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During chemiosmosis, the proton gradient created across a membrane is utilized to synthesize ATP from ADP and inorganic phosphate. This process occurs in the mitochondria during cellular respiration, specifically in the inner mitochondrial membrane, where protons are pumped from the mitochondrial matrix into the intermembrane space, establishing a concentration gradient. As protons flow back into the matrix through ATP synthase, the enzyme harnesses their kinetic energy to convert ADP and inorganic phosphate into ATP, making ATP the direct product of this process.

The other substances mentioned do not directly result from the proton gradient created during chemiosmosis. NADH and FADH2 are electron carriers that are generated in earlier stages of cellular respiration, such as glycolysis and the Krebs cycle. CO2 is a by-product of the Krebs cycle and is not involved in the chemiosmotic process. Hence, ATP is the only substance generated as a direct result of the proton gradient during chemiosmosis.