How Photosynthesis Powers Almost All Life on Earth

Every Calorie You Have Ever Eaten Was Made by Sunlight

The bread, meat, fruit, and vegetables that constitute every meal you have ever eaten trace their energy to a single source: photons emitted by the sun. Photosynthesis—the process by which plants, algae, and cyanobacteria capture light energy and convert it to chemical energy stored in glucose—is the foundational energy transaction of virtually all life on Earth. It produces the oxygen in every breath you take and fixes roughly 120 billion metric tons of carbon from the atmosphere into organic matter each year, according to estimates published in the journal Science. Remove photosynthesis from Earth, and the food web collapses within weeks.

The Two Stages: Light Reactions and the Calvin Cycle

Photosynthesis proceeds in two coupled but distinct stages that together accomplish the overall reaction:

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

The first stage—the light-dependent reactions—captures solar energy. The second stage—the Calvin cycle—uses that captured energy to fix CO₂ into sugar. Both occur in the chloroplast: light reactions in the thylakoid membranes; the Calvin cycle in the stroma.

Light Reactions: Capturing Sunlight in Two Photosystems

Chlorophyll—the pigment that makes leaves green—absorbs light primarily at wavelengths of 430–450 nm (blue) and 640–680 nm (red), reflecting green light that we see. The chloroplast contains two photosystem complexes:

• Photosystem II (PSII): Absorbs photons at 680 nm. Uses the energy to split water molecules (photolysis), releasing oxygen as a byproduct: 2H₂O → 4H⁺ + 4e⁻ + O₂. The electrons extracted from water replace those excited by light and passed down the electron transport chain.
• Photosystem I (PSI): Absorbs photons at 700 nm. Re-energizes electrons that have moved down the electron transport chain from PSII and uses them to reduce NADP⁺ to NADPH.

As electrons flow from PSII to PSI through the electron transport chain, protons are pumped across the thylakoid membrane, creating a proton gradient. ATP synthase harnesses this gradient to synthesize ATP from ADP and phosphate—the same chemiosmotic mechanism used in cellular respiration, run here in reverse. The products of the light reactions—ATP and NADPH—are the energy currency passed to the Calvin cycle.

The Calvin Cycle: Building Sugar from Air

The Calvin cycle (also called the Calvin-Benson-Bassham cycle, after its discoverers who earned the 1961 Nobel Prize in Chemistry) uses ATP and NADPH to convert CO₂ into the three-carbon sugar glyceraldehyde-3-phosphate (G3P), the precursor to glucose and all other organic molecules in the plant.

Three turns of the cycle are required to produce one net G3P molecule, consuming:

• 3 CO₂ molecules (one per turn, added to ribulose-1,5-bisphosphate by the enzyme RuBisCO)
• 9 ATP
• 6 NADPH

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is arguably the most important enzyme on Earth—it performs the carbon fixation step that incorporates atmospheric CO₂ into organic matter. It is also extraordinarily slow and imprecise, catalyzing only 3–10 reactions per second (most enzymes handle thousands) and mistakenly binding O₂ roughly 25% of the time in a wasteful process called photorespiration. This inefficiency has driven the evolution of alternative photosynthetic pathways in certain plant lineages.

C3 vs. C4 Plants: An Evolutionary Arms Race with Oxygen

C4 plants evolved a spatial CO₂ concentrating mechanism: CO₂ is first captured in mesophyll cells by PEP carboxylase (which does not bind O₂) into 4-carbon acids, which are then shuttled to bundle sheath cells where they release CO₂ at high concentrations around RuBisCO, effectively eliminating photorespiration. C4 photosynthesis evolved independently at least 60 times across the plant kingdom—a remarkable example of convergent evolution driven by rising oxygen and falling CO₂ over geological time.

A third pathway, CAM (crassulacean acid metabolism), is used by cacti and succulents: stomata open only at night to take in CO₂, which is stored as malic acid until daytime, when it is released inside the leaf for Calvin cycle fixation while stomata remain closed. This allows photosynthesis in extreme aridity but is slow and suited only to plants growing in very arid environments.

Oxygen: The Photosynthetic Byproduct That Changed the World

All the oxygen in Earth's atmosphere originated from photosynthesis. Before photosynthetic cyanobacteria evolved approximately 2.7 billion years ago, Earth's atmosphere was anoxic—no free oxygen. The Great Oxidation Event, beginning around 2.4 billion years ago, saw atmospheric oxygen rise dramatically as cyanobacteria proliferated. This was catastrophic for existing anaerobic life (oxygen was toxic to it) but created the conditions for the evolution of complex aerobic life, including all animals.

Today, marine phytoplankton—microscopic photosynthetic organisms in the ocean—are responsible for approximately 50% of global oxygen production. The Amazon rainforest, often called the "lungs of the Earth," contributes roughly 9% of terrestrial oxygen production. Terrestrial plants as a whole contribute the other ~50%.

Photosynthetic Efficiency and the Future of Energy

Natural photosynthesis converts sunlight to stored chemical energy with an efficiency of roughly 1–2% for C3 crops and up to 2–4% for C4 crops under optimal conditions—far below the theoretical maximum of 11%. Modern silicon solar panels achieve 20–26% efficiency. Researchers are working on "artificial photosynthesis"—systems that use photocatalysts to split water and reduce CO₂—as well as genetic engineering projects aimed at introducing C4 traits into C3 staple crops like rice (the IRRI C4 Rice Project). The RIPE project at the University of Illinois has already increased tobacco biomass by 40% by engineering enzymes that allow faster recovery from photorespiration. The ancient chemistry of photosynthesis is being redesigned for an energy-constrained century.