CASE HISTORY Water Is the Most Abundant Molecule of Life-But Too Much

question 40

Essay

CASE HISTORY
Water is the most abundant molecule of life-but too much water can be deadly.In the summer of 2010,heavy rains flooded Pakistan,submerging one-fifth of the country.Millions of people were displaced and exposed to water contaminated by microbial pathogens.Diarrheal disease threatened many,especially children.
A ten-year-old boy named Akal developed profuse diarrhea.He showed extreme thirst,but clean drinking water was unavailable.His diarrhea became watery and cloudy with white flecks of intestinal mucus (known as rice-water stool).Akal became listless,his eyes sunken and his skin wrinkled from dehydration.
A nurse observed a sample of Akal's diarrheal fluid under darkfield microscopy.Numerous bacteria swam rapidly,appearing like "twinkling stars." The bacteria,comma-shaped bacilli,stained Gram negative.
Later testing confirmed Vibrio cholerae,the cause of cholera.These bacteria produce cholera toxin,a protein complex that causes intestinal epithelial cells to secrete large amounts of chloride ion (Cl⁻).The high chloride concentration draws out the positively charged sodium ions (Na⁺).The increased extracellular salt (Na⁺ and Cl⁻,or NaCl)causes water to diffuse out of the epithelial cells,leading to dehydration and diarrhea.Without treatment,Akal's chance of death was 50%.
Akal was treated with oral rehydration therapy (ORT),a method invented by Bangladeshi physicians.ORT involves gradual feeding of a solution containing glucose and NaCl to replace ions and water lost through the diarrhea.Within hours,Akal's flesh had rehydrated,and within a week he made a full recovery.
Explain the properties of cell membranes that lead to the loss of water from epithelial cells in response to cholera toxin.

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