# Calcium reabsorption from egg shell during embryogenesis, how is carbonate neutralized?

As the embryo reabsorbs calcium from the shell, it also reabsorbs an equal quantity of carbonate. Calcium is in quite high concentrations in the body, the same molarity for carbonate causes a very alkaline environment. Carbonate has a pKa near 10, seen in that the egg white in "food eggs" (non-incubated and non-impregnated) becomes alkaline as the egg ages, approaching pH 9 to 10.

Throwing some numbers in there, assuming 2.2 gram of CaCO3, 8% reabsorbed to cause the average 8% thinning documented here (this is also just day 3-5 I think. ) 2.2 gram*8% = 0.176 gram carbonate released. The molecular weight of carbonate is 61 g/mol, 0.176 g / 61 g/mol = 0.003 mol = 3 mmol. If newborn chicken weighs 85 gram, and is 70% fluid, 85 gram * 70% = 60 gram. 60 gram is 0.06 L water. 3 mmol carbonate equivalent to 3 mmol/0.06 L = 50 mmol/L. The solution gets a pH of 11, and if you add 25 mmol/L of hydrochloric acid it is still at pH 9.2.

How is that carbonate neutralized?

At least 1 carbonate ion in 5 can be neutralized by bone formation, since hydroxyapatite has a ratio of 5:1 calcium to hydroxide ions, and, 99% of calcium is in bone (assuming somewhat similar ratio in infant as well. ) What about the other 4?

EDIT: A user David emphasizes that Wikipedia's entry on Calcium carbonate gives a pKa of 9. I based pKa closer to 10 on equilibrium between carbonate and bicarbonate. 9 or 10 does not really change the question that much since its about the huge quantity of base, quite strong in either case. I used http://www.aqion.onl/ for pH calculations giving pH 11. Feel free to use the correct value.

EDIT: Carbonic acid from CO2 from cellular respiration was mentioned. Carbonic acid from cellular respiration is at least part of it. But I don't know if it is all of it or close to all of it. 3 mmol of calcium absorbed (see Crooks, 1973) if average egg volume is 50 mL, molarity is 60 mmol/L, and this is as carbonate which saturates pCO2/H2CO3/HCO3-/CO3^2- as well.

EDIT: Also quite a lot of uric acid created, 2.3 gram per L by day 13, jbc.org/content/70/2/535.full.pdf, molarity 14 mmol/L.

SUMMARY: Bone formation likely takes care of 1 in 5 carbonates. Carbonic acid from cellular respiration, uric acid from nitrogen excretion, both contribute. Other factors?

• Comments are not for extended discussion; this conversation has been moved to chat. – Bryan Krause Jul 5 '20 at 14:36

Carbonic acid from cellular respiration, like Trond mentioned, and uric acid from nitrogen metabolism, and, bone mineralization, are large sources of acid, might be enough to neutralize 3 mmol (Crooks, 1973; Tuan, 1986) CO3.

Calculations

3 mmol CO3 in a chicken that weighs 45 gram when newly hatched, 45 gram * 60% = 27 g = 27 mL of water, is 3 mmol / 27 mL = 111 mmol/L. The carbonic acid is likely around 30 mmol/L, and shares an equilibrium with the carbonate ion which might reduce that number. Acid from bone mineralization is 1:5th of CO3, 111 mmol/L / 5 = 22 mmol/L. The upper bound for carbonic acid and mineralization is then 30 mmol/L + 22 mmol/L = 52 mmol/L. This leaves 111 mM - 52 mM = 59 mM. Uric acid is up to 14 mmol/L by day 13 (Fiske, 1926), and, could reasonably increase up to 59 mmol/L.

References

Crooks, R. J., & Simkiss, K. (1974). Respiratory acidosis and eggshell resorption by the chick embryo. The Journal of experimental biology, 61(1), 197–202.

Tuan, Rocky S., and Tamao Ono. ‘Regulation of Extraembryonic Calcium Mobilization by the Developing Chick Embryo’. Development, vol. 97, no. 1, Sept. 1986, pp. 63–74.

Fiske, Cyrus H., and Edward A. Boyden. ‘Nitrogen Metabolism in the Chick Embryo’. Journal of Biological Chemistry, vol. 70, no. 2, Jan. 1926, pp. 535–56.

• I was asked by a little notification to provide references. The point is, everyone knows cellular respiration releases CO2, everyone knows uric acid is how uricotelic animals excrete nitrogen, and everyone knows hydroxyapatite is 5 Ca^2+ +3 PO4^3- + OH-, acid excreted during mineralization therefore. The reference I did provide, I can also include the whole APA citation for it, here: Crooks, R. J., & Simkiss, K. (1974). Respiratory acidosis and eggshell resorption by the chick embryo. The Journal of experimental biology, 61(1), 197–202. – Serena Jul 6 '20 at 2:07
• Obviously, reference standards from 20th century aren't ideal way to record information but so there you go. – Serena Jul 6 '20 at 2:07