Total Energy from Glucose: Videos & Practice Problems

The complete oxidation of one glucose molecule is a crucial process in cellular metabolism, involving several key stages: glycolysis, pyruvate oxidation, and the Krebs Cycle (also known as the Citric Acid Cycle). Each of these stages contributes to the overall energy yield from glucose.

In the first stage, glycolysis, one glucose molecule undergoes a series of 10 enzymatic reactions, resulting in the production of two pyruvate molecules. This process also generates two high-energy NADH molecules and a net gain of two ATP molecules. The overall reaction can be summarized as:

Glucose → 2 Pyruvate + 2 NADH + 2 ATP

Next, during pyruvate oxidation, each of the two pyruvate molecules is converted into one Acetyl CoA, yielding a total of two Acetyl CoA molecules. This stage also produces two NADH molecules and releases two molecules of carbon dioxide (CO₂). The reaction can be expressed as:

2 Pyruvate → 2 Acetyl CoA + 2 NADH + 2 CO₂

Finally, in the Krebs Cycle, each Acetyl CoA enters the cycle, and since there are two Acetyl CoA molecules, the cycle runs twice. This results in the production of six NADH molecules, two FADH₂ molecules, two ATP molecules, and four additional carbon dioxide molecules. The overall reaction for the Krebs Cycle can be summarized as:

2 Acetyl CoA → 6 NADH + 2 FADH₂ + 2 ATP + 4 CO₂

In total, from glycolysis, pyruvate oxidation, and the Krebs Cycle, the energy yield includes:

• 2 ATP from glycolysis
• 2 NADH from glycolysis
• 2 NADH from pyruvate oxidation
• 6 NADH, 2 FADH₂, and 2 ATP from the Krebs Cycle

It is important to note that this summary does not include the additional ATP generated through the electron transport chain (ETC) and oxidative phosphorylation, which significantly increases the total ATP yield from one glucose molecule. Understanding these metabolic pathways is essential for grasping how cells convert glucose into usable energy.

Glycolysis is a crucial metabolic pathway that converts glucose into pyruvate, yielding energy in the form of ATP and NADH. This process can be divided into two main phases: the energy-consuming phase (Phase A) and the energy-producing phase (Phase B).

In Phase A, which involves the initial steps of glycolysis, glucose is phosphorylated, consuming 2 ATP molecules in reactions 1 and 3. This sets the stage for the subsequent reactions. In Phase B, the energy-producing stage, ATP is generated. Specifically, reactions 7 and 10 produce ATP, resulting in a total of 4 ATP molecules being created from the two glyceraldehyde 3-phosphate molecules. However, when calculating the net yield of ATP, we consider the ATP consumed in Phase A. Thus, the net yield of ATP from glycolysis is:

Net ATP = ATP produced - ATP consumed = 4 - 2 = 2 ATP

Additionally, during Phase B, specifically in reaction 6, 2 NADH molecules are produced. Each glyceraldehyde 3-phosphate generates 1 NADH, and since there are 2 molecules, the total yield of NADH from glycolysis is:

Total NADH = 1 NADH per glyceraldehyde 3-phosphate × 2 = 2 NADH

Following glycolysis, pyruvate oxidation occurs, where each of the 2 pyruvate molecules produced from glycolysis generates 1 NADH. Therefore, the total yield from pyruvate oxidation is also:

Total NADH from pyruvate oxidation = 1 NADH per pyruvate × 2 = 2 NADH

In summary, glycolysis yields a net of 2 ATP and 2 NADH molecules, while pyruvate oxidation contributes an additional 2 NADH molecules.

The process of cellular respiration begins with glycolysis, where one glucose molecule is broken down into two pyruvate molecules. This stage yields a net gain of 2 ATP and produces 2 NADH, but no carbon dioxide or FADH₂. The pyruvate then undergoes oxidation, resulting in the formation of 2 carbon dioxide molecules and 2 NADH, while no ATP or FADH₂ is produced. This step converts the pyruvate into 2 Acetyl CoA, which then enters the Krebs cycle.

In the Krebs cycle, each Acetyl CoA combines with oxaloacetate to produce 2 carbon dioxide molecules, 2 ATP, 6 NADH, and 2 FADH₂. The NADH and FADH₂ generated in both the Krebs cycle and pyruvate oxidation are crucial for the next stage, oxidative phosphorylation, which occurs in the electron transport chain (ETC).

During oxidative phosphorylation, the 10 NADH molecules produced contribute approximately 25 ATP (since each NADH yields about 2.5 ATP), and the 2 FADH₂ contribute about 3 ATP (as each FADH₂ yields about 1.5 ATP). This brings the total ATP yield from one glucose molecule to approximately 32 ATP when all stages are considered.

In summary, from one glucose molecule, the overall breakdown yields:

• 6 carbon dioxide molecules
• 32 ATP molecules
• 2 FADH₂ molecules
• 10 NADH molecules

This comprehensive understanding of the energy yield from glucose metabolism highlights the efficiency of cellular respiration in producing ATP, the energy currency of the cell.