Can I just use All-Purpose Flour instead of Cake Flower?
Yes, you could just use all-purpose flour for your cakes, cupcakes, and baking needs, but your cakes, and cupcakes will be much more dense cakes or cupcakes and will not be as light and fluffy as they would be if you used cake flour to make them with.
Over the past 10 years that I have been making cakes I have used both cake flour and all-purpose flour, and I recommend when making a cake or cupcakes you plan a head and have cake flour to make your cakes and cupcakes with, because you will be much happier with the results you get in with the end result. A great place to get some great details about the different types of flour to use from cake flour, all-purpose flour, wheat flour, white flour, and the difference between each and why you should use a certain flour to bake your cakes and cupcakes with. Go to this website on Food Networks website and it gives a great explanation. https://www.foodnetwork.com baking guide flour 101 guide
This is where I first went to learn about the different types of flour and why I needed to use cake flour for my cakes and cupcakes, instead of using all-purpose flour.
A quick explanation about cake flour from what I learned from Food Network is:
Cake flour has less gluten in it than an all-purpose flour, so it can absorb more moisture and sugar than an all-purpose flour will. All-purpose flour has more gluten, and this is what causes bread to be chewier and denser. A really good article that goes more in depth about flour is found at the King Arthur Flour website. https://blog.kingarthurflour.com cake flour
In the article the author talks a little about the bleaching process of flour. So, if you ever wondered how flour was bleached this is a good place to start to learn more about flour and what they do to it to bleach flour.