An overloading of the gel with a soluble protein creates a semicircular band of this protein (e. g. in the marker lane of the image at 66 KDa), allowing other proteins with similar molecular weights to be covered.
Like arrowroot, tapioca also comes from the root of a plant, however, in this case it is the root of the Cassava plant. It is ground into powdered form and is often referred to as tapioca flour.
All-purpose flour is made from a blend of high-and low-gluten wheats, and has a bit less protein than bread flour  11% or 12% vs. 13% or 14%. You can always substitute all-purpose flour for bread flour, although your results may not be as glorious as you had hoped.