Question

the energy stored in the
gradient is used with atp synthase to build atp.

Explanation:

In cellular respiration (especially oxidative phosphorylation) and photosynthesis, the energy stored in the proton ($\ce{H+}$) gradient across a membrane (like the inner mitochondrial membrane or thylakoid membrane) is utilized by ATP synthase to phosphorylate ADP and form ATP. This is a key concept in Biology (a subfield of Natural Science) related to energy conversion in cells.

Answer:

proton (or $\ce{H+}$)