In photosynthesis, the electron that travels through the electron transport chain in the thylakoid membrane comes from in cellular respiration, a molecule of supplies the electron that travels through the electron transport chain.

In the given question, the first blank can be filled with water, and the second blank can be filled with NADH.

Explanation:

In the first step of photosynthesis, the first electron is provided by water, that is, due to the splitting of water. In the ultimate step of cellular respiration, that is, oxidative phosphorylation, the electrons are supplied by the molecules of NADH. In the process, the pumping of hydrogen ions takes place into the inner-membrane space of the mitochondria, and this conduction of hydrogen ions assists in the production of ATP.