A physical perspective on cytoplasmic streaming (invited)

Organisms show a remarkable range of sizes, yet the dimensions of a single cell rarely exceed $100$ $\mu$m. While the physical and biological origins of this constraint remain poorly understood, exceptions to this rule give valuable insights. A well-known counterexample is the aquatic plant $Chara$, whose cells can exceed $10$ cm in length and $1$ mm in diameter. Two spiraling bands of molecular motors at the cell periphery drive the cellular fluid up and down at speeds up to $100$ $\mu$m/s, motion that has been hypothesized to mitigate the slowness of metabolite transport on these scales and to aid in homeostasis. This is the most organized instance of a broad class of continuous motions known as "cytoplasmic streaming", found in a wide range of eukaryotic organisms - algae, plants, amoebae, nematodes, and flies - often in unusually large cells. In this overview of the physics of this phenomenon, we examine the interplay between streaming, transport and cell size, and discuss the possible role of self-organization phenomena in establishing the observed patterns of streaming.