Spatial Organization of Active and Passive Transport Proteins Enables Material to Pass Completely Through the Cell

Although macromolecules and biomembranes clearly underlie physiological function, many phenomena of the intact animal emerge that are not initially apparent as a simple sum of parts.

One interesting example is the spatial organization of plasma membrane transport proteins so that ions move across the cell from one ECF compartment to another. This transcellular transport is important in the kidney (see Chapter 41). The plasma membrane of the epithelial cells in the proximal tubules of the kidney contains two distinct regions. The apical mem­brane regions face the lumen of the tubule and the fluid that will become urine, and the basolateral regions are near the capillaries and the blood. The apical surface contains ungated leak channels for Na, whereas the basolateral surface contains Na∖K'-ATPase molecules. The membrane proteins in one region are prevented from diffusing into the other by mem­brane structures called tight junctions. Na⁺ diffuses into the cell on the apical surface from the urinelike fluid driven by both the concentration gradient and the resting mem­brane potential. Once inside the cell, the Na⁺ is pumped from the basolateral surface, essentially into the blood, by the Na*,K⁴-ATPase. This allows the kidney to reabsorb and thus conserve Na⁺. As long as the Na^f₁K^t-ATPase remains restricted to the basolateral surface and the passive channel to the apical membrane, Naⁱ can move through the cell from the Urinelike fluid in the tubule to the blood in the capillaries. If either protein should lose its spatial restriction, Na^t would be transported into and out of the cell on the same surface, merely consuming ATP₁ with no net transport of Na^t from lumen to capillary.

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