Sunday, July 19, 2009

The cell as computer

Making sense of the material world is a gradual process that we all achieve to different degrees over the course of our lives. The further removed a paradigm is from a day to day human experience, the harder it is to put it in a meaningful context. Whether too vast or too small in size (stellar phenomena or microscopic to sub-atomic action), or too long or short in time (evolution; quantum actions), it helps to find a way to express meaning in terms we live each day.

Living cells are computers. This is a very succinct analogy which can help to conceptualise the complexity of cellular life and action.

Of course this metaphor is not new, but it looks to be very well put in a new book, Wetware: a Computer in Every Living Cell, by Dennis Bray (and reported in New Scientist).

Stentor Roeselii is a pond-dwelling single-celled organism. It exhibits behaviour that is akin to being directed by a brain. If a jet of water is squirted at it, it will duck down, then come back up cautiously. Another, identical squirt would be ignored. Squirt an irritant chemical at it, and it will "arch its stalk-like body out of the way, move from side to side", retreat, then finally tear itself free from its mooring and drift off to a new home.

Bray characterises such an organism as a chemical computer. Ultimately, the programme written out in the DNA (and so it cannot be reprogrammed per se). But the complex set of actions (output) are all induced by chemical reactions that occur as a response to stimulus (input). The chemical reactions are not simple, but they are effectively laid down by DNA, which also dictates the form, that is the composition of this "bag of biochemistry". We cannot yet come close to duplicating this because we haven't broken the barrier to the point where we can build computers at anything more than an electronic level.

In a similar fashion, a multicellular organism could be seen as a large group of chemical computers that includes complex mechanisms (chemical interactions) to bind them to working collaboratively. (By the time we get to the sophistication of an animal with a decent-sized brain, though, the ultimate outcome could be said to be non-deterministic, simply because of the complexity and miniturisation... but that determinism is another debate altogether.)

Comparing biochemical action to advanced miniaturised computing is a very useful way to help understand how life builds up from its building blocks to the level of complexity we now see.

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