are ATP and NADH re-charged during the Krebs cycle?

Answer 1

Let's look at each of the two energy molecules separately in the context of cellular respiration.

First, ATP is made from ADP in Glycolysis, the Krebs cycle, and the Electron Transport Chain (ETC). The ultimate goal of cellular respiration is to create ATP to provide energy for cell processes. Once ATP is made in the ETC, it leaves cellular respiration as a process to power the cells other processes. So ADP/ATP are not recycled in the Krebs cycle. If you would like to go into a little more depth, technically ADP/ATP are not even involved in the Krebs cycle at all. There is another energy molecule that is just about energetically equivalent to ADP/ATP. It is GDP/GTP. So the Krebs cycle actually produces GTP. Teachers and scientists will often just teach that it is ATP for simplification since they are energetically equivalent. However, they are separate molecules, so there cannot be any recycling or "recharging" in the Krebs cycle when it comes to ADP/ATP.

Second, NADH is produced from NAD⁺ in both Glycolysis and the Krebs cycle in order to provide electrons for the ETC. Since the cell needs ATP as its source of energy, NADH is like an energy intermediate that passes its energy off to ATP (although indirectly). So the ETC uses NADH and turns it back into NAD⁺ in order to produce more ATP. The NAD⁺ produced in the ETC can then be recycled or "recharged" by traveling back to the Krebs cycle to be turned into NADH again.

So in summary, ATP is not "recharged" by the Krebs cycle, but NADH is "recharged" by the Krebs cycle.

I hope this was helpful! Please let me know if you need any further clarification.

Answer 2

Hi Konstantina! ADP is phosphorylated in the Krebs cycle, forming ATP and NAD⁺ is reduced to NADH⁺ three times per cycle.