Geology Reference
In-Depth Information
gen atom, gather around and shield the positive and negative charges of their molecular
neighbours, preventing the gumming up and allowing the metabolism of the cell to flow
smoothly.
But, strangely enough, these same attractions between water molecules and positive
and negative ions in the surrounding sea water outside the cell threaten the very life of
our alga because of a peculiar force known to biologists as
osmosis
. Let's swim close
to the alga's cell membrane that encloses the entire cell in a sinuous, filmy, balloon-like
embrace. Notice that the membrane is riddled with tiny pores. If you watch carefully,
you'll notice how the random motions of the water molecules result in many of them
leaving our algal cell through the pores to enter the sea, whilst other water molecules
find their way into the cell from the surrounding sea water, also via the pores. If we were
able to paint the water molecules inside our alga with a red dye, and those in the sea wa-
ter immediately surrounding us with a blue dye, we would quickly notice that more red
water molecules end up outside the cell than are replaced by blue ones coming in from
the outside. In other words, our cell loses more water than it gains from the surrounding
ocean.
Why should this be? Unlike most molecules and ions, water can move freely in
either direction across our alga's semi-permeable cell membrane
.
Let's hitch a ride on
a passing water molecule as it travels out through a pore and into the surrounding sea
water. We look back at our cell hovering before us like a giant planet in the vast spa-
ciousness of the sea, where there are many more ions than inside our cell. Like so many
other water molecules in the ocean, ours is attracted to these more abundant ions to such
an extent that fewer water molecules are free to pass into our alga through its cell mem-
brane. But inside the cell there are fewer ions and charged molecules to attract the water
molecules, and so more of them leave the cell as their random movements tumble them
out into the sea through the pores in the cell membrane. The result is that the entire cell
is in danger of dying, as the unshielded ions gum up the cell's metabolism.
Our alga can prevent water loss to a certain extent by using energy to pump water
back into the cell from the sea, but it can't bring in enough water to prevent an uncom-
fortable situation from developing. And now at last we come to the role of DMSP—a
relatively long molecule with a positive charge in the vicinity of the sulphur atom at one
end, and a negative charge at its opposite end. These two charges attract free ions that
