The active absorption of ions in some freshwater animals

Abstract

1. On frogs, goldfish and the crab Eriocheir, which migrates into freshwater, the active absorption of ions from very dilute (usually millimolar) solutions has been studied. 2. Methods are described for the determination of Cl−, (Br−), I−, NO3−, CNS−, CNO− HCO3−, SO4−, Na+ + K+, K+, NH4+, Ca++ on 1–5 ml of millimolar concentration. 3. When an anion is absorbed against a high concentration gradient and without any kation being absorbed at the same time, like Cl− from NH4Cl or CaCl2 this is taken to prove the working of a definite absorbing mechanism, involving the expenditure of energy, and the same argument holds when kations are absorbed against a pressure gradient and without being accompanied by anions. 4. In the frogs skin a separate mechanism exists for the absorption of Cl− and Br−, but unable to take up other anions like I−, NO3− or CNS−. These will enter slowly by diffusion and may increase the permeability of the skin to anions. Another mechanism is adapted for the absorption of Na+, but fails completely to absorb K+. 5. In the gills of the goldfish the same two absorbing mechanisms are present. The general permeability both to water and anions is much lower than in the frog. 6. In Eriocheir the anion mechanism will absorb indiscriminately Cl−, Br−, CNS− and CNO−, ions which show a high degree of chemical affinity. The kation mechanism does not distinguish between Na+ and K+. The general permeability is much higher. 7. It is assumed, in accordance with Lundegardh, that the absorption of ions takes place by chemical combination at the outer surface of certain cells, that the compound formed is carried by plasma circulation to the inner surface, and that it is there split by a process involving the expenditure of energy. The transport is supposed to involve an exchange with an ion of the same sign. 8. The CO2 produced by metabolism and excreted through the skin or gills is probably sufficient to serve in exchange for Cl− absorbed. In the goldfish and in Eriocheir the ammonia excreted through the gills is generally sufficient to serve in exchange for kations absorbed, but to account quantitatively for the kation absorption in the frogs skin it is necessary to assume that NH3 liberated at the outer surface of an absorbing cell may partly diffuse back and become utilized over again. 9. The range of possible shift in the balance of ions inside the organism between NaCl and NaHCO3 probably sets the limit for the excess or exclusive absorption of negative or positive ions respectively. 10. A programme is sketched for the extension of these studies to other types of freshwater animals, to the energetics of the absorption processes, and to its intimate mechanism. Cooperative or independent work along these lines is invited.