4.14: The second step of cellular respiration: the Krebs cycle extracts energy from sugar.

Cells could stop extracting energy when glycolysis ends, but they rarely do so because that would be like leaving most of your meal on your plate. In glycolysis, only a small fraction of the energy stored in sugar molecules is recovered and converted to ATP and NADH. Cells get much more of an “energy bang” for their “food buck” in the steps following glycolysis, which occur in the mitochondria. This is why mitochondria are considered ATP “factories.” In the mitochondria, the molecules produced from the breakdown of glucose during glycolysis are broken down further, during two steps that are dramatically more efficient at capturing energy: the Krebs cycle (the subject of this section) and the electron transport chain (the subject of the next section). In breaking down the products of glycolysis, the Krebs cycle produces some additional molecules of ATP and, more importantly, captures a huge amount of chemical energy by producing high-energy electron carriers.

Before the Krebs cycle can begin, however, the end products of glycolysis—two molecules of pyruvate for every molecule of glucose used—must be modified. First, the pyruvate molecules move from the cytoplasm into the mitochondria, where they undergo three quick modifications that prepare them to be broken down in the Krebs cycle (FIGURE 4-31).

Modification 1. Each pyruvate molecule passes a pair of its high-energy electrons (and a proton) to the electron-carrier molecule NAD⁺, building two molecules of NADH.

Modification 2. Next, a carbon atom and two oxygen atoms are removed from each pyruvate molecule and released as carbon dioxide. The CO₂ molecules diffuse out of the cell and, eventually, out of the organism. In humans, for example, these CO₂ molecules pass into the bloodstream and are transported to the lungs, from which they are eventually exhaled.

Modification 3. In the final step in the preparation for the Krebs cycle, a giant compound known as coenzyme A attaches itself to the remains of each pyruvate molecule, producing two molecules called acetyl-CoA. Each acetyl-CoA molecule is now ready to enter the Krebs cycle.

There are eight separate steps in the Krebs cycle, but our emphasis is on its three general outcomes (FIGURE 4-32).

Outcome 1. A new molecule is formed. Acetyl-CoA adds its two-carbon acetyl group to a molecule of the starting material of the Krebs cycle, a four-carbon chemical called oxaloacetate, creating a six-carbon molecule.

Outcome 2. High-energy electron carriers (NADH) are made and carbon dioxide is exhaled. The six-carbon molecule then gives electrons to NAD⁺ to make the high-energy electron carrier NADH. (Don’t forget that the main purpose of the Krebs cycle is the capture of energy.) The six-carbon molecule also releases two carbon atoms along with four oxygen atoms to form two carbon dioxide molecules. In mammals (including humans), this CO₂ is carried by the bloodstream to the lungs, from where it is exhaled into the atmosphere.

Outcome 3. The starting material of the Krebs cycle is re-formed, ATP is generated, and more high-energy electron carriers are formed. After the CO₂ is released, the four-carbon molecule that remains from the original pyruvate-oxaloacetate molecule formed in Outcome 1 is modified and rearranged to once again form oxaloacetate, the starting material of the Krebs cycle. In the process of this reorganization, one ATP molecule is generated, and more electrons are passed to NAD⁺ and a molecule called FAD to form NADH and FADH₂, both of which are high-energy electron carriers. The formation of these high-energy electron carriers increases the energy yield of the Krebs cycle. One oxaloacetate is re-formed, and the cycle is ready to break down the second molecule of acetyl-CoA. Two turns of the cycle are necessary to completely dismantle our original molecule of glucose.

Now that we have seen the Krebs cycle in its entirety, let’s trace the path of the original six carbons in the original glucose molecule. In a sense, the carbon atoms that were first plucked from the atmosphere to make sugar during photosynthesis have been exhaled back into the atmosphere as six molecules of carbon dioxide.

• 1. Glycolysis: the six-carbon starting point. Glucose is broken down into two molecules of pyruvate. No carbons are removed.
• 2. Preparation for the Krebs cycle: two carbons are released. Two pyruvate molecules are modified to enter the Krebs cycle, and they each lose a carbon atom in the form of two molecules of carbon dioxide.
• 3. Krebs cycle: the last four carbons are released. A total of four carbon atoms enter the Krebs cycle in the form of two molecules of acetyl-CoA, entering one at a time. For each turn of the Krebs cycle, two molecules of carbon dioxide are released. So the two final carbons are released into the atmosphere during the second turn of the wheel. Poof! The six carbon atoms that were originally present in our single molecule of glucose are no longer present.

So we’ve come full circle. In photosynthesis, carbon atoms from the air were used to build sugar molecules, which had energy stored in the bonds between their carbon, hydrogen, and oxygen atoms. In cellular respiration, the energy previously stored in the bonds of the sugar is converted to molecules of ATP, NADH, and FADH₂. Carbon atoms from sugar are exhaled back into the air as CO₂, and water is produced.

What happens to the high-energy electron carriers, NADH and FADH₂? They eventually give up their high-energy electrons to the final stage of cellular respiration, the electron transport chain. The energy released as those electrons pass through the transport chain is captured in the bonds of more ATP molecules. We explore that process in the next section.

As we explore the important roles of the Krebs cycle and electron transport chain in generating usable energy for organisms, it’s important to note that mitochondrial malfunctions have serious consequences for health. More than a hundred genetic mitochondrial disorders have been identified, all of which can lead to energy shortage, including muscle weakness, fatigue, and muscle pain. And, as noted in Section 3-15, it appears that many cases of extreme fatigue or cramps that occur after only slight exertion may be related to inherited mutations in the mitochondrial DNA. Damage to our cellular powerhouses, not surprisingly, can create a personal energy crisis.