(Solved): 6. The crab-eating frog of Southeast Asia, Rana cancrivora, develops and matures in fresh water but ...

6. The crab-eating frog of Southeast Asia, Rana cancrivora, develops and matures in fresh water but searches for its food in coastal mangrove swamps (composed of $80\%$ to full-strength seawater). When the frog moves from its freshwater home to seawater it experiences a large change in the osmolarity of its environment (from hypotonic to hypertonic). (a) Eighty percent seawater contains $460\mathrm{mM}\mathrm{NaCl},10\mathrm{mM}\mathrm{KCl},10\mathrm{mM}\mathrm{CaCl}$, and $50\mathrm{mM}\mathrm{MgCl}$. What are the concentrations of the various ionic species in this seawater? (b) Ignoring dissolved proteins, amino acids, nucleic acids, and other small metabolites, calculate the osmolarity of the frog's cells based solely on thionic concentrations given below. The chart below lists the cytoplasmic concentrations of ions in the frog. (c) Like all frogs, the crab-eating frog can exchange gases through its permeable skin, allowing it to stay underwater for long periods of time without breaking. How does the high permeability of frog skin affect the frog's cells when it moves from freshwater to seawater? (d) The crab-eating frog uses two mechanisms to maintain its cells in osmotic balance with its environment. First it allows the ${\mathrm{Na}}^{+}$and ${\mathrm{Cl}}^{?}$concentrations in its cells to increase slowly as the ions diffuse down their concentration gradients. Second, like many elasmobranchs (sharks), it retains the waste product urea in its cells. The addition of both $\mathrm{NaCl}$ and urea increases the osmolarity of the cytosol to a level nearly equal to that of the surrounding environment. $?$ Urea $({\mathrm{CH}}_4{\mathrm{N}}_2\mathrm{O}$, molecular mass $64.08\mathrm{g}/\mathrm{mol})$ Assuming the volume of water in a typical frog is nearly 100 $\mathrm{mL}$, calculate how many grams of $\mathrm{NaCl}$ (molecular mass $58.44\mathrm{g}/\mathrm{mol}$ ) the frog must take up to make its tissues isotonic with seawater. (e) How many grams of Urea must it retain to accomplish the same thing?