyi on what led her, beyond her childhood experience, to “Performance with Purpose”: