What molecule is indicated by the letter D? Electrons carried via NADH Electrons carried via NADH and FADH2 A B C Glucose Pyruvate Cytosol O ATP O oxygen O water O pyruvate O glucose Mitochondrion D E CO2 CO2 ATP
The Correct Answer and Explanation is:
The molecule indicated by the letter D is oxygen (O₂).
Explanation
In cellular respiration, glucose is broken down in a series of steps to produce ATP, the energy currency of the cell. This process occurs in three main stages: glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation (electron transport chain and chemiosmosis).
- Glycolysis occurs in the cytosol, where glucose is broken down into pyruvate, producing a small amount of ATP and NADH.
- The pyruvate is transported into the mitochondrion, where it is further processed in the citric acid cycle, releasing CO₂, generating ATP, and producing more NADH and FADH₂.
- These reduced electron carriers (NADH and FADH₂) then donate electrons to the electron transport chain (ETC), which is embedded in the inner mitochondrial membrane. As electrons move through the chain (from A to B to C), they power the pumping of protons into the intermembrane space, creating a proton gradient.
- At the end of the electron transport chain, electrons must be accepted by a final molecule to prevent backup in the chain. This final electron acceptor is oxygen (O₂), the molecule labeled D. Oxygen combines with the electrons and hydrogen ions (protons) to form water (H₂O). O2+4e−+4H+→2H2OO₂ + 4e⁻ + 4H⁺ → 2H₂O
Without oxygen, the electron transport chain would halt, ATP production via oxidative phosphorylation would stop, and cells would have to rely on less efficient anaerobic processes.
Thus, oxygen (D) is crucial because it accepts electrons at the end of the chain, enabling continued operation of the ETC and efficient ATP production. This role makes oxygen vital to aerobic respiration and energy metabolism in most eukaryotic cells.
