Probing the interior of living cells with fluorescence correlation spectroscopy

To a large extent the cellular interior is occupied by two complex fluids, the cytoplasm and the nucleoplasm, both of which show a considerable degree of macromolecular crowding. While it is easy to imagine that the chromosomal DNA provides the nucleoplasm with properties similar to a polymer melt, the material properties of the cytoplasm are also affected by the high amount of dissolved macromolecules, the cytoskeletal network, and dispersed organelles. By virtue of the strongly obstructed random motion, reactions in the cytoplasm and nucleoplasm are not comparable to the aqueous conditions commonly used in biochemical experiments. To overcome this gap, a thorough understanding of the material properties of intracellular fluids, and hence transport properties within the cell, is mandatory. Here, we review some recent results on bulk diffusion in living cells and some generic consequences that arise from these observations.