[TeMPOraL]

Random factoid I picked up from The Machinery of Life: how do various proteins inside a cell know to find and bind to a specific chemical or protein they need? They don't - everything inside a cell just constantly bumps into everything else, and because of how fast things move at this scale, it doesn't take long for any single thing to touch approximately every other thing inside the cell. I.e. turns out random walk is a viable search mechanism at nanometer scale.

[stevesimmons]

And presumably the size of cells is partly determined by constraining enough of the right things close enough together, so they will bump into each other quickly enough with probability approaching 1.

[TeMPOraL]

Right! Cells larger than that stop working reliably and end up self-selecting themselves out of the future generations.

Now I wonder if internal structure of cells can be attributed to that too - i.e. organelles create zones where processes relying on fast random walk can work, enabling the cell itself to grow bigger.