Advanced Placement (AP) Biology Practice Exam

What complex is involved in absorbing light for energy conversion during photosynthesis?

1. Photosystem A

2. Photosystem I

3. Photosystem II

4. Photosystem III

The correct answer is: Photosystem I

The process of photosynthesis involves specific protein complexes that are critical for absorbing light energy and converting it into chemical energy. The correct response identifies Photosystem I as a key player in this process. Photosystem I is essential for the light-dependent reactions of photosynthesis, located in the thylakoid membranes of chloroplasts. It contains chlorophyll and other pigments that absorb light, primarily in the red and far-red wavelengths, capturing the energy from sunlight. When light is absorbed by Photosystem I, it energizes electrons, initiating the process of electron transport. This energized electron is ultimately used in the reduction of NADP+ to NADPH, a key molecule that stores energy for use in the light-independent reactions (Calvin cycle). Therefore, Photosystem I plays an integral role in converting light energy into a form that can be utilized by the plant for producing glucose. While Photosystem II is also integral to photosynthesis, it primarily absorbs light to generate ATP and provide electrons to Photosystem I for further reactions. Understanding the distinct roles of these two photosystems can help clarify their functions in the overall process of photosynthesis.