Chemical composition of the cell; water and salts

The cell contains about 70 chemical elements that also occur in non-living nature; this shows the unity of living and non-living nature, though the proportions of the elements differ. About 98 % of cell mass is made up by four elements: oxygen, carbon, hydrogen and nitrogen; they are the basis of all organic substances. Macroelements (P, S, K, Na, Ca, Mg, Cl, Fe) account for about 2 % of cell mass: Na, K and Cl take part in the passage of substances across membranes and in nerve excitation, Ca and P in bone, Ca in blood clotting, Fe in haemoglobin and Mg in chlorophyll. Microelements (Zn, Cu, I, F, Co, Mo, Mn, B and others), although present in tiny amounts, are part of hormones, enzymes and vitamins: iodine is found in the thyroid hormone thyroxine, cobalt in vitamin B12 and zinc in some enzymes. A long-term lack of iodine can lead to thyroid disease (goitre), which is why iodised salt is recommended.