Understanding the Light Reactions: The Role of Photons in Electron Transfer from Water to NADPH

7.8 Looking at the model of the light reactions in Figure 7.8, explain why two photons of light are required in the movement of electrons from water to NADPH.

In the model of the light reactions, two photons of light are required in the movement of electrons from water to NADPH due to the nature of the electron transfer process

In the model of the light reactions, two photons of light are required in the movement of electrons from water to NADPH due to the nature of the electron transfer process. Let’s break it down step by step:

1. Light absorption: The first step in the light reactions is the absorption of photons by the pigments in the thylakoid membrane of chloroplasts. This primarily involves the pigment molecule chlorophyll a, along with accessory pigments like chlorophyll b and carotenoids.

2. Excitation of electrons: When a photon of light strikes a pigment molecule, it excites one of its electrons to a higher energy level. This energy boost allows the electron to become more reactive and ready for transfer.

3. Electron transport chain: The excited electron is transferred to the primary electron acceptor molecule, which is part of a chain of electron carriers embedded in the thylakoid membrane. Two excited electrons are required to initiate this electron transport chain. Each photon donates one electron to the chain.

4. Splitting of water: The transfer of the two electrons causes a reaction called photolysis, which splits water molecules (H2O) into oxygen (O2) and protons (H+). This occurs in a specific protein complex called photosystem II (PSII).

5. Electron replacement: The electrons lost from PSII are replenished by extracting new electrons from water molecules. This continuous splitting of water ensures a constant supply of electrons for the electron transport chain.

6. Electron transport and ATP synthesis: As electrons move through the electron transport chain, they release energy, which is used to pump protons (H+) from the stroma into the thylakoid lumen. This creates a proton gradient, with a higher concentration of protons inside the thylakoid lumen than in the stroma. The flow of protons back into the stroma through ATP synthase generates ATP (adenosine triphosphate), which is a molecule that stores energy.

7. Formation of NADPH: After the first electron transport chain, the electrons are transferred to a second protein complex called photosystem I (PSI). In PSI, the electrons are re-energized by another photon of light and are eventually used to reduce NADP+ (nicotinamide adenine dinucleotide phosphate) into NADPH. This molecule acts as a powerful reducing agent and is essential for the synthesis of carbohydrates in the next stage of photosynthesis, known as the Calvin cycle.

In summary, two photons of light are required for the movement of electrons from water to NADPH because each photon donates one electron to the electron transport chain. This chain is responsible for creating a proton gradient, producing ATP, and ultimately transferring the electrons to NADP+ to generate NADPH.

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