Can cytoplasm exist without undergoing phase separation?

Studies on such systems as the lens of the eye and theoretical considerations suggest that phase separation may well occur in cytoplasm. In this chapter, several issues relevant to this question are raised. It is suggested that while the interaction between water and the macromolecules in a mixture is proving crucial to their phase separation behavior, the abnormal water that is widely observed in cytoplasm and concentrated protein solutions is unlikely to constitute a thermodynamic phase in the sense of phase separation studies. The role of fixed structures in the cytoplasm, the likelihood that the volume of separated phases would be small and subject to spreading over the fixed structures and the expectation that much of the phase volume could be occupied and dominated by properties of the interface are also discussed. Finally, some experimental approaches to studying the existence of liquid-liquid phases in cytoplasm are proposed. While there is no proof that phase separation exists in cytoplasm, application of some of the techniques outlined might well provide more positive evidence for its presence.