Energized electrons from ____ enter an electron transport chain and are then used to reduce NADP\textsuperscript{+}. Which molecule correctly fills the blank?

• A. cytochrome c
• B. photosystem II
• C. ATP synthase
• D. photosystem I

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the context of the problem. The question is asking about the molecule that provides energized electrons to the electron transport chain, which are then used to reduce NADP⁺ to NADPH. This process is part of the light-dependent reactions of photosynthesis.
2. Step 2: Recall the roles of the listed molecules. Photosystem II is responsible for splitting water and providing electrons to the electron transport chain, but it does not directly reduce NADP⁺. Cytochrome c is involved in the electron transport chain in cellular respiration, not photosynthesis. ATP synthase is an enzyme that synthesizes ATP, not involved in electron donation. Photosystem I is the molecule that energizes electrons to reduce NADP⁺.
3. Step 3: Focus on Photosystem I. Photosystem I absorbs light energy and excites electrons to a higher energy state. These energized electrons are transferred to the electron transport chain and ultimately used to reduce NADP⁺ to NADPH.
4. Step 4: Connect the process to the light-dependent reactions. In photosynthesis, Photosystem II provides electrons to Photosystem I via the electron transport chain. Photosystem I then re-energizes these electrons using light energy, enabling the reduction of NADP⁺.
5. Step 5: Conclude that the correct molecule filling the blank is Photosystem I, as it is directly responsible for providing energized electrons to reduce NADP⁺.