This chapter explores how eukaryotic cells achieved new levels of functional integration through the division of their internal space into intracellular membrane-bound compartments. By comparing the internal membranes of prokaryotes and eukaryotes, it argues that these organelles partition the intracellular space in different ways and perform distinct functions, enhancing overall cellular operations. In prokaryotes, constitutive processes primarily occur in the cytoplasm, which exerts near-total control over the microcompartments. In eukaryotes, however, control over these processes is distributed between the cytoplasm and organelles, leading to a more balanced regulation of the cell’s global physiology. These varying modes of control create distinct forms of integration between the cytoplasm and microcompartments.