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clinic.docx - Connection with clinic The phospholipid...

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Connection with clinic The phospholipid bilayer is responsible for certain passive permeability properties of biological membranes. Substances that are highly soluble in water typically per-meate cellular membranes slowly, whereas nonpolar compounds that are more soluble in nonpolar, hydrophobic organic solvents cross cell membranes more rapidly. High concentrations of barium salts are administered by mouth or by enema to make the interior of the gastrointestinal tract opaque to x-rays and improve the contrast of diagnostic x-ray films of the gastrointestinal tract. Barium ions in this concentration would be highly toxic, but because barium is insoluble in the hydrophobic interior of membranes, it is barely absorbed from the gastrointestinal tract. Hence, the concentration of barium in the blood rises very little after the administration of barium salts. If the motor nerve that innervates a skeletal muscle is accidentally severed, the acetylcholine receptors are no longer sequestered at the motor end plate; instead, they spread over the entire plasma membrane of the muscle cells. The entire surface of the cell then becomes excitable by acetylcholine, a phenomenon known as denervation supersensitivity . The plasma membranes of red blood cells from patients with hereditary spherocytosis are about three times more permeable to Na + than are red blood cells from normal individuals. The level of Na + ,K + -ATPase in the erythrocyte membranes of such patients is also substantially elevated. When these red cells have sufficient glucose to maintain normal ATP levels, they extrude Na + as rapidly as it diffuses into the cell cytosol, and the red cell volume is maintained. However, when such erythrocytes are delayed in the venous sinuses of the spleen, where glucose and ATP are present at low levels, the intracellular ATP concentrations fall; Na + cannot be pumped out by Na + ,K + -ATPase as rapidly as it enters, and the red cells swell. The
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