Refer to the table, which summarizes the inputs and outputs of glycolysis. Glycolysis Inputs Outputs Glucose Pyruvate 2 NAD 2 NADH 2 ATP 4 ATP 4 ADP + 4P 2 ADP + 2P Which group represents the net outputs of glycolysis?
Pyruvate, 2 NADH, 4 ATP
4 NADH, 2 ATP, 2 ADP, 2 Pi
Pyruvate, 2 NADH, 4 ATP, 2 ADP, 2 Pi
Pyruvate, 2 NADH, 2 ATP
2 NADH, 2 ATP, 2 ADP, 2 P
The correct answer and explanation is :
The correct answer is: Pyruvate, 2 NADH, 2 ATP.
Explanation:
Glycolysis is a metabolic pathway that breaks down one molecule of glucose (a 6-carbon sugar) into two molecules of pyruvate (each a 3-carbon molecule). This process occurs in the cytoplasm of the cell and does not require oxygen (anaerobic).
Now, let’s analyze the inputs and outputs:
- Inputs:
- 1 glucose molecule
- 2 NAD⁺ molecules
- 2 ATP molecules (to start the process)
- Outputs:
- 2 pyruvate molecules
- 2 NADH molecules (from the reduction of NAD⁺)
- 4 ATP molecules (produced), but since 2 ATP were used initially, the net gain is 2 ATP.
Why “Pyruvate, 2 NADH, 2 ATP” is correct:
- Pyruvate: Glycolysis converts glucose into two pyruvate molecules.
- 2 NADH: During glycolysis, two NAD⁺ molecules are reduced (gain electrons) to form two NADH molecules, which carry electrons to the electron transport chain if oxygen is available.
- 2 ATP: Although 4 ATP are produced, 2 ATP are consumed early in the pathway (investment phase). So, the net gain is 2 ATP.
Why the other choices are incorrect:
- Pyruvate, 2 NADH, 4 ATP: This counts the total ATP made but ignores that 2 ATP were used, so the net ATP is only 2.
- 4 NADH, 2 ATP, 2 ADP, 2 Pi: Glycolysis produces only 2 NADH, not 4.
- Pyruvate, 2 NADH, 4 ATP, 2 ADP, 2 Pi: Again, counts total ATP without accounting for ATP consumption. Plus, ADP and Pi (phosphate) are not considered net outputs; they are intermediates.
- 2 NADH, 2 ATP, 2 ADP, 2 P: Includes extra items (ADP, Pi) that are not true final outputs but are recycled in the cell.
Summary:
The net outputs of glycolysis are 2 pyruvate, 2 NADH, and 2 ATP. These products are crucial for the cell: pyruvate can enter the mitochondria for aerobic respiration, NADH carries high-energy electrons, and ATP provides energy for cellular processes.