Cellular respiration is the process by which living cells convert nutrients into usable energy, primarily adenosine triphosphate (ATP). Central to this energy production pathway is the Krebs cycle, also known as the citric acid cycle.
The Krebs Cycle’s Role in Energy Production
The Krebs cycle is a series of chemical reactions within the mitochondria of eukaryotic cells. For prokaryotic cells, which lack mitochondria, these reactions occur in the cytoplasm. Its main function involves the oxidation of acetyl-CoA, a molecule derived from carbohydrates, fats, and proteins. This oxidation process breaks down acetyl-CoA into carbon dioxide, releasing energy.
The cycle generates reducing equivalents crucial for the subsequent stages of cellular respiration. While carbon dioxide is a direct byproduct, the cycle’s larger contribution to energy production is indirect.
Direct Energy Generation
Within the Krebs cycle, a small amount of ATP is generated directly through substrate-level phosphorylation. For each turn of the cycle, one molecule of guanosine triphosphate (GTP) is produced. This GTP is functionally equivalent to ATP and converts into ATP through an enzyme-catalyzed reaction.
This direct production represents a minor fraction of the overall energy yield from the Krebs cycle. It provides an energy supply within the mitochondrial matrix where the cycle operates.
Indirect Energy Carriers
The majority of the Krebs cycle’s energy contribution comes from high-energy electron carriers: nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2). Each turn of the cycle produces three NADH and one FADH2. These molecules do not directly provide ATP but are crucial for later stages of energy production.
NADH and FADH2 carry high-energy electrons to the electron transport chain (ETC) on the inner mitochondrial membrane. In the ETC, electrons pass along a series of protein complexes, releasing energy to generate a proton gradient. This gradient drives the synthesis of a large amount of ATP through oxidative phosphorylation.
Calculating Total ATP Production
For each molecule of acetyl-CoA entering the Krebs cycle, one ATP is produced directly via GTP. The cycle also yields three NADH molecules and one FADH2 molecule. These electron carriers contribute to further ATP synthesis in the electron transport chain.
Each NADH molecule typically yields approximately 2.5 ATP, and each FADH2 molecule generally yields about 1.5 ATP. Therefore, from the indirect carriers generated by one turn of the Krebs cycle (3 NADH and 1 FADH2), approximately 7.5 ATP (3 × 2.5) and 1.5 ATP (1 × 1.5) are produced, respectively.
Summing the direct and indirect contributions, one turn of the Krebs cycle yields about 10 ATP (1 direct ATP + 7.5 ATP from NADH + 1.5 ATP from FADH2). Since one glucose molecule is processed into two acetyl-CoA molecules, the cycle runs twice per glucose molecule. Consequently, the total ATP generated from the Krebs cycle’s activity per glucose molecule is approximately 20 ATP. The exact ATP yield can vary slightly depending on cellular conditions and specific shuttle systems.