Kaplan Nursing Entrance Practice Exam

What is the source of oxygen released during photosynthesis?

1. Carbon dioxide

2. Glucose

3. Sunlight

4. Water

The correct answer is: Water

The source of oxygen released during photosynthesis is water. During the process of photosynthesis, plants take in carbon dioxide from the atmosphere and water from the soil. The sunlight provides the energy necessary to drive the photosynthetic reactions, specifically in the chloroplasts of plant cells. When water is split in the light-dependent reactions of photosynthesis, oxygen is released as a byproduct. This splitting of water molecules (photolysis) is essential, as it generates the oxygen that we breathe and also produces the energy carriers (ATP and NADPH) needed for the next stage of photosynthesis, the Calvin cycle. While carbon dioxide and glucose play crucial roles in the overall process, they do not directly contribute to the release of oxygen. Carbon dioxide is a reactant that gets transformed into glucose, but it does not have oxygen to donate, and glucose, a product of photosynthesis, is not a source of oxygen. Sunlight is vital for providing the energy for the chemical processes, but again, it does not produce oxygen directly. The role of water is therefore central to the production of the oxygen we observe as a result of photosynthesis.