Macropinocytosis refers to the non-specific uptake of extracellular fluid, which 
plays ubiquitous roles in cell growth, immune surveillance, and virus entry. 
Despite its widespread occurrence, it remains unclear how its initial cup-shaped 
plasma membrane extensions form without any external solid support, as opposed 
to the process of particle uptake during phagocytosis. Here, by developing a 
computational framework that describes the coupling between the bistable 
reaction-diffusion processes of active signaling patches and membrane 
deformation, we demonstrated that the protrusive force localized to the edge of 
the patches can give rise to a self-enclosing cup structure, without further 
assumptions of local bending or contraction. Efficient uptake requires a balance 
among the patch size, magnitude of protrusive force, and cortical tension. 
Furthermore, our model exhibits cyclic cup formation, coexistence of multiple 
cups, and cup-splitting, indicating that these complex morphologies 
self-organize via a common mutually-dependent process of reaction-diffusion and 
membrane deformation.