This would be best explained by examining the cellular structure of the electron transport chain and its function in multi celled organisms. By the use of specific enzymes, it regulates the use of certain materials, such as glucose, while blocking the other materials, such as waste , until it is ready to move it across the cell wall. An illustration of this would be to think of it as a shape and peg system where enzymes of a certain shape are allowed in, used and then the second shape aka enzyme is used to transport the waste out of the cell, with the electron transport chain acting as a "train conductor" of sorts, providing the early step catalyst for energy distribution and waste removal.
