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. The overall reaction is straightforward: six molecules of carbon dioxide plus six molecules of water, powered by light, yield one molecule of sugar and six molecules of oxygen.
Where It Happens Inside the Cell
Photosynthesis takes place inside chloroplasts, small green structures found in plant and algae cells. Chloroplasts have their own DNA and a layered internal architecture that keeps different stages of photosynthesis physically separated.
The key structures are the thylakoids, flattened disc-shaped membranes that often stack together like coins into columns called grana. The light-capturing machinery sits embedded in these thylakoid membranes. Surrounding the thylakoids is a fluid-filled space called the stroma, where carbon dioxide gets assembled into sugar. This separation matters: the thylakoid membranes handle light energy and water splitting, while the stroma handles carbon building. A double outer membrane, the chloroplast envelope, wraps around everything but doesn’t directly participate in photosynthesis.
The Light Reactions: Capturing Energy
The first stage of photosynthesis uses sunlight directly. Chlorophyll and other pigments in the thylakoid membranes absorb light and funnel that energy into two protein complexes called photosystem II and photosystem I. Despite the confusing numbering, photosystem II acts first.
Photosystem II uses light energy to split water molecules into oxygen, protons, and high-energy electrons. This is where all the oxygen in photosynthesis comes from. The oxygen escapes as a gas, which is why plants release oxygen into the air. The protons accumulate inside the thylakoid, building up pressure like water behind a dam. That pressure difference drives a molecular turbine that produces ATP, the cell’s main energy currency.
The high-energy electrons, meanwhile, pass through a chain of carrier molecules to photosystem I. There, a second dose of light energy boosts the electrons again, and they’re used to produce a second energy carrier called NADPH. Together, ATP and NADPH are the power supply for the next stage. They carry the captured sunlight energy in chemical form from the thylakoid membranes into the stroma.
The Calvin Cycle: Building Sugar
The second stage doesn’t need light directly, though it depends on the ATP and NADPH that the light reactions produce. It takes place in the stroma and is called the Calvin cycle. Its job is carbon fixation: pulling carbon dioxide out of the air and building it into sugar.
The cycle has three main phases. In the first, an enzyme called rubisco (short for RuBP carboxylase/oxygenase) grabs a molecule of carbon dioxide and attaches it to a five-carbon sugar. The resulting six-carbon compound immediately splits into two three-carbon molecules. Rubisco is the most abundant protein on Earth, which gives you a sense of how central this single reaction is to life.
In the second phase, the ATP and NADPH from the light reactions power the conversion of those three-carbon molecules into a simple sugar. In the third phase, most of that sugar is rearranged to regenerate the five-carbon starting molecule so the cycle can continue. For every three turns of the cycle (three CO₂ molecules fixed), the plant nets one three-carbon sugar molecule that it can use for energy or to build larger molecules like glucose and starch.
How Efficient Is It Really?
Despite being the engine of life on Earth, photosynthesis is not particularly efficient at converting sunlight into stored energy. Most plants use what’s called C3 photosynthesis, which converts a maximum of about 4.6% of incoming solar energy into biomass. Plants that use the C4 pathway do slightly better, reaching about 6%. The rest of the energy is lost as heat, reflected away, or falls outside the wavelengths chlorophyll can absorb.
Those numbers represent theoretical maximums. In real field conditions, most crops convert well under 2% of available sunlight into harvestable biomass. That gap between the theoretical ceiling and actual performance is one reason agricultural researchers are so interested in improving photosynthetic efficiency.
Three Ways Plants Handle Carbon
Not all plants photosynthesize the same way. The differences come down to how they deal with a design flaw in rubisco: the enzyme sometimes grabs oxygen instead of carbon dioxide, triggering a wasteful process called photorespiration that costs the plant energy without producing sugar.
C3 plants, which include wheat, rice, and most trees, use the standard Calvin cycle with no special adaptations. They’re the most common type but suffer the most from photorespiration, especially in hot, dry conditions when they close their pores (stomata) to conserve water, trapping oxygen inside the leaf.
C4 plants, like corn, sugarcane, and many tropical grasses, evolved a workaround. They use a preliminary enzyme to grab carbon dioxide in one set of cells, then shuttle it as a four-carbon acid to a second set of cells where rubisco operates in a CO₂-rich environment. This concentration mechanism largely suppresses photorespiration and gives C4 plants better water efficiency because they can keep their stomata more closed while still getting enough carbon dioxide.
CAM plants, including cacti, succulents, and pineapples, take a different approach entirely. They open their stomata at night to collect carbon dioxide, store it as acid, then close up during the day and release the CO₂ internally for the Calvin cycle. This strategy minimizes water loss in arid environments but limits growth rates.
What Controls the Speed
Three main factors determine how fast photosynthesis runs: light intensity, carbon dioxide concentration, and temperature. At any given moment, whichever factor is in shortest supply acts as the bottleneck.
Light is the most obvious driver. More light means more energy for the light reactions, up to a saturation point where the system can’t process photons any faster. Carbon dioxide typically saturates photosynthesis at around 1,000 parts per million, roughly 2.5 times the current atmospheric level. That’s why commercial greenhouses often pump in extra CO₂ to boost plant growth.
Temperature interacts with both. When CO₂ is low, photosynthesis peaks at moderate temperatures. But when CO₂ is abundant, plants can take advantage of higher temperatures, shifting the optimum upward. This is because the enzymes in the Calvin cycle work faster in warmth, but only if they have enough carbon dioxide to work with.
The Global Oxygen Supply
Photosynthesis produces virtually all of Earth’s free oxygen. What might surprise you is where that oxygen comes from. Roughly half of global oxygen production happens in the ocean, driven by photosynthetic plankton, drifting algae, and cyanobacteria. One species of marine cyanobacterium alone produces up to 20% of all the oxygen in the biosphere, more than all tropical rainforests combined, according to NOAA.
Terrestrial forests, grasslands, and crops account for the other half. Together, these photosynthetic organisms also pull roughly 100 billion tons of carbon out of the atmosphere each year, making photosynthesis the largest single mechanism regulating atmospheric CO₂. Every molecule of oxygen you breathe and virtually every calorie you eat traces back, directly or through a food chain, to a photosynthetic organism capturing sunlight.