Figure 4.

Kinetics of the formation of microtubules. After warming to 35°C, the tubulin solution assembles into microtubules whose concentration is proportional to the optical density at 350 nm. The optical pathlength of the sample is 1 mm; the optical density would be 10 times greater for a 1-cm-thick sample. The kinetics show an over-shoot; microtubule assembly is at a maximum between 6 and 10 min after instigating microtubule formation. This demonstrates a chemical instability between the relative proportions of free tubulin and microtubules that coincide approximately with the bifurcation time at which the system is gravity-dependent. Microtubule disassembly produces macroscopic concentration fluctuations that interact with gravity and trigger self-organization.