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Writing Across the Curriculum

The passage of salt (NaCl) across a plasma membrane is of primary importance to most cells. The chloride ion (Cl–) usually crosses the plasma membrane because it is attracted by positively charged sodium ions (Na+). First sodium ions are pumped across a membrane, and then chloride ions simply diffuse through channels that allow their passage. As noted in Figure 4.2a, the genetic disorder cystic fibrosis results from a faulty chloride channel. Ordinarily, after chloride ions have passed though the membrane, sodium ions (Na+) and water follow. In cystic fibrosis, Cl– transport is reduced, and so is.