Question

The complete oxidation of glucose 6 -phosphate derived from free glucose yields 30 molecules ATP, whereas the complete oxidation of glucose 6-phosphate derived from glycogen yields 31 molecules of ATP. Account for this difference.

Short answer

Glucose 6-phosphate from glycogen yields more ATP because it bypasses the ATP-consuming phosphorylation step needed for free glucose.

Step-by-step solution

Introduction to Glucose Oxidation

In the process of cellular respiration, glucose is oxidized to produce ATP, the energy currency of the cell. This oxidation involves several steps, including glycolysis, the citric acid cycle, and oxidative phosphorylation. Glucose 6-phosphate (G6P) is an intermediate in the glycolysis pathway, and its source affects the total ATP output.

Oxidation of Free Glucose

When free glucose is phosphorylated to glucose 6-phosphate, 1 ATP molecule is consumed in the process catalyzed by hexokinase. This reaction is the first step in glycolysis: \[ \text{Glucose + ATP} \rightarrow \text{Glucose 6-phosphate + ADP} \]Because of this initial ATP expenditure, the net yield from the complete oxidation of glucose 6-phosphate derived from free glucose is 30 ATP molecules.

Oxidation of Glycogen-derived Glucose

In contrast, glucose 6-phosphate derived from glycogen does not require the initial ATP-consuming phosphorylation step. Glycogen is broken down via glycogenolysis, where glucose units are converted to glucose 1-phosphate and then to glucose 6-phosphate without using ATP:\[ \text{Glycogen} \rightarrow \text{Glucose 1-phosphate} \rightarrow \text{Glucose 6-phosphate} \]The absence of ATP expenditure in this process explains why the oxidation of glucose 6-phosphate derived from glycogen yields 31 ATP molecules.

Conclusion: Account for ATP Difference

The difference of 1 ATP in the yield between free glucose and glycogen-derived glucose oxidation is due to the ATP used in phosphorylating free glucose. Glycogen-derived glucose 6-phosphate bypasses this ATP-consuming step during glycogenolysis, allowing the complete oxidation to yield one additional ATP molecule.

Key concepts

Glycolysis

Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. It takes place in the cytoplasm of cells and can function with or without oxygen. When a molecule of glucose enters glycolysis, it’s converted into two molecules of pyruvate. During this process, a small amount of energy is captured directly as ATP. Additionally, two molecules of NADH are produced, which can also be used to generate more ATP in later steps of cellular respiration. This pathway is crucial because it begins the energy extraction process and fuels further stages of metabolism. - Glycolysis consumes 2 ATP but generates 4 ATP, having a net gain of 2 ATP per glucose molecule. - Producing 2 NADH molecules sets up for more energy extraction later in mitochondrial oxidative phosphorylation.

ATP Production

ATP, or adenosine triphosphate, is the energy currency of the cell. It powers nearly all cellular activities that require energy. The process of ATP production begins with glycolysis, resulting in a modest ATP gain. The majority of ATP is produced later during processes like the citric acid cycle and oxidative phosphorylation. - Oxidative phosphorylation, which takes place in mitochondria, generates the most ATP. It uses the electron transport chain and a process known as chemiosmosis. - The entire process of glucose oxidation can produce up to 30 or 31 ATP, depending on the origin of glucose phosphate.

Glycogenolysis

Glycogenolysis is the process by which glycogen is broken down into glucose to be used for energy. Glycogen serves as a storage form of glucose found in the liver and muscles. When energy is needed, glycogenolysis converts it into glucose 1-phosphate, which is then transformed into glucose 6-phosphate. This step is notable because it bypasses the ATP consumption step required in free glucose breakdown. - Bypassing the ATP use in glycolysis leads to an extra ATP molecule being conserved in glycogen-derived glucose oxidation. - Glycogenolysis is crucial during periods when glucose is not readily available from the diet, such as between meals or during intense physical activity.

Cellular Respiration

Cellular respiration is a multi-step process where cells convert biochemical energy from nutrients into ATP, and it mainly involves glycolysis, the citric acid cycle, and oxidative phosphorylation. Each of these steps contributes to the overall efficiency of ATP synthesis from glucose. - The citric acid cycle processes the byproducts of glycolysis further, generating more NADH and FADH2 to fuel oxidative phosphorylation. - Oxidative phosphorylation then takes the high-energy electrons from NADH and FADH2 to drive ATP synthesis through the electron transport chain, yielding the majority of ATP captured from glucose. - Understanding cellular respiration is key to grasping how biological cells maximize energy from nutrients, as well as understanding the difference in ATP yield depending on the glucose source. This is because glycogen-derived glucose ultimately bypasses an ATP-consuming step and thus offers slightly more ATP output.