CHAPTER SIX

PROKARYOTES AND EUKARYOTES

The Potassium world

Life was born in the sea, and even the organisms which invaded the land could do so only by carrying with them an internal sea that enabled their cells to continue to live in water. This liquid that floods every cell has still values of pH and osmotic pressure which are similar to those of sea water, and contains likewise high concentrations of sodium and potassium ions. The really extraordinary thing, however, is that inside all cells (including those that live in the sea) the concentrations of sodium and potassium are totally different from those of the external liquid.
A first explanation of this strange experimental fact came into view when it was found that sodium ions (Na+) produce very high osmotic pressures inside the cell, and, without defence mechanisms, a cell would swell to bursting point and nothing could save it. The osmotic defence mechanisms can be of three kinds: (1) an external wall that prevents swelling from the outside, (2) an internal web of filaments that ties the cell membrane from the inside, or (3) a battery of ion pumps on the cell membrane which contually drains out the excess sodium.
For a long time it has been tought that the cell’s osmotic problems are caused by sodium alone, but things turned out to be more complicated. The real problem is that cells need very high concentrations of potassium ions (K+) in their interior, and to this end are continually importing potassium from the outside. The cell can cope with potassium osmotic pressure, but not with much higher values, and it is for this reason that the addition of sodium ions would be lethal.