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The proton motive force arises from a combination of the concentration gradient of protons and a voltage gradient across the membrane (the protons are positively charged, so an imbalance of protons means that the side of the membrane with more protons has a positive charge relative to the side with fewer protons).
This gradient is composed of both the pH gradient and the electrical gradient. The pH gradient is a result of the H+ ion concentration difference. Together the electrochemical gradient of protons is both a concentration and charge difference and is often called the proton motive force (PMF). In mitochondria the PMF is almost entirely made up of the electrical component but in chloroplasts the PMF is made up mostly of the pH gradient.