2) Why can it be said that each glucose molecule runs the Krebs cycle twice?

To understand why it can be said that each glucose molecule runs the Krebs cycle twice, let's first review what the Krebs cycle is. The Krebs cycle, also known as the citric acid cycle or the tricarboxylic acid cycle, is a series of chemical reactions that occur in the mitochondria and is an important part of cellular respiration.

During the Krebs cycle, a series of reactions break down glucose and produce energy-rich molecules such as ATP, NADH, and FADH2, which play a crucial role in generating energy for the cell. The Krebs cycle is a cyclic pathway, meaning that it begins and ends with the same molecule: oxaloacetate.

Now, when we consider glucose, it is a six-carbon molecule, and it needs to be completely broken down to release its stored energy. To break down a glucose molecule, a series of reactions occur where the glucose is converted into two molecules of pyruvate through a process known as glycolysis. Each pyruvate molecule is then further processed through another cycle, called the pyruvate dehydrogenase complex, which produces acetyl-CoA.

Now, here comes the link to the Krebs cycle. Each acetyl-CoA molecule enters the Krebs cycle, combining with oxaloacetate to form citric acid, which then undergoes a series of reactions to generate energy-rich molecules like NADH and FADH2. Eventually, the cycle produces oxaloacetate again, ready to begin the next round.

Since there are two pyruvate molecules derived from the breakdown of each glucose molecule, it means that there are two acetyl-CoA molecules that enter the Krebs cycle. As a result, it can be said that each glucose molecule runs the Krebs cycle twice because both pyruvate molecules eventually undergo the Krebs cycle. Therefore, we can think of it as each glucose molecule going through two rounds of the Krebs cycle to fully extract the energy stored within it.

In summary, each glucose molecule undergoes glycolysis to produce two pyruvate molecules, and both these pyruvate molecules proceed through the pyruvate dehydrogenase complex to generate two acetyl-CoA molecules. These two acetyl-CoA molecules enter the Krebs cycle separately, leading to the statement that each glucose molecule runs the Krebs cycle twice.