Size- and speed-dependent mechanical behavior in living mammalian cytoplasm
Significance Although the driving force generated by motor proteins to deliver intracellular cargos is widely studied, the mechanical nature of cytoplasm, which is also important for intracellular processes by providing mechanical resistance, remains unclear. We use optical tweezers to directly characterize the resistance to transport in living mammalian cytoplasm. Using scaling analysis, we successfully distinguish between the underlying mechanisms governing the resistance to mechanical deformation, that is, among viscosity, viscoelasticity, poroelasticity, or pure elasticity, depending on the speed and size of the probe. Moreover, a cytoplasmic state diagram is obtained to illustrate different mechanical behaviors as a function of two dimensionless parameters; with this, the underlying mechanics of various cellular processes over a broad range of speed and size scales is revealed.