Which of the following best describes how ATP is generated by ATP synthase?

ATP is generated by ATP synthase primarily through the movement of hydrogen ions (H+) down their concentration gradient. This process occurs during cellular respiration and photosynthesis, where a proton gradient is established across a membrane. In cellular respiration, this gradient is created by the electron transport chain, which actively transports hydrogen ions from the mitochondrial matrix to the intermembrane space. In photosynthesis, a similar gradient is established in the thylakoid membranes of chloroplasts.

As hydrogen ions flow back into the mitochondrial matrix or the stroma of chloroplasts, ATP synthase utilizes this kinetic energy to convert adenosine diphosphate (ADP) and inorganic phosphate (Pi) into adenosine triphosphate (ATP). This process is known as chemiosmosis and is crucial for ATP production in living organisms, making it the primary means of ATP generation in oxidative phosphorylation and photophosphorylation.

Other options relate to different processes: breaking down glucose molecules pertains to glycolysis and the Krebs cycle, direct utilization of sunlight refers to photolysis and light reactions in photosynthesis without ATP synthase involvement, and hydrolysis of ATP is the reverse process, where ATP is broken down to release energy.