Energy relations number of ATP molecules generated

Cellular respiration is the process of breaking down glucose to generate ATP (adenosine triphosphate), the primary energy currency of the cell. The complete breakdown of one glucose molecule yields a significant amount of ATP through a series of interconnected pathways: glycolysis, oxidative decarboxylation, the Krebs cycle, and the electron transport system (ETS). The exact number of ATP molecules generated is a subject of discussion, but the theoretical maximum can be calculated.

1.0ATP: The Energy Currency of the Cell

ATP stands for Adenosine Triphosphate, a molecule composed of:

• Adenine (a nitrogenous base)
• Ribose (a five-carbon sugar)
• Three phosphate groups

When ATP breaks down into ADP (Adenosine Diphosphate) and Pi (inorganic phosphate), it releases energy:

ATP → ADP + Pi + Energy

Each molecule of ATP releases approximately 7.3 kcal/mol of energy.

ATP Production from Glycolysis

Glycolysis is the initial breakdown of glucose in the cytoplasm. It doesn't require oxygen.

• Substrate-level phosphorylation: 4 ATP molecules are produced, but 2 are consumed in the initial steps, leading to a net gain of 2 ATP.
• NADH Production: 2 molecules of NADH are generated. Each NADH can produce 3 ATPs later in the ETS. Still, since glycolysis occurs in the cytoplasm and ETS is in the mitochondria, the NADH produced by glycolysis must be transported into the mitochondria. This transport process consumes energy. Depending on the shuttle system used (malate-aspartate or glycerol phosphate), yields vary. The malate-aspartate shuttle yields 3 ATP per NADH, while the glycerol phosphate shuttle yields 2 ATP per NADH.

Summary for Glycolysis:

• Net ATP: 2
• NADH: 2 (Yielding 4-6 ATP in ETS)

ATP Production from Pyruvate to Acetyl CoA

This is a link reaction between glycolysis and the Krebs cycle. For each glucose molecule, two pyruvate molecules are formed.

• Oxidative Decarboxylation: Each pyruvate molecule is converted into an acetyl CoA molecule. This step produces one molecule of NADH.
• Since there are two pyruvate molecules per glucose, this stage yields 2 NADH in total.

Summary for Oxidative Decarboxylation:

• NADH: 2 (Yielding 6 ATP in ETS)

ATP Production from the Krebs Cycle

The Krebs cycle (or TCA cycle) occurs in the mitochondrial matrix. For each glucose molecule, two acetyl-CoA molecules enter the cycle.

• ATP/GTP Production: The cycle directly produces one GTP molecule per turn, which is equivalent to one ATP. For two turns of the cycle, this is 2 ATP.
• NADH and FADH2​ Production: Each turn of the cycle produces 3 NADH and 1 FADH₂
• For two turns of the cycle (from one glucose), this means a total of 6 NADH and 2 FADH₂​ are generated.

Summary for Krebs Cycle (per glucose):

• ATP: 2
• NADH: 6 (Yielding 18 ATP in ETS)
• FADH2​: 2 (Yielding 4 ATP in ETS)

The Electron Transport System (ETS)

The ETS is where the majority of ATP is produced through oxidative phosphorylation. The NADH and FADH_2​molecules from the previous stages are used here.

• Each NADH molecule provides energy to generate approximately 3 ATP.
• Each FADH_2​ molecule provides energy to generate approximately 2 ATP.

Total ATP Yield Calculation (Theoretical Maximum):

1. From Glycolysis: 2 ATP (net) + (2 NADH x 3 ATP/NADH) = 8 ATP
2. • Note: The NADH from glycolysis may yield less depending on the shuttle, but here we consider the theoretical max.
2. From Oxidative Decarboxylation: 2 NADH x 3 ATP/NADH = 6 ATP
3. From Krebs Cycle: 2 ATP + (6 NADH x 3 ATP/NADH) + (2 FADH_2​ x 2 ATP/FADH_2​) = 2 + 18 + 4 = 24 ATP

Total ATP from one glucose molecule = 8 (from glycolysis) + 6 (from link reaction) + 24 (from Krebs cycle) = 38 ATP

Note: A more accurate, but less simplified, count is 36 ATP, acknowledging the energy cost of transporting NADH into the mitochondria.