Na⁺ TRANSPORT SYSTEMS IN PROKARYOTES

All living cells, whether eukaryotic or prokaryotic, maintain a Na+ cycle across the cytoplasmic membrane. This cycle is driven by Na+ extruding systems which excrete the ion and maintain a Na+ concentration gradient (Δμ̂ na +) directed inward (reviews in References 1 through 14). Na+ reenters the cells via Na+ gradient consumers, Na+-coupled cotransport systems being the most widespread route (reviews in References 8 and 15). Na +-motive flagella 16, 17 and Na+ gradient-driven ATP synthase are known for certain bacteria. 12, 18 The energy stored in the Na+ gradient, in the first half of the Na+ cycle, is transduced to osmotic, mechanical, or chemical work in its second part. Hence, the Δμ̂ na +, like Δμ̂ h +, is a convertible energy currency. No ion, other than H+ and Na+, is known to play such a central role in bioenergetics.