In the process of photosynthesis, what is the primary source of hydrogen ions (protons) within the thylakoid lumen?

• A. Carbon dioxide (CO_2) absorbed from the atmosphere
• B. Glucose (C_6H_{12}O_6) produced in the Calvin cycle
• C. Oxygen (O_2) released during photosynthesis
• D. Water molecules (H_2O) split during photolysis

Answer: (D)

Verified step by step guidance

1. Understand the context of the problem: Photosynthesis occurs in the chloroplasts of plant cells, specifically within the thylakoid membranes. The thylakoid lumen is the space inside these membranes, and hydrogen ions (protons) accumulate here during the light-dependent reactions of photosynthesis.
2. Identify the process responsible for generating hydrogen ions: During the light-dependent reactions, photolysis occurs. Photolysis is the splitting of water molecules (H₂O) into oxygen (O₂), electrons, and hydrogen ions (protons) under the influence of light energy absorbed by chlorophyll.
3. Break down the chemical reaction of photolysis: The reaction can be represented as: $H_{2}O\to O+4H+4e$ This reaction shows that water molecules are split into oxygen gas, protons (H⁺), and electrons.
4. Explain the role of hydrogen ions: The protons generated during photolysis are released into the thylakoid lumen, contributing to the proton gradient across the thylakoid membrane. This gradient is essential for ATP synthesis during the light-dependent reactions.
5. Clarify why other options are incorrect: Carbon dioxide (CO₂) is used in the Calvin cycle, not the light-dependent reactions. Glucose (C₆H₁₂O₆) is a product of the Calvin cycle, not a source of protons. Oxygen (O₂) is released as a byproduct of photolysis but does not contribute hydrogen ions directly.