Question

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

Short answer

The stage of cellular respiration that produces the most ATP is oxidative phosphorylation, which generates approximately \(25\) ATP molecules per glucose molecule. This occurs as the electron transport chain and chemiosmosis utilize the energy from high-energy electrons carried by NADH and FADH2 to create a proton-motive force, powering ATP synthesis.

Step-by-step solution

Glycolysis

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.

Citric Acid Cycle

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

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.

Determine the stage that produces the most ATP

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.

Conclusion

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.

Key concepts

Glycolysis

Glycolysis is the initial pathway of cellular respiration and occurs in the cytoplasm of the cell. It doesn’t require oxygen, making it an anaerobic process. The primary role of glycolysis is to break down glucose, a 6-carbon sugar, into two molecules of pyruvate, each with three carbons. This process involves a series of ten enzyme-catalyzed reactions, which can be categorized into two main phases:

• The energy-investment phase, where the cell uses 2 ATP molecules to modify glucose into a more reactive form.
• The energy-payoff phase, where energy is harvested as ATP and NADH. Specifically, 4 ATP molecules are produced, resulting in a net gain of 2 ATP molecules, since 2 ATP were used up initially.

Glycolysis also generates 2 NADH molecules by transferring electrons from glucose to NAD⁺, which will later play a crucial role in oxidative phosphorylation.
While its ATP yield is modest, glycolysis is vital as it initiates the breakdown of glucose and provides intermediates for other metabolic pathways.

Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle or TCA cycle, operates in the mitochondrial matrix. It is crucial for the aerobic conversion of energy and works as a continuous cycle processing Acetyl-CoA derived from pyruvate, which is produced during glycolysis. Before entering the cycle, pyruvate undergoes oxidative decarboxylation to transform into Acetyl-CoA, releasing one carbon dioxide molecule. During each turn of the cycle, several energy-rich molecules are generated:

• 1 ATP (or GTP, depending on the cell type)
• 3 NADH
• 1 FADH2
• 2 CO2 (as waste)

Considering that one glucose molecule creates two Acetyl-CoA molecules, the cycle turns twice per glucose, resulting in a production of 2 ATP, 6 NADH, and 2 FADH2 per glucose molecule.
This stage completes the oxidation of glucose, contributing many high-energy electron carriers that are vital for the upcoming oxidative phosphorylation stage. Although it directly yields only a small amount of ATP, its value lies in the electron carriers it supplies to the electron transport chain.

Oxidative Phosphorylation

Oxidative phosphorylation represents the final and most ATP-productive stage of cellular respiration, occurring on the inner mitochondrial membrane. It consists of two major components: the electron transport chain (ETC) and chemiosmosis.
The electron transport chain is a series of protein complexes that transfer high-energy electrons from NADH and FADH2 to molecular oxygen, forming water. As electrons flow through these complexes, protons (H⁺ ions) are pumped from the mitochondrial matrix to the intermembrane space, creating a significant electrochemical gradient known as the proton-motive force.

• Each NADH can lead to the production of approximately 2.5 ATP molecules.
• Each FADH2 contributes about 1.5 ATP molecules.

The energy from the proton gradient powers ATP synthase, an enzyme that synthesizes ATP as protons flow back into the matrix through it. This process, chemiosmosis, acts like a turbine driven by the proton flow.
Oxidative phosphorylation produces the majority of ATP generated in cellular respiration, approximately 25 ATP molecules per glucose molecule, making it the most effective ATP generator. It efficiently harnesses the energy stored in electrons transported by NADH and FADH2 from glycolysis and the citric acid cycle.