Metabolism
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Bloating, flatulence and diarrhoea after drinking milk - but not yogurt or cheese
Key points from this exercise:
It is normal that lactase activity in the small intestine is lost during late adolescence; among people of northern European origin (and some other groups) the enzyme persists into adult life.
People who lack lactase are intolerant of lactose, and a moderately large amount causes bloating, flatulence and diarrhoea because of the metabolism of unabsorbed lactose by intestinal bacteria.
Under anaerobic conditions, yeast ferments glucose to yield 2 mol of ethanol and 2 mol of carbon dioxide per mol of glucose. The initial series of reactions leads to formation of 2 mol of a 3-carbon compound (pyruvate), which then undergoes decarboxylation and reduction to ethanol and carbon dioxide.
Fluoride inhibits the metabolism of glucose in bacteria and red blood cells. Blood samples for measurement of glucose are collected in tubes containing sodium fluoride.
Red blood cells metabolise glucose to form 2 mol of lactate per mol of glucose. The amount of glucose metabolised depends on the amount of ADP provided.
Two mol of ATP are formed for each mol of glucose metabolised to lactate. However, in red cells that have been depleted of ATP there is no metabolism of glucose - ATP is also required for glucose metabolism.
Glucose is taken up by red blood cells by metabolic trapping - it crosses the cell membrane by facilitated diffusion and is then phosphorylated to glucose 6-phosphate, which cannot cross the cell membrane, at the expense of ATP.
Inorganic phosphate is also required for glucose metabolism and ATP formation in red blood cells. At low concentrations of phosphate glucose metabolism is impaired. At high concentrations of phosphate a number of phosphorylated intermediates accumulate. You will investigate these intermediates and deduce the pathway of glucose metabolism in later exercises.