Anomalous Diffusion in Living Yeast Cells

The viscoelastic properties of the cytoplasm of living yeast cells were investigated by studying the motion of lipid granules naturally occurring in the cytoplasm. A large frequency range of observation was obtained by a combination of video-based and laser-based tracking methods. At time scales from ${10}^{-4}$ to ${10}^2\mathrm{s}$, the granules typically perform subdiffusive motion with characteristics different from previous measurements in living cells. This subdiffusive behavior is thought to be due to the presence of polymer networks and membranous structures in the cytoplasm. Consistent with this hypothesis, we observe that the motion becomes less subdiffusive upon actin disruption.

Figure 1

Image of a fission yeast cell. Small black spheres are lipid granules in the cytoplasm (arrow).

Figure 2

Power spectra of thermal motion of a free granule in water (marked “in water”) and a granule inside the cell (marked “in the cell”). The data points shown were blocked with equal bin width on the logarithmic frequency axis. Error bars are so small that they are not visible at high frequencies. The lines show the fit of the theory to the data (see text) from $f_{\min \mathrm{}}=200\mathrm{H}\mathrm{z}$ to the Nyquist frequency, 11 kHz. The fit to the data of the granule inside the cell gave an exponent $\alpha =0.762\pm 0.006$. For the granule in water, the fit yielded $\alpha =1.00\pm 0.01$.

Figure 3

Mean squared displacement, $\langle |\Delta \overrightarrow{r}(t){|}^2\rangle$, of granules as a function of time lag. The short-time part of the data was obtained by OT ($N=10$ randomly chosen granules), whereas the long-time part was obtained by MPT (other ten granules). Lines with a slope of 0.75 are drawn to guide the eye. Lines with symbols show examples of different types of motion: normal diffusion ($\Delta$), subdiffusion with a plateau (●), and directed motion (□).