Thursday, January 14, 2010

Osmosis in Fish

My son has some biology coursework about osmosis in plants, so he can probably ignore all the interesting facts on Wikipedia about osmosis in fish ("Effect on cells").

Salt water fish lose water through osmosis, and need a mechanism to rehydrate and desalinate their cells. Meanwhile, freshwater fish gain water through osmosis, and need an entirely different mechanism to dehydrate their cells. So that appears to be two different systems with two different purposes.

But what if we look at a species of fish that thrives both in salt water and in freshwater, such as salmon. Presumably such fish need both mechanisms. The purpose now is not adding or subtracting but regulating. (Scientists call it osmoregulation.)

So we have different ways of thinking about the purpose of these mechanisms. If we are solely looking at freshwater fish, we see the purpose in a specific way; if we are looking at all kinds of fish, then we see the purpose of the mechanism in a much more general way. This relates to @seabird20's point that POSIWID should be plural.

This means that we have to be careful when using POSIWID as a simple systems thinking heuristic. We get different answers depending (if I may use a fishing metaphor here) on how widely we cast the net.

1 comment:

Scribe said...

For some aquaticy reason, this brings to mind ballast tanks in submarines for me. Is the purpose of the tank to fill with water and sink, or empty of water and rise?

From this, the main lesson is that purpose is contextual. The purpose of a system depends on what other systems it is within or interacting with. A system that 'wants' to save money is dependent on the wider money-value system, and the emotions that go with it. We can conjecture that the purpose of the first will only change once the wider 'purpose' undergoes change, for either evolutionary/intended or revolutionary/unintended reasoning.

The plurality of purposes shouldn't be confused with the plurality of a) effects and b) rules for stability, I think. Effects can be thought of as "other outcomes" other than the intended purpose. Rules for stability are influences needed to keep the system together, so paying people a required amount maintains stability by keeping people working for an organisation.

Here my thinking runs out. Is there a clear split between these "internal" purposes of stability, and the P of POSIWID?