Question

select the correct answer from each drop - down menu.
compare the electron transport chain in photosynthesis with the one in cellular respiration by identifying sources of the high - energy electrons for each.
in photosynthesis, electrons that travel through the electron transport chain in the thylakoid membrane come from
in cellular respiration, molecules of supply some of the electrons that travel through the electron transport chain.

Explanation:

For the first drop - down (photosynthesis electron source): In photosynthesis, during the light - dependent reactions, the electrons for the electron transport chain in the thylakoid membrane come from water ($\ce{H_2O}$). When water is split, it releases electrons, protons, and oxygen.

For the second drop - down (cellular respiration electron source): In cellular respiration, specifically in the process of oxidative phosphorylation, molecules of NADH (and also FADH₂, but NADH is a major source) supply electrons to the electron transport chain. NADH is produced during glycolysis, the Krebs cycle, and other preparatory steps. These molecules carry high - energy electrons that are donated to the electron transport chain to generate a proton gradient for ATP synthesis.

Answer:

First drop - down: $\ce{H_2O}$ (water)
Second drop - down: NADH (or FADH₂, with NADH being a primary source)