Sunday, August 15, 2010

Free Lunch

I think it is pretty amazing that cells have evolved to take advantage of the thermal motion of molecules. As Nelson says on page 128, diffusion is free. Cellular organisms can transport molecules across themselves by diffusion, which costs them no energy at all.

In some cases, where there is an abundance of a particular substrate, bacteria can survive by consuming all of this substrate that is in close proximity to it, then waiting for the substrate to diffuse back to it, rather than moving itself to the higher concentration of substrate. So it seems, at least on the bacterial world, there is such thing as a free lunch.

It makes me wonder though, what if molecules were bigger, or if thermal motion was weaker, in such a way that diffusion wasn’t an efficient way for molecules to be transported in life processes? In section 4.6.2 we calculated an upper limit on the metabolism of a bacterium. What if atoms were bigger relative to the diffusion constant, such that the minimum size of a bacterium was too big for oxygen metabolism to occur at a significant rate?

These kind of thought experiments can never be tested, but it is interesting to wonder at how likely life as we know it is. Furthermore, what even more complex and incredible forms of life are not and will never be in existence because the values of universal constants are the way they are?


  1. I think if thermal motion was weaker or if molecules were bigger that it would be difficult to get the conditions in which life evolved. The precursor molecules of DNA/RNA may never aggregate. Also if diffusion is the simplest way of getting nutrients - and if this was no longer the case - how would the bacteria get energy to divide and thereby evolve more advanced ways of getting these chemicals i.e. flaggelum?

    It seems that heat is so fundamental that if you changed it you may change the way energy levels are distributed in molecules and thereby change the chemistry entirely.

  2. It's like the straw man all over again.

    It said in the book (section 4.4.1) that eukaryotic cells cannot rely on simple diffusion, they instead employ a transport infrastructure. Taking this into account I would assume that organisms would simply evolve the means to survive if life is feasible.

    Actually in doing some background I found this other blog post from science 2.0 which is both relevant and interesting.

    The quote at the end is particularly good.