The Membrane Potential

As a result of the permeability properties of the plasma membrane, the presence of nondiffusible negatively charged molecules inside the cell, and the action of the Na+/K+ pumps, there is an unequal distribution of charges across the membrane.As a result, the inside of the cell is negatively charged compared to the outside.This difference in charge, or potential difference, is known as the membrane potential.

In the preceding section, the action of the Na+/K+ pumps was discussed in conjunction with the topic of active transport, and it was noted that these pumps move Na+ and K+ against their concentration gradients. This action alone would create and amplify a difference in the concentration of these ions across the plasma membrane. There is, however, another reason why the concentration of Na+ and K+ would be unequal across the membrane.

Cellular proteins and the phosphate groups of ATP and other organic molecules are negatively charged at the pH of the cell cytoplasm. These negative ions (anions) are "fixed" within the cell because they cannot penetrate the plasma membrane. As a result, these anions attract positively charged inorganic ions (cations) from the extracellular fluid that are small enough to diffuse through the membrane pores. The distribution of small inorganic cations (mainly K+, Na+, and Ca2+) between the intra-cellular and extracellular compartments is thus influenced by the negatively charged fixed ions within the cell.

Since the plasma membrane is more permeable to K+ than to any other cation, K+ accumulates within the cell more than the others as a result of its electrical attraction for the fixed an-

Q Q Q Electrical attraction

Q Q Q Electrical attraction

Plasma membrane

■ Figure 6.21 The effect of fixed anions on the distribution of cations. Proteins, organic phosphates, and other organic anions that cannot leave the cell create a fixed negative charge on the inside of the membrane. This negative charge attracts positively charged inorganic ions (cations), which therefore accumulate within the cell at a higher concentration than is found in the extracellular fluid. The amount of cations that accumulates within the cell is limited by the fact that a concentration gradient builds up, which favors the diffusion of the cations out of the cell.

■ Figure 6.21 The effect of fixed anions on the distribution of cations. Proteins, organic phosphates, and other organic anions that cannot leave the cell create a fixed negative charge on the inside of the membrane. This negative charge attracts positively charged inorganic ions (cations), which therefore accumulate within the cell at a higher concentration than is found in the extracellular fluid. The amount of cations that accumulates within the cell is limited by the fact that a concentration gradient builds up, which favors the diffusion of the cations out of the cell.

ions (fig. 6.21). So, instead of being evenly distributed between the intracellular and extracellular compartments, K+ becomes more highly concentrated within the cell. The intracellular K+ concentration is 150 mEq/L in the human body compared to an extracellular concentration of 5 mEq/L (mEq = milliequivalents, which is the millimolar concentration multiplied by the valence of the ion—in this case, by one).

As a result of the unequal distribution of charges between the inside and outside of cells, each cell acts as a tiny battery

Interactions Between Cells and the Extracellular Environment 141

with the positive pole outside the plasma membrane and the negative pole inside. The magnitude of this charge difference is measured in voltage. Although the voltage of this battery is very small (less than a tenth of a volt), it is of critical importance in such physiological processes as muscle contraction, the regulation of the heartbeat, and the generation of nerve impulses. In order to understand these processes, then, we must first examine the electrical properties of cells.

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Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • Sam
    What is the distribution of charges through a membrane?
    8 years ago
  • lawana
    Why negatively charged organic ions (anions) fixed?
    8 years ago

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