"Student-Generated Analogies: Another Way of Knowing?"
by: Kim Pittman
The conversion from ADP to ATP is similar to recharging a rechargeable battery. You can use a rechargeable battery in your "boom box" over and over, as long as it is recharged after losing power each time. Your cells use the ADP "battery" over and over again. ADP is "recharged" in the mitochondria by replacing the third phosphate, the one removed as energy was used in the cytoplasm.
Inside each mitochondrion is an energy-releasing pathway known as the electron transport system (ETS). The ETS process is like a series of small waterfalls on a creek. However, instead of water flowing down the creek and waterfalls, electrons move from one specialized molecule to another. Each time electrons move to the next molecule in the ETS, energy is released.
Some of the "waterfalls" between ETS molecules on this electron "creek" are too short to provide enough energy to add a phosphate group to a dead ADP battery. Three of the "waterfalls" in the pathway are the correct height. At each of these steps in the ETS process, the energy yield is the amount required to recharge one ADP. A single ADP is recharged at each of these three spots on the pathway. See Figure 1.
Figure 1. The Electron Transport System (ETS). The cyt represents a type of molecule known as cytochrome. Electrons enter the ETS at flavin mononucleotide (FMN) or at Coenzyme Q (CoQ). As the electrons flow "down" the pathway, they lose energy. The steps indicated by ADP-->ATP have enough energy to charge-up one discharged ADP "battery" each.
From: Making Connections Integrating the Science of The Human Machine Grossmont Union High School District 1999.