ATP bound G-actin monomers are added to the fast growing barbed ends of both mother and daughter filaments and the polymerization of these filaments drives membrane protrusion. In the process of phagocytosis, pseudopodia extend around the antibody-bound particle to form the phagocytic cup. This elongation continues until the filament reaches steady state equilibrium with free G-actin monomers (Millard et al. 2004, Le Clainche et al. 2008).