AP Biology 2.8.A Active Transport and Ion Gradients

Polycystic kidney disease (PKD) is an inherited disease that causes water loss from the body and affects cell division in the kidneys. Because water movement across cell membranes is related to ion movement, scientists investigated the role of the $\mathrm{Na}^{+} / \mathrm{K}^{+}$ATPase (also known as the sodium/potassium pump) in this disease. Ouabain, a steroid hormone, binds to the $\mathrm{Na}^{+} / \mathrm{K}^{+}$ATPase in plasma membranes. Individuals with PKD have a genetic mutation that results in an increased binding of ouabain to the $\mathrm{Na}^{+} / \mathrm{K}^{+}$ATPase. The scientists treated normal human kidney (NHK) cells and PKD cells with increasing concentrations of ouabain and measured the number of cells (Figure 1) and the activity of the $\mathrm{Na}^{+} / \mathrm{K}^{+}$ATPase (Figure 2) after a period of time. The scientists hypothesized that a signal transduction pathway that includes the protein kinases MEK and ERK (Figure 3) may play a role in PKD symptoms.

Describe the characteristics of the plasma membrane that prevent simple diffusion of $\mathrm{Na}^{+}$and $\mathrm{K}^{+}$across the membrane. Explain why ATP is required for the activity of the $\mathrm{Na}^{+} / \mathrm{K}^{+}$ATPase.