<P> A common feature of all electron transport chains is the presence of a proton pump to create a transmembrane proton gradient . Bacterial electron transport chains may contain as many as three proton pumps, like mitochondria, or they may contain only one or two . They always contain at least one proton pump . </P> <P> In the present day biosphere, the most common electron donors are organic molecules . Organisms that use organic molecules as an energy source are called organotrophs . Organotrophs (animals, fungi, protists) and phototrophs (plants and algae) constitute the vast majority of all familiar life forms . </P> <P> Some prokaryotes can use inorganic matter as an energy source . Such an organism is called a lithotroph ("rock - eater"). Inorganic electron donors include hydrogen, carbon monoxide, ammonia, nitrite, sulfur, sulfide, manganese oxide, and ferrous iron . Lithotrophs have been found growing in rock formations thousands of meters below the surface of Earth . Because of their volume of distribution, lithotrophs may actually outnumber organotrophs and phototrophs in our biosphere . </P> <P> The use of inorganic electron donors as an energy source is of particular interest in the study of evolution . This type of metabolism must logically have preceded the use of organic molecules as an energy source . </P>

Where does nadph build up during electron transport