In a large saucepan, bring the chicken stock to a boil. Add the meatballs and simmer over moderate heat until no longer pink on the outside, about 2 minutes. Add the diced tofu and the chopped Chinese cabbage and simmer for 2 minutes longer. Add the cellophane noodles and soy sauce. Season the soup generously with freshly ground pepper and simmer for 3 minutes longer. Stir in the chopped scallions and the chopped celery leaves. Ladle the soup into warmed bowls and serve piping hot.