Energy and Metabolism

ID #2197

Why exactly are CO2 molecules released in pyruvate oxidation/Krebs Cycle? I know he said something about C-H bonds being high energy, but how does that have to do with CO2 being released?

Remember that the ultimate goal of cellular respiration is to extract the most amount of energy from our starting material, a six-carbon glucose molecule, and convert that energy to ATP. This means that we have to break down our 6-carbon molecule into 1-carbon molecules, and the most stable form (i.e. unreactive form) of carbon is carbon dioxide (CO2). Also recall that at the end of glycolysis, the cell has only managed to convert the 6-C glucose to two molecules of 3-C pyruvate. The cell has not yet maximized the amount of energy that it can extract from a molecule of glucose, and this is where pyruvate oxidation and the Krebs cycle come in. At this point, it is important that you keep track of where your carbon molecules are going, and also keep in mind that there are two pyruvate molecules per molecule of glucose entering Phase II.

In pyruvate oxidation, the 3-C pyruvate reacts with coenzyme A (CoA). As pyruvate is processed, one of the carbons in pyruvate is oxidized to CO2 and NAD+ is reduced to NADH. The remaining 2-C acetyl unit is transferred to CoA to form Acetyl CoA.

To follow energy generation in the Krebs Cycle, it is important to keep in mind what is produced along the way. The Krebs cycle (at a couple of points) generates CO2 while reducing NAD+ to NADH. At the end of the cycle, All C-H bonds have been converted to C-O bonds. The NADH molecules, meanwhile, are then able to function as electron donors in the electron transport chain (Phase III of aerobic respiration), allowing more efficient generation of energy.

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