When a specimen is to be observed under the light microscope, scientists usually stain the specimens using different type of chemical stains depending on the type of specimen to be observed. Staining of specimen make the structures that are to be observed in the specimen very visible under the microscope, this make it easy for scientists to observe their structures.
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.
