ABSTRACT
Sodium (Na), like potassium (K), is an alkali metal belonging to the rst group in the periodic table of the chemical elements. Both of the elements are abundant in the earth’s crust, which is 2.6% Na and 2.4% K. They occur in various salts, and after dissolution of the salts form monovalent cations. Despite these striking similarities, K and Na differ considerably in their signicance for the biosphere. Whereas K is considered a plant nutrient (see Chapter 4), Na, depending on the plant species, may act as a benecial or toxic element (Kronzucker et al., 2013). The difference between the two monovalent cations is based on the ionic diameter, which in turn determines the surface charge. Because the ionic diameter of the nonhydrated Na+ is smaller (0.20 nm) than that of the nonhydrated K+ (0.27 nm), the surface charge of Na+ is higher, resulting in a larger diameter of the hydration shell. This size difference is the reason why K+ in most cases cannot be substituted by Na+ in the three major physiological functions, namely,
1. Charge balance 2. Enzyme activation 3. Osmotic effects
Charge balance requires high permeability of cell membranes to allow free movement of the ion following an electrochemical gradient. In most of the plant cell membranes, this action is facilitated by cation channels with a high specicity for K+. The larger hydration shell that is not abandoned during channel-mediated transport hinders the passage of Na+. The same applies to enzyme activation.
