If animal cells are to survive, it is essential that they regulate the movement of water across the plasma membrane. Given that proteins and other organic constituents of the ICF cannot be allowed to cross the membrane, diffusion of water becomes a problem. Animal cells have solved this problem by excluding a compensating extracellular solute, sodium ions. We'll discuss in more detail later exactly how they go about excluding Na+.

Diffusion equilibrium is reached when internal and external concentrations are equal for all substances that can cross the membrane. For uncharged substances, such as those we have considered in our examples so far, we do not have to consider the influence of electrical force on the equilibrium state. However, the solutes of the ICF and ECF of real cells bear a net electrical charge. In the next chapter, we will consider what role electric fields play in the movements ofthese charged substances across the membranes of animal cells.

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