Identify the two photosystems and their primary pigments.

• A. Photosystem II (P680) and Photosystem I (P700); chlorophyll a is the primary pigment
• B. Photosystem II (P680) and Photosystem I (P700); chlorophyll b is the primary pigment
• C. Photosystem II only; chlorophyll a is the primary pigment
• D. Photosystem I only; carotenoids are the primary pigment

Answer: (A)

Two photosystems work together in the light-dependent reactions, each with a reaction center built around a special pair of chlorophyll a molecules. The first system’s reaction center is called P680, reflecting its peak absorption around 680 nm, while the second system’s reaction center is P700, absorbing best near 700 nm. The primary pigment in both reaction centers is chlorophyll a, the molecule that donates electrons once excited by light. Other pigments like chlorophyll b and carotenoids act as light-harvesting helpers, broadening the range of light absorbed and funneling that energy to chlorophyll a, but they are not the primary pigments in the reaction centers. So the correct description is two photosystems, P680 and P700, with chlorophyll a as the primary pigment.