7.4 Electrons “fall” from food to oxygen during cellular respiration

Objectives Relate breathing and cellular respiration. Summarize the cellular respiration equation. Tell how "falling" electrons are a source of energy. Explain the role of electron transport chains

Key Terms aerobic electron transport chain

Relationship of Cellular Respiration to Breathing Cellular respiration is an aerobic process, meaning that it requires oxygen. You have probably heard the word respiration used to describe breathing. Although breathing for a whole organism is not the same as cellular respiration, the two processes are relatedaerobic

During cellular respiration, a cell exchanges two gases with its surroundings. The cell takes in oxygen and releases carbon dioxide.

Overall Equation for Cellular Respiration Glucose is a common fuel for cellular respiration

Cellular respiration's main function is to generate ATP for cellular work. In fact, the process can produce up to 38 ATP molecules for each glucose molecule consumed. Notice that cellular respiration also transfers hydrogen and carbon atoms from glucose to oxygen atoms, thus forming water and carbon dioxide

"Falling" Electrons as an Energy Source Why does the process of cellular respiration release energy? For an analogy, recall the water slide. At the top, your potential energy is high. As you are pulled down the slide by the force of gravity, the potential energy is converted to kinetic energy. Similarly, an atom's positively charged nucleus exerts an electrical "pull" on negatively charged electrons. When an electron "falls" toward the nucleus, potential energy is released.

Oxygen attracts electrons very strongly, similar to how gravity pulls objects downhill. In contrast to oxygen, carbon and hydrogen atoms exert much less pull on electrons. A sugar molecule has several carbon-hydrogen bonds. During cellular respiration, the carbon and hydrogen atoms change partners and bond with oxygen atoms instead. The carbon- hydrogen bonds are replaced by carbon- oxygen and hydrogen-oxygen bonds.

As the electrons of these bonds "fall" toward oxygen, energy is released. In your cells, some of the released energy is used to generate ATP molecules instead of being converted to heat and light.

Cellular respiration involves breaking down glucose in several steps. Oxygen only enters as an electron acceptor in the final electron transfer. During the breakdown, molecules called electron carriers accept many of the high-energy electrons from the glucose molecule. The electron carriers pass the electrons on to other carriers in a series of transfers called an electron transport chainelectron transport chain

Each carrier holds the electrons more strongly than the carrier before it. At the end of the chain, oxygen—the electron grabber—pulls electrons from the final carrier molecule and joins them with hydrogen ions, forming water.

As electrons undergo each transfer in the chain, they release a little energy. The cell has a mechanism that traps this released energy and uses it to make ATP