Model is underconstrained.
The following variables couldn't be defined:
 * x3 in component single_bound_receptor
 * scatchard in component Insulin
 * d/dt x1 in component Insulin
 * d/dt x2 in component Unbound_receptor
 * d/dt x3 in component single_bound_receptor
 * d/dt x4 in component double_bound_receptor