How Photosynthesis Works: The Process That Powers the Planet
Photosynthesis is the chemical process by which plants, algae, and some bacteria convert light energy into chemical energy stored in sugars. It is the foundation of nearly every food chain on Earth and the reason the atmosphere contains oxygen at all.
The Two Stages
Photosynthesis happens in two coupled stages inside the chloroplasts of plant cells. The light-dependent reactions take place in the thylakoid membranes. Chlorophyll absorbs photons, which excite electrons. The energy from those electrons splits water molecules into oxygen (released as a byproduct), protons, and electrons, while also producing the energy carriers ATP and NADPH.
The Calvin cycle, the light-independent stage, occurs in the stroma. Using the ATP and NADPH from the first stage, the cycle fixes carbon dioxide into three-carbon sugars that the plant later assembles into glucose, starch, and cellulose. No light is required directly, but the cycle stalls without the products of the light reactions.
Why It Matters
The net reaction — six molecules of carbon dioxide and six of water plus light, yielding one glucose and six oxygen molecules — sets the budget for life on Earth. Photosynthesis produces the oxygen we breathe, the food we eat, and over geological time the fossil fuels that, when burned, return ancient carbon to the atmosphere.
This article is an educational overview and simplifies a great deal of biochemistry. For depth, consult a physiology or plant biology textbook.