How Photosynthesis Works
BLUF: Photosynthesis converts light energy into chemical energy stored in sugars, using a two-stage process where light reactions generate ATP and NADPH, which the Calvin cycle uses to fix atmospheric CO2 into organic molecules.
Understanding photosynthesis explains how plants fuel nearly all life on Earth and how CO2 is removed from the atmosphere.
Capturing light energy
Photosynthesis occurs in chloroplasts within plant cells. Light-dependent reactions happen in thylakoid membranes, where chlorophyll and accessory pigments absorb photons. When light hits Photosystem II, it energizes electrons, which are stripped from water molecules (H2O), releasing oxygen as a byproduct—Earth's atmospheric oxygen comes from this process. These high-energy electrons travel through an electron transport chain, pumping protons across the thylakoid membrane to create a gradient. ATP synthase uses this gradient to generate ATP (energy currency). The electrons reach Photosystem I, where another photon boosts them again, allowing them to reduce NADP+ to NADPH (electron carrier). The net result: light energy is converted into chemical energy stored in ATP and NADPH.
Building sugars from CO2
The Calvin cycle (light-independent reactions) occurs in the chloroplast stroma, using ATP and NADPH to fix CO2 into organic molecules. The cycle has three phases: (1) Carbon Fixation—the enzyme RuBisCO combines CO2 with ribulose bisphosphate (RuBP, a 5-carbon sugar), producing two 3-carbon molecules (3-phosphoglycerate). (2) Reduction—ATP and NADPH convert 3-phosphoglycerate into glyceraldehyde 3-phosphate (G3P), a sugar precursor. (3) Regeneration—most G3P molecules are recycled to regenerate RuBP, allowing the cycle to continue. Every 3 CO2 fixed produces 1 net G3P; two G3P combine to form glucose. RuBisCO is the most abundant protein on Earth but is inefficient—it sometimes grabs O2 instead of CO2 (photorespiration), wasting energy. C4 and CAM plants evolved mechanisms to concentrate CO2, boosting efficiency in hot, dry climates.
Photosynthesis and the biosphere
Photosynthesis is the primary energy input to the biosphere, converting ~100 terawatts of solar energy into chemical energy annually. It removes ~120 gigatons of CO2 from the atmosphere yearly (roughly balanced by respiration and decomposition). This carbon fixation built the fossil fuels we burn today—ancient photosynthetic organisms captured solar energy that stored for millions of years. Terrestrial plants (especially tropical rainforests) and oceanic phytoplankton (cyanobacteria, diatoms) drive the process. Deforestation and ocean warming threaten these carbon sinks. Enhancing photosynthetic efficiency (e.g., engineering better RuBisCO) could boost crop yields and carbon sequestration, addressing food security and climate simultaneously.
Common misconceptions
Myth: Plants breathe in CO2 and breathe out oxygen all day. Reality: They photosynthesize during the day (releasing O2) but respire 24/7 (consuming O2, releasing CO2), though net O2 production is positive. Myth: More CO2 automatically means more plant growth. Reality: CO2 fertilization has limits—plants also need nutrients, water, and appropriate temperatures; excess CO2 with other constraints doesn't help. Myth: All plants use the same photosynthesis pathway. Reality: C4 and CAM plants evolved alternative strategies to minimize photorespiration and water loss in harsh environments. Myth: Artificial photosynthesis can replace oil immediately. Reality: Lab systems achieve <10% efficiency; scaling to industrial levels requires major breakthroughs in catalyst design and energy storage.