Natural convection in the cytoplasm: Theoretical predictions of buoyancy-driven flows inside a cell

TL;DR

This study uses numerical and theoretical methods to reassess the strength of natural convection in cells caused by temperature gradients, finding it to be much weaker than earlier predictions and unlikely to significantly aid intracellular transport.

Contribution

The paper provides a semi-analytical solution for cytoplasmic convection, demonstrating that previous models overestimated flow strength and showing convection's limited role in solute redistribution.

Findings

Flow strength is an order of magnitude weaker than prior estimates.

Natural convection has negligible impact on intracellular solute transport.

Theoretical and numerical results are in strong agreement.

Abstract

The existence of temperature gradients within eukaryotic cells has been postulated as a source of natural convection in the cytoplasm, i.e. bulk fluid motion as a result of temperature-difference-induced density gradients. Recent computations have predicted that a temperature differential of $\Delta T \approx 1$ K between the cell nucleus and the cell membrane could be strong enough to drive significant intracellular material transport. We use numerical computations and theoretical calculations to revisit this problem in order to further understand the impact of temperature gradients on flow generation and advective transport within cells. Surprisingly, our computations yield flows that are an order of magnitude weaker than those obtained previously for the same relative size and position of the nucleus with respect to the cell membrane. To understand this discrepancy, we develop a semi-analytical solution of the convective flow inside a model cell using a bi-spherical coordinate framework, for the case of an axisymmetric cell geometry (i.e. when the displacement of the nucleus from the cell centre is aligned with gravity). We also calculate exact solutions for the flow when the nucleus is located concentrically inside the cell. The results from both theoretical analyses agree with our numerical results, thus providing a robust estimate of the strength of cytoplasmic natural convection and demonstrating that these are much weaker than previously predicted. Finally, we investigate the ability of the aforementioned flows to redistribute solute within a cell. Our calculations reveal that, in all but unrealistic cases, cytoplasmic convection has a negligible contribution toward enhancing the diffusion-dominated mass transfer of cellular material.