frog skin - Na Hye Eunice Kim Thur Section - Keiling LAB4:...

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Na Hye Eunice Kim Thur Section - Keiling LAB4: Ion Transport by Frog Skin I. Introduction Background : The skin of frog is permeable to respiratory gases and to water. As a result, it continually takes up water from the environment since its blood has a higher osmotic pressure than pond water. This gain in water must be balanced by a loss of Na+, which is compensated for partly by Na+ uptake from food ingestion and partly but most importantly by active uptake of Na+ directly through the skin from pond water through the epithelial cells of the skin. The epithelial cells play an important role in keeping the internal environment and by, conserving Na+, achieve similar functions as kidney tubules in other species. In addition, just like kidney, its skin is reactive to many of drugs and hormones. In order for Na+ to move across an epithelia cell layer in a frog skin, it must go through two membranes. Na+ moves: 1. From pond (mucosal) side of the epithelial layer to the inside of the cell via sodium channels which allow diffusion down its concentration gradient. 2. From the inside of the cell to the blood (serosal) side of the epithelial layer via Na+- K+-ATPase which actively transports Na+ against its concentration gradient by using energy. Transport of charged Na+ via both Na+ channels on the mucosal side and the sodium pumps on the serosal side creates potential difference across the epithelia layer (Voltage on the serosal side of the skin – Voltage on the mucosal side of the skin). As a result, serosal side will be more positive. Furthermore, an increase in pump activity will cause more potential difference. Though not as permeable as to Na+, the frog skins also transport Cl-, making serosal side more negative. The purpose of this experiment is to examine how ions (mostly Na+) set up potential differences and how they move across a cell layer present in the frog skin. This movement of ions across the epithelial layer is observed by examining the voltage difference over time under the following conditions: changing the concentration of sodium ions, replacing chloride ions, and adding various drugs to alter the pathway for Na+ transport. Hypothesis : Since sodium channels allow diffusion down its concentration gradient, an increase of the sodium concentration on the mucosal side will increase the flux across the mucosal membrane and thus increase the sodium concentration inside the cell. And an increase in intracellular sodium concentration will increase the activity of the Na+-K+ ATPase. As a result, serosal side becomes more positive over the time, increasing the voltage difference across the epithelial layer. Also replacing Cl- with the impermeant anion sulfate definitely makes it difficult to offset the effect of Na+ transport, and thus causing greater potential difference. Since the skin of the frog is responsive to many of drugs and hormons, adding various drugs which block or change the pathway for Na+ transport across the epithelial layer causes the change in
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frog skin - Na Hye Eunice Kim Thur Section - Keiling LAB4:...

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