The pyruvate and NADH output from glycolysis is moved into the mitochondria and undergo further processing to produce more ATP. The processes that take place inside the mitochondria are the Kreb Cycle and the Electron Transport Chain. Recall that these are both aerobic processes, oxygen is an essential ingredient for them to proceed. If there is no oxygen available the processes do not begin.
Depending on the type of cell and its energy requirements, a cell may have tens, hundreds, or thousands of mitochondria continually working, helping to convert glucose to ATP. (The diagram shows a single mitochondria for illustration purposes only.)
The pyruvate enters the mitochondria and is first converted to Acetyl CoA and then processed in the Kreb Cycle where it is converted to more NADH. This NADH and the NADH produced during glycolysis are then further processed in the Electron Transport Chain. It is here that the greatest amount of ATP is produced. An additional 34 units of ATP are produced in the mitochondria from the pyruvate and NADH that was generated in glycolysis.
The oxygen is essential because it is a receptor for the hydrogen ions, H+, that are removed from the NADH. Each atom of oxygen receives 2 hydrogen ions to form H2O, or water. (This is fascinating.) The cell takes oxygen gas that we breathe in from the air, adds hydrogen, and converts the oxygen to water. If oxygen is not present the hydrogen ions from NADH do not have a receptor to attach to. This is not desirable so the process halts until oxygen is present.
If the process halts, the NADH is unable to return to NAD+ and is not available for use in other glycolysis processes. Glycolysis would become impeded as NAD+ availablity is reduced. It is important that NAD+ is recycled in this process because there is a fixed amount of this ingredient in each cell. As we will soon see, there are mechanisms in the cell that are designed to prevent this from happening.
A by-product of the Kreb Cycle is Carbon dioxide gas, CO2. Unless, oxygen is available at the mitochondria the cell does not produce CO2 in the Kreb Cycle.
The next page shows greater detail in the happenings inside the mitochondria. It is not necessary that you understand the smallest detail. The key concept to grasp is the important role that oxygen plays in the process and that oxygen is essential for the process to start.
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