Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, using carbon dioxide and water to produce sugars and releasing oxygen as a byproduct. The overall reaction:
In eukaryotic organisms, photosynthesis occurs in chloroplasts — organelles containing the pigment chlorophyll, which absorbs light primarily in the red and blue wavelengths (reflecting green, which is why plants appear green). The process has two stages. The light-dependent reactions occur in the thylakoid membranes, where chlorophyll absorbs photons and uses their energy to split water molecules, producing ATP and NADPH (energy carriers) and releasing oxygen. The light-independent reactions (Calvin cycle) occur in the stroma, where ATP and NADPH drive the fixation of carbon dioxide into three-carbon sugars, which are then assembled into glucose and other organic molecules.
Photosynthesis is the energetic foundation of nearly all life on Earth. The sugars it produces are the primary source of chemical energy for plants themselves (through cellular respiration) and for virtually all other organisms — directly through herbivory or indirectly through food webs. The oxygen it releases transformed Earth’s atmosphere beginning roughly 2.4 billion years ago (the Great Oxidation Event) and made aerobic metabolism possible.
Photosynthetic efficiency varies with environmental conditions — light intensity, temperature, water availability, carbon dioxide concentration — and plants have evolved different strategies to cope. C3 photosynthesis (the standard Calvin cycle) is efficient in cool, moist conditions but loses energy to photorespiration in hot, dry environments. C4 plants (maize, sugarcane) and CAM plants (cacti, succulents) have evolved modifications that concentrate carbon dioxide, reducing photorespiration at the cost of additional energy expenditure.
Related terms
- Metabolism — the broader set of chemical reactions that photosynthesis feeds into
- Cell — the structure containing the chloroplasts where photosynthesis occurs
- Ecosystem — the ecological context sustained by photosynthetic energy capture
- Symbiosis — the endosymbiotic origin of chloroplasts