By Dr. David Warmflash
Updated Aug 30, 2022

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What Is Photosynthesis?

Photosynthesis is the biological process that transforms light energy into chemical energy stored in the bonds of molecules, sustaining life on Earth. It is the mechanism that replenishes atmospheric oxygen and fuels cellular metabolism in plants, algae, and many bacteria.

Oxidation and Reduction in Photosynthesis

In chemical terms, carbon dioxide (CO₂) is highly oxidized, whereas glucose (C₆H₁₂O₆) is reduced. During photosynthesis, electrons derived from sunlight reduce CO₂, enabling the assembly of glucose—a molecule rich in usable energy. The electrons that drive this reduction originate in the light reactions, while the actual construction of glucose occurs in the dark reactions (Calvin cycle).

The Light Reactions

The light reactions take place in the thylakoid membranes of chloroplasts, where pigments such as chlorophyll absorb photons from the sun. This excitation initiates a series of events:

• Photosystem II splits water molecules, releasing O₂ as a by‑product and providing electrons.
• These electrons travel through the electron transport chain, pumping protons across the membrane and generating a proton motive force that synthesizes ATP via ATP synthase.
• At the end of the chain, electrons reduce NADP⁺ to NADPH, a high‑energy carrier that delivers reducing power to the Calvin cycle.

In short, the light reactions convert solar photons into two key energy carriers—ATP and NADPH—while simultaneously releasing oxygen into the atmosphere.

From Light to Glucose: The Dark Reactions

During the dark reactions, also called the Calvin cycle, ATP and NADPH produced above power the fixation of CO₂ into organic molecules, eventually forming glucose. This cycle does not require light directly but depends entirely on the products of the light reactions.

Understanding these two interconnected stages clarifies how plants and photosynthetic organisms sustain themselves—and how the Earth’s biosphere remains oxygenated.