For the San Diego Zoo, I'd say it wouldn't be quite enough. We were there yesterday, got there at 11:00. We were able to see the gorillas, monkeys, cats and koala's. That was it. You'd probably be able to get it all done if you do the bus tour.
San Diego Zoo is ALOT of walking.
It would also depend on when the park closes, yesterday they closed at 4:30.
LA zoo you probably can get it done in that amount of time.