Molluscs shell formation

A number of suggestions have been made that calcium may be transported because it is coupled to the movement of other ions or because it moves passively down an electrochemical gradient established by the movement of some other ions600. Thus, a sodium-induced potential has been found which was sufficient to account for the passive movement of calcium into the shell gland of the domestic fowl during egg shell formation. In the mollusc, the shell side of the mantle is normally positive relative to blood and a potential of this type would, of course, tend to move calcium away from the extrapallial fluid. A potential of this orientation could be produced by the movement of an anion into the animal (mollusc) and the low chloride concentration of the extrapallial fluid could be accounted for on this basis. 

A variation of the proton removal hypothesis of shell formation has been proposed603, 604. It has been shown that in molluscs and birds the sites of shell formation contain high concentrations of ammonia and they suggest that this may be an initiating phenomenon for calcification, i. e. 

Weiner, S., Hood, L. Soluble protein of the organic matrix of mollusc shells A potential template for shell formation. Science 190, 987 (1975)... 

Wilbur, K. M. Shell formation in molluscs. Chemical Zoology 7, 103 (1972)... 

Bevelander, G., and Nakahara, H. An electron microscope study of the formation of the nacreous layer in the shell of certain bivalve molluscs. Calc. Tiss. Res. 3, 84-92 (1969). 

Ammonites are extinct, marine molluscs, distantly related to nautilus. When cut in half and polished they often display attractive formations of crystals within the outline of the original shell. Some contain crystals of the brassy-coloured mineral pyrite. Ammonites occur worldwide and are regarded as collectors items, or are used for ornamental purposes. In Yorkshire, England, they are found in the same area as jet, and are sometimes combined with that material and made into items such as chessboards. 

Let us see what effect a CO2 pressure of 10 atm has on the solubility compared with that found above in the closed system. This P is a little above normal atmospheric conditions, 0.031 % C02-Substituting P = 10 into the equation just obtained shows that the first and third terms are major ones under these conditions, assuming the S is near 10" M as obtained in the closed system. We try successive approximations starting with this value, substituting the trial value of S into the term and solving for a new y from the y term. This rapidly leads to y = 10 . This is also S since Xj, is less than 1 % of y. Now we can use set (11-2) to obtain the other species concentrations and find that the pH is 7.98. Thus, by the addition of only 10" atm CO2, we increase the solubility about fourfold and change the pH from about 10 to 8. This shows the powerful effect of CO2 on the weathering of limestones, and also in the formation of shells by molluscs, who must precipitate CaCOj from the ions of their tissue fluids. 

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