Figure 1.  The Chemiosmotic Paradigm.  The two basic energy sources in living systems--respiration and light--lead to the generation of a proton electrochemical gradient () across the appropriate energy-transducing membrane which is the immediate driving force for a wide range of seemingly unrelated processes like oxidative or photophosphorylation, active transport, transhydrogenation of NADP by NADH and many other cellular phenomena.  It is also important to note that the arrows shown point in both directions.  Thus, ATP hydrolysis, movement of accumulated substrates down a concentration gradient or reduction of NAD by NADPH can lead to generation of

Since protons (i.e., hydrogen ions or H+) are being pumped vectorally by either the respiratory chain or the photosynthetic electron transfer chain, positive charge is being pumped, and an electrical potential (DY) is generated, the polarity of which depends upon which direction the protons are pumped (when protons are pumped out, DY is interior negative; when pumped in, DY is interior positive).  In addition to DY, a chemical concentration gradient of protons (DpH) may also be established, the polarity of which is also dependent upon the vectorality of the proton pump (when protons are pumped out, DpH is interior alkaline; when pumped in, DpH is interior acid).  Don’t worry about the constants (R, T and F) which are there simply to put both components into the same units [in this case, millivolts (mV)].  The important take-home message is that electrochemical proton gradient means just what it says; there are two components, DY and DpH, that are important energetically.