Cells expend a large amount of their free energy currency keeping the appropriate environment inside the cell. Thus, for example, Ca²⁺is present intracellularly at < 10^-7 M, while extracellular Ca²⁺ is present in millimolar (10^-3 M) concentrations, that is, 10,000-fold higher. The free energy difference due to the difference in [Ca²⁺], sometimes termed its chemical potential, can be calculated. The difference of the ΔG°’ values when Ca²⁺ is at the same concentration (1M) on each side of a membrane is, of course, zero, so the free energy is given by:

Converting from natural to base 10 logarithms, and substituting values for the gas constant, and a standard temperature of 25° C. (298° K.):

This expression means that the influx of Ca²⁺ into a cell is highly exergonic. If a channel is opened into a cell to allow Ca²⁺ across the membrane, it will flood into the cell. In muscle cells, this influx of Ca²⁺ is the signal for contraction. Cells, especially muscle cells, have a Ca²⁺ active transport system, which transports two Ca²⁺ ions out of the cell for every ATP hydrolyzed. The ΔG°’ of ATP hydrolysis is enough to do transport only a single ion. Because, however, the ATP/ADP ratio is kept very high during active metabolic conditions, the concentration gradient of higher [Ca²⁺] outside the cell is maintained.

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