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Electrochemical potential energy and transport processes


Objectives

  • 1.

    Recognize that concentration gradients and electrical potential gradients store chemical and electrical potential energy, respectively.

  • 2.

    Recognize that electrochemical potential energy drives all transport processes.

  • 3.

    Use the concept of electrochemical potential energy to analyze transport processes.

Electrochemical potential energy drives all transport processes

In Chapter 2 , by examining the permeability of biological membranes to various solutes, we concluded that, with the exception of simple, small, and typically lipid-soluble molecules (e.g., O 2 , CO 2 , ethanol), most biologically important solutes (e.g., sugars, amino acids, inorganic ions) cannot readily traverse cellular membranes. Therefore special transport mechanisms are required to move these impermeant solutes from one side of a membrane to the other. An important class of special transport mechanisms—the ion channels—has already been discussed in Chapter 5, Chapter 6, Chapter 7, Chapter 8 . There we observed that differences in ion concentrations and electrical potential across a membrane can drive the movement of ions through channels. In this chapter we introduce the concepts of chemical and electrical potential energy, which are stored in concentration gradients and electrical potential gradients, respectively. We also demonstrate that electrochemical potential energy drives all solute transport processes.

The relationship between force and potential energy is revealed by examining gravity

Experience tells us that the gravitational force acts on an object at any height so that, when the object is released, it is pulled toward the ground. Because force is mass ( m ) times acceleration ( a ), the gravitational force ( F G ) must be:


F G =- m a G

where a G is the acceleration caused by gravity and the minus sign indicates that the force is directed downward (in the negative y direction).

Lifting an object of mass, m , from the ground to some height, y , requires an investment of energy. The amount of energy invested in lifting an object is directly proportional to the mass of the object and the height to which the object is lifted. One way to conceptualize this is to say that the object has greater potential energy when it is at a greater height from the surface of the Earth. These considerations are neatly summarized in the definition of gravitational potential energy (PE G ):


PE G = m a G y

Implicit in this equation is the fact that ground level is the reference point against which gravitational potential energy is measured; that is, at ground level ( y = 0), PE G = 0. From this definition, it is clear that a change in height, Δ y , causes a corresponding change in gravitational potential energy, ΔPE G = ma G Δ y. In other words, a gradient of gravitational potential energy exists in the y direction. The gravitational potential energy gradient is ΔPE G /Δ y, or, when written as a derivative, d PE G / dy :


d PE G d y = m a G =- F G

We interpret this equation as saying that a gradient in gravitational potential energy gives rise to the gravitational force, which moves an object down the potential energy gradient, from some height toward ground level. This example provides an important and general insight: a gradient of potential energy gives rise to a force that will tend to move material down the potential energy gradient.

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