When light strikes chlorophyll molecules in the reaction-center complex, they lose electrons, which are ultimately replaced by
a. Splitting water
b. Oxidizing NADPH
c. The primary electron acceptor
d. The electron transport chain

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Understand the context: The question is about the light-dependent reactions of photosynthesis, specifically what happens when chlorophyll molecules in the reaction-center complex lose electrons.

Recall the process: In the light-dependent reactions, chlorophyll molecules absorb light energy, which excites their electrons to a higher energy state. These high-energy electrons are transferred to the primary electron acceptor in the reaction-center complex.

Identify the problem: When chlorophyll loses electrons, it becomes oxidized and needs to replace the lost electrons to continue the process of photosynthesis.

Determine the source of replacement electrons: In photosystem II, the lost electrons are replaced by splitting water molecules (a process called photolysis). This reaction produces electrons, protons (H⁺), and oxygen (O₂) as byproducts.

Eliminate incorrect options: Oxidizing NADPH (b) is incorrect because NADPH is a product of the light-dependent reactions, not a source of electrons. The primary electron acceptor (c) and the electron transport chain (d) are involved in electron transfer but do not replace the lost electrons. Therefore, the correct answer is splitting water (a).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Chlorophyll and Photosynthesis

Chlorophyll is a green pigment found in plants that plays a crucial role in photosynthesis by absorbing light energy, primarily from the blue and red wavelengths. This energy is used to excite electrons in the chlorophyll molecules, initiating a series of reactions that convert light energy into chemical energy.

Electron Transport Chain

The electron transport chain (ETC) is a series of protein complexes located in the thylakoid membrane of chloroplasts. It facilitates the transfer of electrons from excited chlorophyll to various acceptors, ultimately leading to the production of ATP and NADPH, which are essential for the Calvin cycle in photosynthesis.

Water Splitting (Photolysis)

Water splitting, or photolysis, occurs during the light-dependent reactions of photosynthesis, where water molecules are split into oxygen, protons, and electrons. This process replenishes the electrons lost by chlorophyll, ensuring a continuous flow of energy and maintaining the cycle of electron transport necessary for the synthesis of energy-rich molecules.

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