Diffusiophoresis in Cells: A General Nonequilibrium, Nonmotor Mechanism for the Metabolism-Dependent Transport of Particles in Cells

The more we learn about the cytoplasm of cells, the more we realize that the cytoplasm is not uniform but instead is highly inhomogeneous. In any inhomogeneous solution, there are concentration gradients, and particles move either up or down these gradients due to a mechanism called diffusiophoresis. I estimate that inside metabolically active cells, the dynamics of particles can be strongly accelerated by diffusiophoresis, provided that they are at least tens of nanometers across. The dynamics of smaller objects, such as single proteins, are largely unaffected.

Figure 1

Schematic of a particle (blue), immersed in a cytoplasm with gradients in the concentrations of two metabolites (red and green). Proteins are magenta. Small ions are not shown. The dotted line indicates the approximate extent of the particle-cytoplasm interface, where slip occurs, creating a gradient in velocity, and hence the diffusiophoretic slip velocity $\mathbf{U}$.

Figure 2

Schematic of part of a prokaryote cell, with an ATP concentration gradient indicated by shading. Sources of the gradient are ATP synthases, in magenta, while our model assumes that sinks (ATP-consuming proteins) are uniformly distributed in the cytoplasm. We show one particle moving up the concentration at a diffusiophoretic velocity $\mathbf{U}$.