Photosynthesis, Explained
How plants, algae, and some bacteria turn sunlight, water, and carbon dioxide into sugar and oxygen — the light-dependent reactions, the Calvin cycle, and why this single process underwrites nearly all life on Earth.
Updated 2026-06-01
What photosynthesis actually does
Photosynthesis is the process by which plants, algae, and certain bacteria capture energy from sunlight and use it to build sugar from carbon dioxide and water, releasing oxygen as a by-product. The overall reaction is deceptively simple: six molecules of carbon dioxide plus six of water, powered by light, yield one molecule of glucose and six of oxygen.
That one reaction is the foundation of almost every food chain on the planet and the source of the oxygen you are breathing right now. Understanding it means understanding two linked stages — the light-dependent reactions that capture energy, and the Calvin cycle that uses that energy to assemble sugar.
The key players
Where it happens and what each part contributes.
Chloroplast
The organelle where photosynthesis occurs, containing stacked membranes (thylakoids) and a fluid interior (stroma).
Chlorophyll
The green pigment that absorbs red and blue light and reflects green, driving energy capture.
Thylakoid membrane
Site of the light-dependent reactions; houses the photosystems and the electron transport chain.
Stroma
The fluid surrounding the thylakoids, where the Calvin cycle builds sugar.
ATP and NADPH
The energy and electron carriers produced in stage one and spent in stage two.
Rubisco
The enzyme that fixes carbon dioxide into an organic molecule — the most abundant protein on Earth.
Two stages, one assembly line
In the light-dependent reactions, photons strike chlorophyll in the thylakoid membrane, exciting electrons that travel down an electron transport chain. This splits water (releasing oxygen), pumps protons to build a gradient, and powers the synthesis of ATP and NADPH. These two molecules are the energy currency handed off to the next stage.
In the Calvin cycle (the light-independent reactions), the ATP and NADPH are spent to fix carbon dioxide into a three-carbon sugar via the enzyme rubisco. Several turns of the cycle produce glucose. The two stages are tightly coupled: stage one captures energy, stage two converts it into stored chemical form.
How to study it (the efficient way)
Learn the overall equation cold
Reactants in, products out — anchor everything else to carbon dioxide plus water yielding glucose plus oxygen.
Separate the two stages
Know exactly where each happens (thylakoid vs. stroma) and what each produces or consumes.
Trace the energy carriers
Follow ATP and NADPH from where they are made to where they are spent — this is the most-tested link.
Drill with active recall
Flashcards on inputs, outputs, and locations expose gaps fast; quiz yourself before re-reading.
Common questions
Why do leaves look green?
Chlorophyll absorbs red and blue wavelengths for energy and reflects green light, which is why foliage appears green to your eyes.
Does photosynthesis stop at night?
The light-dependent reactions stop without light, but the Calvin cycle can run briefly on the ATP and NADPH already produced. Sustained sugar production requires light.
How is this related to cellular respiration?
They are near-opposites: photosynthesis stores energy in glucose using carbon dioxide and water, while respiration releases that energy and regenerates carbon dioxide and water.
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