Final answer:
NADH is the molecule that is reduced as it gains electrons and a hydrogen atom, transitioning from NAD+ to NADH in redox reactions during cellular respiration.
Step-by-step explanation:
The student asked which of the following molecules is reduced: NADH, CO2, O2, or ATP. In redox reactions, reduction refers to the gain of electrons. The molecule that is reduced in this context is NADH because it has gained electrons and a hydrogen atom, transitioning from NAD+ to NADH in the process. The role of NAD+ in redox reactions is crucial as it acts as an oxidizing agent, accepting electrons and protons from organic molecules. This process of electron transfer is what leads to the production of NADH, which then carries this stored energy to later stages of cellular respiration for ATP generation.
During the catabolism of glucose, known as glycolysis, NAD+ is reduced to NADH, indicating the gain of electrons. Furthermore, in the Krebs cycle, other molecules like FAD are also reduced to FADH2, showcasing additional redox reactions in cellular respiration.