Because one side of cellulose mould is soaked with flat acetic anhydride to make cellulose acetate film, the surface of acetic acid is smooth, while the cellulose gap on the other side is reduced (protein can't get in) and the surface of cellulose is rough (protein can get in), so the spots are on the rough surface and face down during electrophoresis (to prevent gravity from invading the surface of cellulose acetate). Serum protein (protein number 1E7i) is the carrier of fatty acids in blood. When the body needs energy or building materials, fat cells release fatty acids into the blood, and fatty acids are obtained from serum proteins and transported to the needed parts.