Which stage of cellular respiration produces the most ATP? Explain. (pages \(58-59\) )

Glycolysis is the first stage of cellular respiration that takes place in the cytoplasm of the cell. It involves the breakdown of glucose (a 6-carbon sugar) into two molecules of pyruvate (a 3-carbon compound). In this step, 4 ATP molecules are produced but 2 ATP molecules are consumed, resulting in a net gain of 2 ATP molecules.

The citric acid cycle, also known as the Krebs cycle or TCA cycle, takes place in the matrix of the mitochondria. In this stage, each pyruvate from glycolysis is converted into Acetyl-CoA, which enters the cycle and generates energy in the form of ATP, NADH, and FADH2. Each turn of the cycle (for each Acetyl-CoA) produces 1 ATP molecule. Since there are two pyruvate molecules produced from one glucose molecule, the citric acid cycle will produce a total of 2 ATP molecules per glucose molecule.

Oxidative phosphorylation takes place in the inner mitochondrial membrane and includes the electron transport chain (ETC) and chemiosmosis. In this stage, the high-energy electrons carried by NADH and FADH2 from the previous stages (glycolysis and citric acid cycle) are passed along the electron transport chain, which creates a flow of protons (H+) across the membrane, establishing an electrochemical gradient called the proton-motive force. This force is used by ATP synthase to generate ATP as the protons flow back across the membrane. It is estimated that 1 NADH molecule produces about \(2.5\) ATP, and 1 FADH2 molecule produces about \(1.5\) ATP. For one glucose molecule, glycolysis produces \(2\) NADH, the citric acid cycle produces \(6\) NADH and \(2\) FADH2. Therefore, the oxidative phosphorylation stage generates about \(2\times2.5 + 6\times2.5 + 2\times1.5 = 25\) ATP molecules per glucose molecule.

Now that we've looked at the production of ATP in each stage, we can determine which stage produces the most ATP: 1. Glycolysis: \(2\) ATP 2. Citric Acid Cycle: \(2\) ATP 3. Oxidative Phosphorylation: \(25\) ATP Oxidative phosphorylation produces the most ATP (approximately \(25\) molecules) compared to the other stages.

The stage of cellular respiration that produces the most ATP is oxidative phosphorylation, with approximately \(25\) ATP molecules produced per glucose molecule. This is because the electron transport chain and chemiosmosis harness the energy of the high-energy electrons carried by NADH and FADH2 from the previous stages to create a proton-motive force that drives ATP synthesis.