Position rack in center of the oven.
Preheat oven to 350° F.
Separate 6 of the eggs. Place yolks in a large bowl and whites in a medium bowl.
Add the 1 whole egg to the egg yolks and beat 4-5 minutes until frothy and doubled in volume.
Gradually add the sifted sugar, lemon zest and lemon juice. Mix thoroughly.
Gradually add the sifted potato flour until mixed thoroughly. Set aside.
Beat the egg whites until soft peaks form.
Gently fold thoroughly into the egg yolk mixture until no white can be seen.
Spoon into and ungreased 10 inch tube pan. Place in the oven and bake for 50-55 minutes or until the cake springs back when touched gently.
Invert pan and cool completely before removing the cake.