The energy lost in the electron chain transfers is used to produce ATP (adenosine triphosphate), the molecule universally used by organisms as a quick energy source. To do so, proton pumps in the thylakoid membrane are activated by the high-energy electrons. They pump protons (H + ) into the intermembrane space creating a (H + ) gradient across the thylakoid membrane. As protons diffuse back down the gradient into the stroma (the space in the chloroplast), they pass through pores lined with ATPsynthase, the enzyme that catalyzes the formation of ATP from ADP (adenosine diphosphate). In the passage, a phosphate group (P i ) is added to ADP forming a terminal high energy bond and ATP. The general term for this process is photophosphorylation ( photo meaning light energy; phosphorylation indicating a phosphate group is added to an organic molecule).
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