Lighting up mitochondrial membranes: Reading and writing the physical state of the inner mitochondrial membrane.

The mitochondrial inner membrane (IMM) is depicted as a scaffold housing the enzymes of the electron transport chain, allowing electron transfer and generation of the electrochemical gradient that drives respiration and adenosine triphosphate (ATP) synthesis. However, evidence from imaging, structural biology, and novel chemical probes reveals that the IMM is not a mere passive backdrop: its physical properties change with the metabolic state, and together these changes influence mitochondrial function. Here, we summarize how IMM microviscosity, lipid packing, and mesoscale organization might shape electron transfer kinetics, reactive oxygen species (ROS) production, and mitochondrial form/function. We discuss new chemical probes that reveal IMM organization, alongside novel chemical/genetic approaches to alter IMM organization and mitochondrial function. Finally, we outline current challenges in measuring or modulating the state of the IMM, suggesting approaches required to address how the IMM state is coupled with electron entry routes, ROS dynamics, and mitochondrial morphology.