Question

The skin of frogs is quite permeable to water, and because frogs that live in freshwater are hypertonic to the surrounding water, water will tend to enter the animal across the skin by osmosis. To avoid overhydration, frogs produce large amounts of very dilute urine, but this causes loss of Na⁺ in the urine. Frogs do not drink, so how do they replace the lost Na⁺? Can they obtain it from pond water across their skin? To test this hypothesis, you place a piece of skin from the abdomen of a frog in diffusion chamber, such that the apical surface of the skin is facing half-chamber A and the basolateral surface is facing half-chamber B. You then fill both half-chambers with frog Ringer's solution (containing 85 mM NaCl, 25 mM NaHCO₃, 2 mM KCl and 1 mM CaCl₂). Under these conditions, you measure a net Na⁺ flux from A to B of 1.5 nanomole/sec. Based on the results of your experiment, which of the following statements is most likely FALSE?

A) Reversing the orientation of the skin (such that the apical surface faces half-chamber B) would reverse the direction of net Na⁺ flux.
B) The K⁺ flux rate in this system is -1.0 nanomole/sec
C) Replacing the Ringer's solution with "potassium-free Ringer's solution" (87 mM NaCl, 25 mM NaHCO₃ and 1 mM CaCl₂) would reduce the Na+ flux rate
D) The Na⁺ flux rate could also be calculated from the Nernst equilibrium potential for Na⁺, provided Na⁺ conductance (gNa) is known

Answer 1

From the experimental results, the statement likely to be FALSE is that the K+ flux rate in this system is -1.0 nanomole/sec because the experiment focused on Na+ flux and does not provide direct data on K+ flux rates.

Step-by-step explanation:

To address the hypothesis of how frogs replace lost Na+ without drinking, an experiment placing frog skin in a diffusion chamber showed a net Na+ flux from one half-chamber to another. From the described experimental results, one can infer certain outcomes. Firstly, the orientation of the skin would indeed likely reverse the direction of net Na+ flux due to the polarity of epithelial cells. Na+ flux rate could also be calculated using the Nernst equilibrium potential if Na+ conductance is known. However, the statement about the K+ flux rate being -1.0 nanomole/sec cannot be directly concluded from this experiment, as the potassium (K+) flux would require its own measurements and considerations separate from sodium (Na+).

The experiment demonstrates that frogs can potentially gain sodium across their skin from their freshwater environment, maintaining electrolyte balance through mechanisms other than direct consumption of water, akin to how they manage water influx by producing dilute urine.