Abstract: Association constants of the complexes CaATP~(2-) and Ca_2ATP, K_1, K_2 and the product of solubility of CaCO_3, K_(sp), in the lecithin-water order system (PC-H_2O) have been measured by potentionmeter using a calcium ion selection membrance electrode. We have observed that formation constant of the complex CaATP~(2-), K_1, in the PC-H_2O order system is larger about an order of magnilude than K_1 in the H_2O, and K_1 (PC-H_2O) increases with the concentration of phospholipid in the PC-H_2O system. But the rate of increase of the K_1(PC-H_2O) is reduced with the increase of the concentration of the lecithin beyend(cmc). However, the formation constant of Ca_2ATP, K_2, does not change obviously in the process of increasing [PC]. When [PC]>[cmc], the K_(sp)(PC-H_2O) of CaCO_3 in the PC-H_2O system is larger than K_(sp)(H_2O) of CaCO_3 in the water. It illustrates that the solubility of CaCO_3 is increased in the liguid crystal order system.
It is expected that the liquid crystal state in biological body is favorable for the absorbtion of calcium.

Key words: Order molecular system, Egg yolk phospholipid, Formation constant of Ca-ATP, The product of solubility of CaCO₃