Question

Which is used as a source of ATP during Krebs cycle?

Answer 1

The Krebs cycle, also known as the citric acid cycle or TCA cycle, is a critical step in cellular respiration that takes place in the mitochondria. It plays an essential role in producing ATP, the primary energy source used by cells for various functions.
Role of NADH in ATP Production:
During the Krebs cycle, the main function is to oxidize acetyl-CoA, a derivative of glucose, fatty acids, or proteins. As the cycle progresses, energy from these substrates is captured in the form of high-energy molecules, namely NADH and FADH₂.
Steps Leading to NADH Formation:
1. Oxidation of Isocitrate: Isocitrate, formed from citrate, is oxidized by NAD⁺, reducing it to NADH while converting isocitrate into α-ketoglutarate. This step also releases CO₂.
2. Oxidation of α-Ketoglutarate: In the next step, α-ketoglutarate is further oxidized, producing another NADH molecule while forming succinyl-CoA. Another CO₂ is released.
3. Succinate Dehydrogenation: Although not directly part of the Krebs cycle, the electron carrier FADH₂ is generated in this part of the process, but NADH remains the major player.
At each of these points, NAD⁺ is reduced to NADH, capturing energy that will later be used for ATP production.
How NADH Contributes to ATP Production:
The NADH molecules formed during the Krebs cycle are crucial for the production of ATP. These NADH molecules do not directly generate ATP in the Krebs cycle itself. Instead, they transfer their high-energy electrons to the electron transport chain (ETC) located in the inner mitochondrial membrane. The ETC consists of a series of protein complexes that pass the electrons through their structures, releasing energy in small, controlled steps.
The Electron Transport Chain and Oxidative Phosphorylation:
Once NADH donates electrons to Complex I (NADH dehydrogenase) in the ETC, the energy released is used to pump protons (H⁺) across the inner mitochondrial membrane, creating a proton gradient. This proton gradient generates a proton-motive force (PMF), which powers the enzyme ATP synthase. ATP synthase allows protons to flow back into the mitochondrial matrix, and the energy released from this flow is used to convert ADP and inorganic phosphate (Pi) into ATP. This process is called oxidative phosphorylation and is where most ATP is produced.
Each NADH molecule can generate approximately 2.5 ATP molecules via this process, depending on the efficiency of the electron transport chain and ATP synthase. This makes NADH one of the most important molecules for ATP production in the cell.
Additional Notes:
While NADH is the primary electron donor in oxidative phosphorylation, FADH₂ also contributes to ATP production, but it donates electrons at a later stage in the electron transport chain, resulting in fewer ATP molecules generated per FADH₂ (around 1.5 ATP). Therefore, NADH is more efficient in ATP production compared to FADH₂.
Conclusion:
NADH is a critical source of ATP during cellular respiration. It captures energy from food molecules in the Krebs cycle and donates this energy to the electron transport chain, where it powers ATP production through oxidative phosphorylation. Without NADH, the cell would not be able to produce sufficient ATP for energy-demanding processes.