Soundings: New uses for old parts
I recently rafted down the lower Yuba River with folks from SYRCL – the South Yuba River Citizens League.
As we humans, led by naturalists Steve Darden and Joel Passovoy, paddled downstream, Chinook salmon swam upstream to spawn.
We share a many-times-great ancestor with these animals that lived hundreds of millions of years ago. Since then, we and fish have both evolved. Fish kept their fins; our fins evolved into arms and legs.
And both of us have found new uses for the hormones – chemical signals – that we inherited. One such hormone ensures that there will be future generations of both fish and humans, but in very different ways.
In fish, the use of this hormone comes from the need to deal with water’s ability to pass through membranes, and how the consequences differ in fresh water and ocean water.
Open a bottle of perfume here, and soon you’ll smell it over there. Whenever there’s a lot of something in one place, nature re-distributes it to everywhere else.
Membranes work against that natural tendency for things to distribute themselves evenly. The membranes you feel with your tongue in your cheek protect the cells of which your cheek is made, keeping what’s inside in and what’s outside out.
But cell membranes are leaky: Water molecules can pass through them.
The cells contain water with all sorts of salts and sugars and proteins dissolved in it.
Distilled water, on the other hand, is pure; it has nothing dissolved in it. Distilled water is “concentrated” water, more “concentrated” than the water within in our cells.
Imagine holding some distilled water in your mouth. Like perfume drifting across a room, some of this “concentrated” water would leak through the cell membranes, into your cheek cells.
Filling with water, your cheek cells would swell, maybe even burst.
Now imagine holding saltwater in your mouth. The water in your cheek cells is more concentrated than saltwater. The water would try to be evenly concentrated across both, flowing through your cell membranes and out of your cells.
Losing water, your cheek cells would shrink.
This seepage of water through leaky membranes is called osmosis.
Freshwater fish have a problem: The water in their bodies is saltier than the water in the river in which they live. River water “wants” to seep into their body water … but that water would then seep into their … cells, which would swell, maybe burst, if enough water seeped in.
Solution: Freshwater fish have microscopic pumps in the cells of their gills that can draw salts from the river into the water that bathes their cells. With body water and cell water equally salty, osmosis doesn’t happen.
Ocean fish have the opposite problem. Their body fluids are much less salty than the sea. Cell water “wants” to seep across the membranes into the body water, which would make the cells shrink (and die, just as plants wilt and die when they lose water).
Solution: Microscopic pumps in the cells of their gills, pump salts out of the body fluids that bathe the cells, until cell water and body water are equally salty.
These tiny “salt pumps” handle salts atom-by-atom to do their work.
Anadromous (an-ADD-roe-mus) fish – fish that live in the ocean, but migrate to fresh water to spawn – must deal with both problems.
When it’s time for a young salmon to head for the ocean, hormones signal the fish’s DNA to disassemble the freshwater-type pumps (that pump salt in), and assemble saltwater-type pumps (that pump salt out).
Some years later, when it’s time for the adult salmon to return to its birth stream to spawn, another hormone kicks in, signaling DNA to disassemble the saltwater-type pumps, and assemble freshwater-type pumps.
The change from saltwater pump to freshwater pump is triggered by a hormone: Prolactin.
Mammals don’t need to make new salt pumps, but we’ve found other uses for prolactin. Most prominent is the one for which the hormone was named: Stimulating, in a new mom’s body, lactation – the production of milk.
Alan Stahler trained as a biologist and is an amateur astronomer. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).
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