To address the question of which chemical reaction in the cycle transfers energy to an energy carrier, it is important to understand the basics of the reactions provided:
1. Option A: ATP → ADP + Pi
- This reaction represents the breakdown of ATP (adenosine triphosphate) into ADP (adenosine diphosphate) and an inorganic phosphate (Pi). This process releases energy, which is then available for cellular processes.
2. Option B: NAD+ + H+ → NADH
- This reaction involves the reduction of NAD+ (nicotinamide adenine dinucleotide) by adding a hydrogen ion (H+) to form NADH. This process is crucial for storing energy, which is later used in the electron transport chain to produce ATP.
3. Option C: Pyruvate ion → acetyl-CoA + CO2
- This reaction is a decarboxylation step in metabolism known as the pyruvate dehydrogenase complex reaction. Pyruvate is converted into acetyl-CoA and CO2, which does not directly involve an energy carrier but prepares molecules for the citric acid cycle.
4. Option D: 2-carbon molecule (acetyl-CoA) + 4-carbon molecule (oxaloacetate) → 6-carbon molecule (citrate)
- This reaction is part of the citric acid cycle (Krebs cycle) where acetyl-CoA combines with oxaloacetate to form citrate. This step is crucial in metabolism and can be considered part of the energy production pathway, but it does not specifically describe the transfer of energy to an energy carrier.
From the information provided:
- Option A is about releasing energy from ATP.
- Option C and Option D are intermediate steps in metabolism but do not specifically describe energy carrier formation or direct energy storage.
The reaction that specifically involves transferring energy to an energy carrier is when NAD+ picks up electrons and a hydrogen ion to form NADH. NADH is a high-energy molecule that will later be used to produce ATP through oxidative phosphorylation in the electron transport chain.
Therefore, the chemical reaction in the cycle that transfers energy to an energy carrier is:
B. NAD+ + H+ → NADH
This reaction stores energy by forming NADH, which is the correct answer to the given question.
