This blog has been created partly as a companion to Chemistry for the Biosciences, the textbook that I co-author with Tony Bradshaw, and to act as an archive of posts I write for other sites (particularly the OUPblog). Like the book itself, it explores how life on the scale of atoms and molecules has an impact on biology - at the scale of cells, tissues, and organisms - and seeks to demystify a range of biological and chemical concepts.

The blog's name takes as its inspiration the cover of the first edition of Chemistry for the Biosciences, which depicts a gecko seemingly clinging to its surface. To find out what links geckos to chemistry, read this.

Thursday, 3 February 2011

Entropy: just go with the flow

It began with the sound of a tyre rim grinding on the surface of the cycle path I’d been travelling along, and a sudden sensation of being on a bike that was moving through treacle rather than through air. My rear tyre had punctured and, not for the first time of late, I found myself resenting the seeming futility of life: of having the bad luck to get the puncture, of having to spend time and effort buying and fitting a new inner tube - of my life being enriched not one iota by the whole experience.

As I trudged home that evening, wheeling the now-useless bike beside me, I reflected on the many situations we encounter that mirror this experience – when we find ourselves having to invest energy, only to be no further forward, in real terms, having done so.

Why is it that we have to invest energy merely to maintain the status quo? Why do we find ourselves running, effectively only to stand still? The answer lies in an intrinsic property of all matter, a universal truth so fundamental to our existence that it is captured by its own law: the Second Law of Thermodynamics. This law tells us, in a nutshell, that we are living in a perpetual downward spiral, in which things just get worse. A cheery outlook on life, if ever there was one. But it is an outlook from which there is no escape: the universe, and everything in it, is gradually crumbling into a state of ever-increasing disorder.

This property of all matter – this collapse into disorder – is given a name: entropy. Things that are disordered have greater entropy than things that are relatively more organized. A glass of water, in which the molecules of water itself can move around relatively freely, is more disorganized – has greater entropy – than a block of ice, in which the molecules of water are trapped into a rigid, organized array.

A process that increases disorder, with its associated increase in entropy, is a spontaneous one: one that happens without having to do work to bring it about. This fact has one important corollary: a decrease in entropy – a move towards a more organized state – requires us doing work to bring it about. This is arguably why housework feels like a chore: a living room doesn’t spontaneously tidy itself. We need to invest effort to reverse the spread of disorder, and bring order to whatever degree of chaos had befallen our living space since we last made the effort to tidy up. We are essentially swimming against the natural tide of entropy, with disorder setting in the moment we take our foot off the pedal.

When we look at life at the scale of the molecules and cells of our bodies we continue to see an ongoing battle with entropy: a tussle between order and disorder. Consider proteins, the molecular machines that carry out many important functions in the cell. As they are first being manufactured (or ‘synthesised’) in the cell, proteins exist as elongated chains of conjoined amino acid subunits, much like links of sausages as they are extruded from a sausage-making machine. However, these elongated protein chains must fold into specific three-dimensional shapes to function correctly. This folding represents an increase in order, and hence a decrease in entropy. As we note above, though, swimming against the tide of entropy comes at a cost: the cell must do work to drive such a process forward.

This battle against entropy is essentially why we must eat on a regular basis: to give the cells of our body the energy they need to drive forward those processes that won’t happen spontaneously.

Even the very continuation of life is a battle against disorder. Successful reproduction relies on the passing of biological information from one generation to the next. Every time a cell divides, it must pass on a copy of its DNA to its progeny; the copy must be a faithful replica of its parent for the information it contains to remain intact. But this copying process is not immune from the eroding effects of entropy. Errors – themselves manifestations of disorder - can creep in, just as if you tried to re-type this blog post word-for-word, letter-for-letter. Errors in biological information can scarcely be tolerated, however, so the cell invests energy and resources to overcome them, employing sophisticated proof-reading apparatus to error-check DNA as it is copied, and repair as many molecular infelicities as possible before the copying process is complete.

Despite the best efforts of our cells to resist its effects, however, the Second Law still ultimately reigns supreme, and the relentless march towards disorder continues.

So next time you’re faced with an office strewn with paperwork, or an inbox you’re no longer in control of, ask yourself whether it’s really worth struggling against the flow of entropy, or whether you shouldn’t just go with the flow...

This post originally appeared on the OUPblog

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