Photosynthesis is the process plants, algae, and some bacteria use to convert sunlight, water, and carbon dioxide into sugar and oxygen. It’s the foundation of nearly all life on Earth, producing the food that feeds ecosystems and the oxygen that fills the atmosphere. Roughly half of the world’s oxygen comes from photosynthetic organisms in the ocean, with the other half generated by plants on land.
The Basic Equation
The overall reaction is straightforward: six molecules of carbon dioxide plus six molecules of water, powered by sunlight, produce one molecule of glucose sugar and six molecules of oxygen. In chemical shorthand, that’s 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The plant keeps the sugar as fuel for growth and releases the oxygen into the air. Every breath you take depends on this reaction happening billions of times across the planet.
Where It Happens Inside the Plant
Photosynthesis takes place inside tiny structures called chloroplasts, found mainly in leaf cells. A single leaf cell can contain dozens of them. Each chloroplast has an internal system of flattened, disc-shaped sacs called thylakoids, stacked like coins. The membranes of these thylakoids hold the machinery that captures light and converts it into usable energy. Surrounding the thylakoids is a fluid-filled space called the stroma, where the plant actually builds sugar molecules from carbon dioxide.
This two-compartment design matters because photosynthesis happens in two distinct stages, each in a different part of the chloroplast.
Stage One: Capturing Light Energy
The first stage, called the light-dependent reactions, occurs in the thylakoid membranes. It begins when a particle of light (a photon) hits a pigment molecule, most commonly chlorophyll. That energy bounces from one pigment molecule to the next in roughly a millionth of a second until it reaches a reaction center, where it boosts an electron to a higher energy state. That energized electron then passes along a chain of proteins embedded in the thylakoid membrane, like a ball rolling downhill through a series of gates.
As electrons move through this chain, they power tiny molecular pumps that push hydrogen ions (protons) across the thylakoid membrane into the interior of the thylakoid sac. This creates a concentration difference: many more protons on one side of the membrane than the other. The protons then flow back out through a protein called ATP synthase, and that flow of ions spins the protein like a turbine, generating ATP, the cell’s universal energy currency. For every pair of electrons that completes the full journey, the cell produces roughly one to one and a half molecules of ATP.
Meanwhile, water molecules are split to replace the electrons that were boosted away from chlorophyll. This splitting releases oxygen as a byproduct, which is why plants give off O₂. The electrons eventually reach a second light-capturing system, get re-energized by another photon, and are used to produce a second energy carrier called NADPH. Together, ATP and NADPH are the power supply for the next stage.
Stage Two: Building Sugar
The second stage takes place in the stroma, the fluid surrounding the thylakoids. Known as the Calvin cycle, it doesn’t require light directly but depends entirely on the ATP and NADPH generated in the first stage. The cycle has three phases: fixation, reduction, and regeneration.
During fixation, an enzyme grabs a molecule of CO₂ from the air and attaches it to an existing five-carbon molecule inside the chloroplast. This enzyme, commonly called RuBisCO, is the most abundant protein on Earth, which makes sense given how much carbon fixation the planet performs every day. The result of this reaction is an unstable six-carbon compound that immediately splits into two three-carbon molecules.
In the reduction phase, ATP and NADPH donate their energy to convert those three-carbon molecules into a simple sugar building block. Some of these building blocks exit the cycle to be assembled into glucose, sucrose, starch, and other carbohydrates the plant needs. In the regeneration phase, the remaining molecules are rearranged and recycled, using more ATP, to recreate the original five-carbon molecule so the cycle can grab another CO₂ and start again.
Why Plants Are Green
Chlorophyll absorbs light most strongly at both ends of the visible spectrum. Chlorophyll a has its peak absorption in the red range (around 640 to 690 nanometers), while chlorophyll b absorbs strongly in the blue range (around 460 nanometers). Both types absorb very little green light (500 to 600 nanometers), only a few percent. That unabsorbed green light bounces off or passes through the leaf, which is why plants look green to your eyes.
Plants also contain pigments called carotenoids, which absorb blue and blue-green light (400 to 520 nanometers) and pass that energy along to chlorophyll. Carotenoids are yellow and orange pigments, and they become visible in autumn when chlorophyll breaks down and stops masking them. Together, this mix of pigments allows plants to harvest a broad range of the light spectrum, though the efficiency is still modest. The maximum conversion of solar energy to plant biomass tops out at about 4.6% for most plants and around 6% for certain tropical grasses and crops.
What Controls the Speed of Photosynthesis
Three environmental factors have the biggest influence on how fast photosynthesis runs: light intensity, carbon dioxide concentration, and temperature. At any given moment, whichever factor is in shortest supply acts as a bottleneck. Increasing light will speed up photosynthesis, but only until CO₂ or temperature becomes the limiting factor. Raising CO₂ levels boosts the rate, but only when light and temperature are adequate.
Temperature affects the enzymes that drive the process. Most common plants (called C3 plants, which include wheat, rice, and most trees) photosynthesize best between 18 and 24°C (roughly 65 to 75°F). Beyond that window, enzyme activity drops off. Water availability matters too, since water is both a raw ingredient and essential for keeping leaf pores (stomata) open to let CO₂ in. Nutrient supply, especially nitrogen and phosphorus from the soil, determines how much photosynthetic machinery a plant can build in the first place.
How Some Plants Adapt to Heat and Drought
Not all plants photosynthesize the same way. The standard pathway (C3) works well in cool, moist conditions, but it has a weakness: RuBisCO sometimes grabs oxygen instead of CO₂, triggering a wasteful process called photorespiration that burns energy without producing sugar. In hot, dry climates, this problem gets worse because plants close their leaf pores to conserve water, trapping oxygen inside the leaf.
C4 plants, which include corn, sugarcane, and many tropical grasses, evolved a workaround. They use an extra biochemical step to concentrate CO₂ around RuBisCO, effectively suppressing photorespiration almost entirely. This lets them keep photosynthesizing efficiently even with their stomata partially closed, giving them more than triple the water efficiency of their C3 relatives. C4 plants thrive in hot conditions, with optimal temperatures between 32 and 55°C (90 to 130°F).
CAM plants, including cacti and succulents, take water conservation even further. They open their stomata only at night, when it’s cool and humidity is higher, to collect CO₂. They store it chemically and then run the Calvin cycle during the day with stomata sealed shut. This strategy sacrifices speed for survival, making CAM plants slower growers but remarkably drought-tolerant.
Photosynthesis and Earth’s Oxygen
According to NOAA, roughly half of all oxygen production on Earth comes from the ocean, driven primarily by phytoplankton, drifting algae, and photosynthetic bacteria. One species of ocean cyanobacteria alone produces up to 20% of the oxygen in the biosphere, more than all tropical rainforests combined. The other half comes from terrestrial plants. This means photosynthesis isn’t just a plant process; it’s a planetary system that spans both land and sea, sustaining the atmosphere that makes animal life possible.