Pressure dependence gravitational potential

Figure 6-5 indicates that the C>2-evolution step and the electron flow mediated by the plastoquinones and the Cyt b(f complex lead to an accumulation of H+ in the lumen of a thylakoid in the light. This causes the internal H+ concentration, c, or activity, to increase. These steps depend on the light-driven electron flow, which leads to electron movement outward across the thylakoid in each of the two photosystems (see Fig. 5-19). Such movements of electrons out and protons in can increase the electrical potential inside the thylakoid (E ) relative to that outside ( °), allowing an electrical potential difference to develop across a thylakoid membrane. By the definition of chemical potential (fij = jx + RT In cij 4- ZjFE Eq. 2.4 with the pressure and gravitational terms omitted see Chapter 3, Section 3.1), the difference in chemical potential of H+ across a membrane is...

The forms for the gravitational contribution (rrijgh) and the electrical one (ZjFE) can be easily understood. We showed in Chapter 3 (Section 3.2 A) that RT n cij is the correct form for the concentration term in jij. The reasons for the forms of the pressure terms in a liquid (V)/>) and in a gas [RT In (Pj/P j] are not so obvious. Therefore, we will examine the pressure dependence of the chemical potential of species / in some detail. 

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