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V2N1 (pp. 121-23)

In the course of evolution, there are spectacular spin-offs. Typically, each macromolecule (such as a protein or a nucleic acid) inside a living cell tends to carry many excess negative charges. These charges get balanced by positive ions (especially potassium) dissolved in the water within the cell. But the presence of these ions means that water tends to be drawn into the cell by osmosis. (This osmosis is not on account of electrical charge on the potassium ions, but merely because potassium is not water and nature abhors concentration gradients among species of chemicals.) What keeps the cell from swelling and finally bursting as more and more water is taken in?

The walls of plant cells are made pretty strong . . . , the pressure inside the cells can therefore be higher than outside, and this higher pressure inside opposes the osmotic flow of water into the plant’s cells. (Notice the passivity of this solution for the problem.) That is one stable solution to the problem that began with the fact that the molecules of life have excess negative charge. It is a rather simple-minded solution, however, and this is why to this day plants do so poorly on IQ tests (J. Enright, personal communication).

The membrane forming the boundary of an animal cell is hardly a wall. The membrane is so thin it cannot withstand any pressure difference across it. Such a cell must live a bit more dynamically with its surround. It will be surrounded by water molecules just teeming to get inside and dilute the concentrations of dissolved chemical species (especially ions of potassium, but also sodium and chlorine). What to do?

First, take stock: the cell membrane is permeable to water, to potassium, to sodium, . . . and not permeable to chlorine. The membrane is more permeable to the potassium than to the sodium. Hummm. Try this: pump sodium ions out. As it happens, doing that will simultaneously pump more potassium ions in from the outside. Then the pump (if it reaches a steady state before burning up) will be able to maintain a higher concentration of sodium ions outside than inside. Then the sodium ions outside will be diffusing across the membrane, trying to sneak back in, and the potassium ions on the inside will be diffusing across the membrane, trying to get out to the suburbs. Voila! Since the membrane allows potassium to get out more freely than it allows sodium to get in, the net effect of the pump will be to increase the concentration of nonwater particles on the outside, thereby making the water molecules content to just stay out there. Wait another paragraph before starting the pump.

Consider the electrical situation. Both the potassium ions and the sodium ions carry an excess positive charge. Since the pump will be decreasing the overall concentration of these (the nonwater pawn in this game) on the inside of the cell, the excess negative charge on the inside (macromolecules and chlorine) will not be entirely cancelled out by the dissolved positive ions inside. Then the cell membrane will have an electrical potential difference across it. The cell can live with that provided the pump speed is restricted to a certain range implicated by the membrane’s electrical conductance with respect to sodium ions relative to its electrical conductance with respect to potassium ions. Whew! Now, start the pump.

The momentous spin-off in evolutionary history was that this membrane potential, in some animal cells, could be briefly changed by adjustments in the membrane conductances with respect to sodium ions and with respect to potassium ions. Thus the animal-cell solution to the problem that the molecules of life (the macros inside the cell) carry excess electrical charge made possible the essential signaling mechanism (brief change in membrane potential) for muscle cells and for nerve cells (neurons). And that is how it came about that some animals today can study philosophy. —SB 1994

 

Cells of the Five Kingdoms*

Monera (eg. bacteria)

no nucleus, no organelles, circular DNA without histones, cell membrane, cell wall (mostly peptidoglycon)

Protista (eg. amoeba, paramecium, algae)

nucleus, organelles, vacuole, chloroplasts (some), cell membrane, cell wall (some)

Fungi (eg. mushrooms, molds, yeasts)

nucleus, organelles, cell membrane, cell wall (chitin)

Plants (eg. mosses, ferns, trees)

nucleus, organelles, vacuole, chloroplasts, cell membrane, cell wall (cellulose)

Animals (eg. worms, snails, insects, birds, mammals)

nucleus, organelles, cell membrane

Edited by Boydstun
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