Thus, although membranes might not have been a strict requirement at the very early stages of the origin of life, they were certainly important to maintain and sustain it at some point during life's development, as membranes may be more effective at certain essential primitive functions. For example, as chemical evolution within non-membrane-bound compartments leads to the formation of more complex chemical networks, it became more important to separate the concentrated internal milieu from the dilute exterior, maintain gradients between inside and outside, and to generate energy. For example, ATP synthase, a molecular machine embedded in membranes of modern cells, is responsible for the synthesis of ATP in the form of energy. If some simple cross-boundary ATP-generating machinery were required in primitive cells, then the chemical gradient required for ATP generation might have been larger in membrane-bound components.