What is Photosynthesis in Reaction?

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In the context of photosynthesis, “reaction” usually refers to two specific stages within the overall process:

Light-dependent Reactions

This stage relies on the direct capture of sunlight energy by pigment molecules, primarily chlorophyll, within specialized structures called thylakoids in plant cells (specifically, chloroplasts). Here’s a breakdown of the key reactions:

Photosystem II

  • Absorbs red and blue wavelengths of sunlight, causing an electron to be ejected from a chlorophyll molecule. This energized electron is transferred through a series of electron carriers, ultimately generating ATP (adenosine triphosphate), the cell’s energy currency.


  • Water molecules are split using energy from Photosystem II, releasing oxygen as a byproduct. The remaining hydrogen ions (protons) and electrons contribute to the generation of a proton gradient across the thylakoid membrane.

Photosystem I

  • Absorbs primarily red light, again ejecting an electron. This electron fuels further ATP production and also reduces NADP+ (nicotinamide adenine dinucleotide phosphate) to NADPH, an electron carrier used in the next stage.

Light-independent Reactions (Calvin Cycle)

This stage, also known as the Calvin cycle, does not directly require sunlight but utilizes the ATP and NADPH generated in the light-dependent reactions. It occurs in the stroma of the chloroplast and focuses on fixing carbon dioxide into organic molecules. Here’s how it works:

Carbon fixation

  • CO2 molecules from the atmosphere are captured by an enzyme called RuBisCO, forming an unstable intermediate compound.


  • Using energy from ATP and electrons from NADPH, this intermediate is converted into glycerate-3-phosphate (G3P), a simple organic molecule.


  • Some G3P molecules are used to build carbohydrates like glucose (sugar), the plant’s primary energy source. The remaining G3P molecules are recycled to RuBP, regenerating the initial acceptor for CO2 fixation, thus perpetuating the cycle.

Therefore, “photosynthesis in reaction” encompasses these two sets of interconnected reactions that ultimately convert light energy into chemical energy stored in sugars, while also producing oxygen as a crucial byproduct. It’s the foundation of life on Earth, powering the entire food chain and shaping the composition of our atmosphere.

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