Answer:
Creating a high concentration
Towards the low concentration
Explanation:
First, he should measure how long it takes for the liquid to become clear if X and Y are mixed together. Then, he should measure how long it takes if he also adds substance A to X and Y. He will find out if substance A is a catalyst.
So other scientists can try the exact same experiment and see if they both got the same results.
I think the correct term to fill in the blanks would be glycerol phosphate electron shuttle. This shuttle is responsible of transporting agents that are reducing into the inner membrane of the mitochondria. Since NADH cannot enter, it is reduced so that the electrons could go in to the transport chain.