A pate sucre or sweet pie crust is made from flour, butter, egg, sugar and salt. During the cooking process the butter melts into the other ingredients and cooks the starch out of the flour and the eggs coagulate. For lemon meringue the filling is lemon juice and zest, sugar, butter and eggs. The butter is melted and the remaining ingredients added and cooked gently over a low heat until eggs coagulate and mixture becomes a thick custard. This is poured into the pie shell and during the baking process the eggs completely coagulate and firm up the lemon mixture. For the meringue it's made from egg whites and sugar. The egg whites are beaten until they are white and fluffy. What is happening is during the beating on the egg whites air is forced into the cells of the egg whites causing them to expand. Sugar is added near the end and beat in all the way. The meringue is put on the pie before baking and it will brown to a golden brown in the oven. Thats carmelization.
