Life in crowded conditions

Molecular crowding is ubiquitous in cells, which are rather densely packed with macromolecules. The effects of such crowded conditions on biophysical processes can be complex and puzzling. Here, we review these effects in a step-by-step manner. We start with excluded volume effects on elementary physical processes: diffusion, binding, reactions, and polymer compaction. We then discuss the binding of a transcription factor to a binding site on DNA as an example of a more complex process and consider effects of attractive interactions and active processes. We also give an outlook to larger-scale crowded systems such as suspensions of cells, biofilms, and tissues, which can be described using similar approaches as molecular crowded systems.