Explain the biological importance of sodium and potassium.

(N/A) typical $70 \ kg$ man contains about $90 \ g$ of $Na$ and $170 \ g$ of $K$ compared with only $5 \ g$ of iron and $0.06 \ g$ of copper.
Sodium ions $(Na^+)$ are found primarily on the outside of cells,being located in blood plasma and in the interstitial fluid which surrounds the cells.
These ions participate in the transmission of nerve signals,in regulating the flow of water across cell membranes,and in the transport of sugars and amino acids into cells.
Potassium ions $(K^+)$ are the most abundant cations within cell fluids,where they activate many enzymes,participate in the oxidation of glucose to produce $ATP$,and,with sodium,are responsible for the transmission of nerve signals.
There is a considerable variation in the concentration of $Na^+$ and $K^+$ ions across cell membranes. In blood plasma,$Na^+$ is present at $143 \ mmol \ L^{-1}$,whereas $K^+$ is only $5 \ mmol \ L^{-1}$. Inside red blood cells,these concentrations change to $10 \ mmol \ L^{-1}$ for $Na^+$ and $105 \ mmol \ L^{-1}$ for $K^+$.
These ionic gradients are maintained by a discriminatory mechanism called the sodium-potassium pump,which consumes more than one-third of the $ATP$ used by a resting animal.