Why do multicellular organisms possess a large number of cells rather than having large sized cells?

Large sized cells would be very inefficient because the surface area to volume ratio in those cells would be very low. This means that they wouldn't be able to interact with their environment efficiently with large cells.

The electron transport chain is located on the inner membrane of the mitochondria, as shown below. The electron transport chain contains a number of electron carriers. These carriers take the electrons from NADH and FADH2, pass them down the chain of complexes and electron carriers, and ultimately produce ATP.