Question: Can I substitute...whole wheat for all-purpose flour?

Answer: Replacing white flour with whole wheat generally makes baked goods denser, drier, and more crumbly because the germ and bran in whole wheat absorbs more water. So in the case of baked good where you want a light and tender texture, like cake, it's just a bad idea. In the case of pancakes, muffins, and quick breads, though, where the batter tends to have more liquid, you can usually get away with substituting up to 25% of the white flour for whole wheat. It will definitely have an impact on the texture, but usually in a way that is intriguing rather than disastrous, and will add some flavor and nuttiness to the equation.