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we should consider what ultimately drives all the frantic activity and turnover within each and every cell. If there was a candidate for a vital force to animate life, it is the one that first entranced Robert Brown (1773–1858) in 1827, when the Scottish botanist became fascinated by the incessant zigzag motion of fragments in pollen grains, a phenomenon that would come to be named after him (unless you are French, that is—they argue that similar observations were reported in 1828 by botanist Adolphe-Théodore Brongniart, 1801–1876).
Brownian movement What puzzled Brown was that this microscopic motion did not arise from currents in the fluid, or from evaporation, or from any other obvious cause. At first he thought that he had glimpsed “the secret of life,” but after observing the same kind of motion in mineral grains he discarded that belief. The first key step in our current understanding of what Brown had witnessed came more than seventy-five years after his discoveries, when Albert Einstein [1879–1955] demonstrated how the tiny particles were being shoved about by the invisible molecules that made up the water around them.
Brownian movement of a certain molecular Until Einstein’s 1905 paper, a minority of physicists (notably Ernst Mach [1838–1916]) still doubted the physical reality of atoms and molecules. Einstein’s notion was eventually confirmed with careful experiments conducted in Paris by Jean Baptiste Perrin (1870–1942), who was rewarded for this and other work with the Nobel Prize in Physics in 1926. Brownian motion has profound consequences when it comes to understanding the workings of living cells. Many of the vital components of a cell, such as DNA, are larger than individual atoms but still small enough to be jostled by the constant pounding of the surrounding sea of atoms and molecules. So while DNA is indeed shaped like a double helix, it is a writhing, twisting, spinning helix as a result of the forces of random Brownian motion. The protein robots of living cells are only able to fold into their proper shapes because their components are mobile chains, sheets, and helices that are constantly buffeted within the cell’s protective membrane. Life is driven by Brownian motion, from the kinesin protein trucks that pull tiny sacks of chemicals along microtubules to the spinning ATP synthase.31 Critically, the amount of Brownian motion depends on temperature: too low and there is not enough motion; too high and all structures become randomized by the violent motion. Thus life can only exist in a narrow temperature range. Within this range, the equivalent of a Richter 9 earthquake rages continuously inside cells. “You would not need to even pedal your bicycle: you would simply attach a ratchet to the wheel preventing it from going backwards and shake yourselves forward,” according to George Oster and Hongyun Wang, of the Department of Molecular and Cellular Biology at the University of California, Berkeley.32 Protein robots accomplish a comparable feat by using ratchets and power strokes to harness the power of Brownian motion. Due to the incessant random movement and vibrations of molecules, diffusion is very rapid over short distances, which enables biological reactions to occur with tiny quantities of reactants in the extremely confined volumes of most cells. Now that we know that the linear code of DNA determines the structure of the protein robots and RNAs that run our cells and, in turn, that the structure determines the functions of the protein and RNAs, the next question is obvious: how do we read and make sense of that code so that we can understand the software of life?
Fractal Brownian Treehttp://blog./2013/06/06/fractal-brownian-tree/ |
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