Advanced Placement (AP) Biology Practice Exam

In photosynthesis, what is the primary input needed for the process to occur?

1. Oxygen

2. Glucose

3. Carbon dioxide

4. ATP

The correct answer is: Carbon dioxide

The primary input needed for photosynthesis to occur is carbon dioxide. During the process, plants, algae, and some bacteria utilize carbon dioxide from the atmosphere to produce glucose and oxygen through a series of reactions that occur primarily in the chloroplasts of plant cells. In the light-dependent reactions of photosynthesis, sunlight provides the energy required to split water molecules, which releases oxygen as a byproduct and generates ATP and NADPH. However, these energy carriers are not inputs; rather, they are produced during the light-dependent phase to drive the subsequent light-independent reactions (Calvin cycle). While glucose is a product of photosynthesis, it is not an input in the process itself as it is synthesized from carbon dioxide and other components during the reactions. Oxygen, although important for cellular respiration and the overall gas exchange in ecosystems, is not utilized as a reactant in photosynthesis but is rather a byproduct released during the process. In summary, carbon dioxide is crucial as it serves as a raw material that, when combined with water and sunlight, ultimately leads to the formation of glucose, supporting the plant's energy needs and growth.