How is light energy converted to chemical energy in carbon compounds during photosynthesis?

Sourav PanNovember 8, 2024

Answered

The conversion of light energy to chemical energy during photosynthesis is a complex process that occurs in two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle). Here’s a detailed breakdown of how this energy transformation takes place:

Light-Dependent Reactions

1. Absorption of Light:
   • The process begins when chlorophyll, the green pigment in chloroplasts, absorbs sunlight. This occurs primarily in the thylakoid membranes of the chloroplasts.
2. Water Splitting:
   • The absorbed light energy excites electrons in chlorophyll, leading to the splitting of water molecules (photolysis). This reaction produces oxygen as a by-product and releases electrons and protons (hydrogen ions) into the thylakoid lumen.
3. Electron Transport Chain:
   • The excited electrons are transferred through a series of proteins known as the electron transport chain. As electrons move through this chain, their energy is used to pump protons into the thylakoid lumen, creating a proton gradient.
4. ATP and NADPH Formation:
   • The proton gradient drives ATP synthesis through a process called chemiosmosis, where protons flow back into the stroma through ATP synthase, producing ATP from ADP and inorganic phosphate.
   • Additionally, at the end of the electron transport chain, electrons are used to reduce NADP+ to NADPH, another important energy carrier.

Light-Independent Reactions (Calvin Cycle)

1. Carbon Fixation:
   • Carbon dioxide enters the plant through stomata and is fixed into organic molecules in a series of reactions known as carbon fixation. The enzyme Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes this initial step by combining CO2 with ribulose bisphosphate (RuBP) to form 3-phosphoglycerate (3-PGA).
2. Reduction Phase:
   • ATP and NADPH produced in the light-dependent reactions are utilized in this phase to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. ATP provides energy while NADPH provides reducing power for this conversion.
3. Regeneration Phase:
   • Some G3P molecules are used to regenerate RuBP, allowing the cycle to continue, while others are utilized to synthesize glucose and other carbohydrates that serve as chemical energy sources for the plant and other organisms