Write the net biochemical equation for the metabolism of a molecule of glucose by glycolysis and the citric acid cycle, including all cofactors.
The correct answer and explanation is :
Net Biochemical Equation for the Metabolism of Glucose by Glycolysis and the Citric Acid Cycle:
Glycolysis:
[
\text{Glucose} (C_6H_{12}O_6) + 2 \text{NAD}^+ + 2 \text{ADP} + 2 \text{P}_i \rightarrow 2 \text{Pyruvate} (C_3H_4O_3) + 2 \text{NADH} + 2 \text{H}^+ + 2 \text{ATP}
]
Citric Acid Cycle (for 2 Pyruvate molecules):
[
2 \text{Pyruvate} (C_3H_4O_3) + 8 \text{NAD}^+ + 2 \text{FAD} + 2 \text{ADP} + 2 \text{P}_i + 6 \text{H}_2O \rightarrow 6 \text{CO}_2 + 8 \text{NADH} + 2 \text{FADH}_2 + 2 \text{ATP}
]
Overall Net Equation:
[
\text{Glucose} (C_6H_{12}O_6) + 10 \text{NAD}^+ + 2 \text{FAD} + 4 \text{ADP} + 4 \text{P}_i + 6 \text{H}_2O \rightarrow 6 \text{CO}_2 + 10 \text{NADH} + 2 \text{FADH}_2 + 4 \text{ATP}
]
Explanation:
The complete metabolism of a glucose molecule involves two main metabolic pathways: Glycolysis and the Citric Acid Cycle.
1. Glycolysis:
- Glycolysis occurs in the cytoplasm and breaks down one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons each).
- During glycolysis, 2 NAD+ molecules are reduced to form 2 NADH, and 2 ATP molecules are consumed and 4 ATP molecules are produced (net gain of 2 ATP).
- The products of glycolysis (2 pyruvate, 2 NADH, 2 ATP) are then transported into the mitochondria.
2. Citric Acid Cycle (Krebs Cycle):
- Each pyruvate molecule is converted to acetyl-CoA, which then enters the citric acid cycle.
- The citric acid cycle involves a series of enzymatic reactions that oxidize acetyl-CoA to produce energy-rich molecules, including NADH and FADH2, and release carbon dioxide (CO2).
- For each glucose molecule (which generates 2 pyruvate), the citric acid cycle goes through two rounds, generating 8 NADH, 2 FADH2, and 2 ATP (directly through substrate-level phosphorylation).
3. Overall Energy Balance:
- In total, glycolysis and the citric acid cycle produce 10 NADH, 2 FADH2, 4 ATP, and 6 CO2 molecules from one molecule of glucose.
- NADH and FADH2 are used in the electron transport chain to generate more ATP via oxidative phosphorylation, leading to a total yield of up to 38 ATP per glucose molecule.
This biochemical pathway is a central part of cellular respiration, converting glucose into energy that cells can use for various functions.