4.15: The third step of cellular respiration: ATP is built in the electron transport chain.

How do we finally get a big payoff of usable energy from our glucose molecule? Glycolysis and the Krebs cycle produce a few molecules of ATP for each molecule of glucose broken down, but it is the energy held in the high-energy electron carriers NADH and FADH₂ that are formed in these processes that ultimately generates the largest amount of usable energy as ATP. In fact, almost 90% of the energy payoff from a molecule of glucose is harvested in the final step of cellular respiration, when the electrons from NADH and FADH₂ move along an electron transport chain. This process, like the Krebs cycle, takes place in the mitochondria.

In a manner similar to that seen in the chloroplast during photosynthesis, mitochondria convert kinetic energy (from electrons) into potential energy (a concentration gradient of protons). Two structural features of mitochondria are essential to their impressive ability to harness energy from molecules.

Feature 1. Mitochondria have a “bag-within-a-bag” structure that makes it possible for the regions inside and outside the “inner bag” to have different concentrations of molecules and makes it possible to harness the potential energy in the bonds of NADH and FADH₂ molecules to produce ATP (FIGURE 4-33).

Material inside the mitochondrion can lie in one of two spaces: (1) in the intermembrane space, which is outside the inner bag, or (2) in the mitochondrial matrix, which is inside the inner bag. With two distinct regions separated by a membrane, the mitochondrion can create higher concentrations of molecules in one area or the other, creating a concentration gradient. And because a concentration gradient is a form of potential energy—molecules move from the high-concentration area to the low-concentration area the way water rushes down a hill—once a gradient is created, the energy released as the gradient dissipates can be used to do work. In the electron transport chain, this energy is used to build the energy-rich molecule ATP.

Feature 2. The inner bag of the mitochondrion is studded with molecules, mostly electron carriers, which are sequentially arranged as a “chain.” This arrangement makes it possible for the molecules to hand off electrons in an orderly sequence.

Now let’s explore how these features of mitochondria make it possible to harness energy from high-energy electron carriers (FIGURE 4-34).

Step 1 of the electron transport chain begins with NADH and FADH₂ in the mitochondrial matrix (inside the inner bag) moving to the membrane. There, the high-energy electrons they carry are transferred to molecules embedded within the membrane. After they donate their electrons, the molecules that remain, NAD⁺ and FAD, are recycled back to the Krebs cycle.

The membrane-embedded molecules pass the electrons to the next carrier, which passes the electrons to the next, and so on. At each handoff, a bit of energy is released. Thus, as electrons move from one carrier to another through the electron transport chain, they lose energy at each handoff.

At the end of the chain (step 2 in Figure 4-34), the lower-energy electrons are handed off to oxygen, which then combines with free H⁺ ions in the mitochondrial fluid to form water.

As shown in step 3 of Figure 4-34, most of the energy released at each handoff from one electron carrier to another in the electron transport chain is used to pump protons (H⁺ ions) from the mitochondrial matrix across the membrane and into the intermembrane space. As more and more protons are pumped across the membrane and packed into the intermembrane space, a concentration gradient is created. This gradient represents a significant source of potential energy.

If this description seems familiar, it is. In chloroplasts, during photosynthesis, great numbers of protons are pumped from the stroma outside the thylakoid sacs to the inside of the thylakoids. We likened this potential energy to the potential energy of water in an elevated tower, which can be released with great force. Similarly, in step 4 of the mitochondrial electron transport chain, the protons pumped into the intermembrane space rush back into the mitochondrial matrix through channels in the inner mitochondrial membrane. And as the protons pass through, the force of their flow fuels the attachment of free-floating phosphate groups to ADP to produce ATP.

In the end, the number of ATP molecules generated from the complete dismantling of one molecule of glucose is about 36, most of which are produced with the energy harnessed from high-energy electron carriers as they pass their electrons down the electron transport chain (FIGURE 4-35).

Given the central role of the electron transport chain in the generation of usable energy from the breakdown of food molecules, any interference in its functioning has dire consequences. And in fact, murder by cyanide poisoning, an old tradition in detective stories, is just such an interference. When cyanide gets into the mitochondria, it binds to a molecule in the electron transport chain, preventing it from accepting electrons. This halts the transfer of electrons and the pumping of protons across the mitochondrial membrane. As a consequence, the production of ATP that would occur when protons rushed back across the membrane down their concentration gradient ceases. Halting the production of ATP removes a cell’s energy source, starving it very quickly. For this reason, cyanide poisoning can cause death within minutes.