Aerobic Respiration

The pyruvate produced in glycolysis undergoes further breakdown through a process called aerobic respiration in most organisms. This process requires oxygen and yields much more energy than glycolysis. Aerobic respiration is divided into two processes: the Krebs cycle, and the Electron Transport Chain, which produces ATP through chemiosmotic phosphorylation. The energy conversion is as follows:

C₆H₁₂O₆ + 6O_<2 -> 6CO₂ + 6H₂O + energy (ATP)

Krebs Cycle

The pyruvate molecules produced during glycolysis contain a lot of energy in the bonds between their molecules. In order to use that energy, the cell must convert it into the form of ATP. To do so, pyruvate molecules are processed through the Kreb Cycle, also known as the citric acid cycle.

1. Prior to entering the Krebs Cycle, pyruvate must be converted into acetyl CoA (pronounced: acetyl coenzyme A). This is achieved by removing a CO₂ molecule from pyruvate and then removing an electron to reduce an NAD⁺ into NADH. An enzyme called coenzyme A is combined with the remaining acetyl to make acetyl CoA which is then fed into the Krebs Cycle. The steps in the Krebs Cycle are summarized below:

2. Citrate is formed when the acetyl group from acetyl CoA combines with oxaloacetate from the previous Krebs cycle..

3. Citrate is converted into its isomer isocitrate..

4. Isocitrate is oxidized to form the 5-carbon α-ketoglutarate. This step releases one molecule of CO₂ and reduces NAD⁺ to NADH₂⁺.

5. The α-ketoglutarate is oxidized to succinyl CoA, yielding CO₂ and NADH₂⁺.

6. Succinyl CoA releases coenzyme A and phosphorylates ADP into ATP.

7. Succinate is oxidized to fumarate, converting FAD to FADH₂.

8. Fumarate is hydrolized to form malate.

9. Malate is oxidized to oxaloacetate, reducing NAD⁺ to NADH₂⁺.

We are now back at the beginning of the Krebs Cycle. Because glycolysis produces two pyruvate molecules from one glucose, each glucose is processes through the kreb cycle twice. For each molecule of glucose, six NADH₂⁺, two FADH₂, and two ATP.

Electron Transport Chain

What happens to the NADH₂⁺ and FADH₂ produced during the Krebs cycle? The molecules have been reduced, receiving high energy electrons from the pyruvic acid molecules that were dismantled in the Krebs Cycle. Therefore, they represent energy available to do work. These carrier molecules transport the high energy electrons and their accompanying hydrogen protons from the Krebs Cycle to the electron transport chain in the inner mitochondrial membrane.

In a number of steps utilizing enzymes on the membrane, NADH₂⁺ is oxidized to NAD⁺, and FADH₂ to FAD. The high energy electrons are transferred to ubiquinone (Q) and cytochrome c molecules, the electron carriers within the membrane. The electrons are then passed from molecule to molecule in the inner membrane of the mitochondron, losing some of their energy at each step. The final transfer involves the combining of electrons and H₂ atoms with oxygen to form water. The molecules that take part in the transport of these electrons are referred to as the electron transport chain.

The process can be summarized as follows: the electrons that are delivered to the electron transport system provide energy to "pump" hydrogen protons across the inner mitochondrial membrane to the outer compartment. This high concentration of hydrogen protons produces a free energy potential that can do work. That is, the hydrogen protons tend to move down the concentration gradient from the outer compartment to the inner compartment.

However, the only path that the protons have is through enzyme complexes within the inner membrane. The protons therefore pass through the channel lined with enzymes. The free energy of the hydrogen protons is used to form ATP by phosphorylation, bonding phosphate to ADP in an enzymatically-mediated reaction. Since an electrochemical osmotic gradient supplies the energy, the entire process is referred to as chemiosmotic phosphorylation.

Once the electrons (originally from the Krebs Cycle) have yielded their energy, they combine with oxygen to form water. If the oxygen supply is cut off, the electrons and hydrogen protons cease to flow through the electron transport system. If this happens, the proton concentration gradient will not be sufficient to power the synthesis of ATP. This is why we, and other species, are not able to survive for long without oxygen!