Correct option is D
Explanation of the correct answer:
The correct answer is Option 4 because the establishment of a proton gradient across the thylakoid membrane is directly involved in the formation of ATP during the light reaction of photosynthesis.
During the light reactions, light energy is absorbed by Photosystem II and Photosystem I. This energy is used to split water molecules (photolysis) into oxygen, protons (H⁺), and electrons. These protons accumulate in the thylakoid lumen.
The electrons from the water molecules pass through the electron transport chain, creating an electrochemical proton gradient (proton gradient) across the thylakoid membrane. This gradient creates a potential energy difference that drives the synthesis of ATP through chemiosmosis via the enzyme ATP synthase.
Information Booster:
ATP Synthesis: The proton gradient generated across the thylakoid membrane provides the energy necessary for ATP synthesis. As protons flow back through ATP synthase (down their electrochemical gradient), this flow drives the conversion of ADP and inorganic phosphate (Pi) into ATP. This process is known as photophosphorylation.
The Role of the Proton Gradient: The proton gradient is critical because it provides the necessary energy for ATP synthase to produce ATP from ADP and Pi. Without this gradient, ATP synthesis would not occur during the light reactions.
Additional Information about Incorrect Options:
Option 1: The splitting of water molecules into oxygen, protons, and electrons occurs during the light reactions, but it primarily provides electrons to replace those lost from Photosystem II and protons for the gradient. While it is important for the light reaction, it does not directly contribute to ATP formation. It plays a crucial role in maintaining the electron flow, but not in ATP synthesis directly.
Option 2: The transfer of electrons from Photosystem I to NADP⁺ to form NADPH is a key process in the light reactions, but it is not directly involved in ATP formation. NADPH is used in the Calvin cycle (dark reactions) for carbon fixation, not directly for ATP synthesis.
Option 3: Carbon fixation by RuBisCO occurs in the Calvin cycle (the dark reaction), which takes place after the light reactions. It is not part of the light reactions, nor does it directly contribute to ATP formation during the light reaction.
