Photosynthesis runs on two linked sets of reactions: the light-dependent reactions, which capture sunlight and convert it into chemical energy, and the Calvin cycle (sometimes called the light-independent reactions), which uses that energy to build sugar from carbon dioxide. The overall equation is simple: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Six molecules of carbon dioxide plus six molecules of water yield one molecule of glucose and six molecules of oxygen. But behind that tidy summary lies a surprisingly intricate chain of events.
Where the Reactions Take Place
Both sets of reactions happen inside chloroplasts, the small green organelles in plant cells. A chloroplast has two distinct compartments that matter here. The first is a system of folded internal membranes called thylakoids, stacked like coins into columns. The light-dependent reactions occur along these thylakoid membranes. The second compartment is the stroma, the fluid that fills the space around the thylakoids. The Calvin cycle runs in the stroma, using the energy products that the thylakoid reactions hand off to it.
The Light-Dependent Reactions
The light-dependent reactions do exactly what the name suggests: they require sunlight. Their job is to convert light energy into two chemical energy carriers, ATP and NADPH, while releasing oxygen as a byproduct. Five protein complexes embedded in the thylakoid membrane coordinate this work, but the two most important are Photosystem II and Photosystem I.
Photosystem II: Splitting Water
The process starts at Photosystem II (numbered “II” for historical reasons, even though it acts first). When chlorophyll molecules in this complex absorb photons, the energy is funneled to a reaction center that uses it to split water molecules into oxygen, protons, and high-energy electrons. This water-splitting step is the source of all the oxygen released during photosynthesis. The protons released from water accumulate inside the thylakoid, building up a concentration difference across the membrane that will later drive ATP production, much like water pressure behind a dam drives a turbine.
The high-energy electrons, meanwhile, pass along a chain of carrier molecules to another membrane complex called the cytochrome bf complex. As electrons move through this chain, additional protons are pumped into the thylakoid interior, strengthening the proton gradient. A dedicated enzyme then lets those protons flow back out through it, harvesting their energy to assemble ATP.
Photosystem I: Making NADPH
From the cytochrome bf complex, electrons travel to Photosystem I. Here, a second round of light absorption re-energizes the electrons. But instead of pumping more protons, Photosystem I channels those electrons to a small protein on the outer side of the thylakoid membrane, which passes them to an enzyme that combines them with protons and a molecule called NADP⁺ to produce NADPH. Think of NADPH as a rechargeable battery loaded with electrons, ready to power the sugar-building reactions in the stroma.
Together, the two photosystems generate both ATP (energy currency) and NADPH (electron carrier). These two molecules are the entire energy output of the light reactions, and the Calvin cycle depends on a steady supply of both.
The Calvin Cycle
The Calvin cycle takes carbon dioxide from the air and, using the ATP and NADPH produced by the light reactions, constructs three-carbon sugar molecules that the plant can later assemble into glucose, starch, or other carbohydrates. The cycle has three stages.
Carbon Fixation
In the first stage, CO₂ is attached to a five-carbon molecule called RuBP. The enzyme responsible for this step, called Rubisco, is the most abundant protein on Earth. There are roughly 5 kilograms of it for every person alive, and it fixes more than 90% of all the inorganic carbon that ends up in living biomass. Despite its importance, Rubisco is remarkably slow, processing only about 1 to 10 molecules per second. It also makes mistakes, sometimes grabbing oxygen instead of CO₂, which triggers an energy-wasting side process called photorespiration that can reduce a plant’s overall photosynthetic output by 20% to 50%.
When Rubisco does grab CO₂ successfully, the six-carbon product immediately splits into two three-carbon molecules called 3-PGA. This is why standard photosynthesis is called “C3” photosynthesis: the first stable product has three carbons.
Reduction
In the second stage, ATP and NADPH donate their stored energy to convert 3-PGA into another three-carbon molecule called G3P. This is where the energy captured from sunlight actually gets locked into chemical bonds. Some of the G3P molecules exit the cycle and become the raw material for glucose and other sugars the plant needs for growth and energy storage.
Regeneration
Most of the G3P molecules, however, stay in the cycle. In this final stage, they are rearranged, with additional ATP input, to regenerate RuBP so the cycle can grab another round of CO₂. Without this regeneration step, the cycle would stall after a single turn.
How C4 and CAM Plants Modify the Reactions
About 85% of plant species use the standard C3 pathway, but some plants have evolved workarounds for Rubisco’s tendency to grab oxygen. C4 plants, which include corn, sugarcane, and many tropical grasses, separate carbon fixation into two cell types. In one set of cells, CO₂ is first captured by a different, faster enzyme and converted into a four-carbon compound (hence “C4”). That compound is shuttled to a second set of cells, where it releases CO₂ directly to Rubisco at high concentration. This built-in CO₂-concentrating mechanism suppresses photorespiration and makes C4 plants more efficient in hot, sunny conditions. Their optimal temperature range is 32–55 °C, compared to 18–24 °C for C3 plants.
CAM plants, such as cacti and succulents, use a similar chemical trick but separate the steps by time rather than by location. They open their pores at night to collect CO₂ (minimizing water loss in desert heat), store it as an organic acid, and then release it internally during the day for Rubisco to use. The light reactions and the Calvin cycle themselves are unchanged in both C4 and CAM plants. Only the delivery route for CO₂ differs.
What Controls the Speed of Photosynthesis
Three environmental factors most strongly affect how fast these reactions run: light intensity, CO₂ concentration, and temperature. Increasing any one of them speeds up photosynthesis, but only up to a point, and only if the other two are not already limiting. Bright light accelerates the light-dependent reactions, producing more ATP and NADPH. Higher CO₂ concentrations give Rubisco more substrate and reduce its error rate. Warmer temperatures speed enzyme activity until the optimum is exceeded, after which enzymes begin to lose their shape and efficiency drops sharply.
Even under ideal conditions, photosynthesis converts only a small fraction of the light energy it absorbs. The maximum quantum yield for C3 leaves is about 0.095 molecules of CO₂ fixed per absorbed photon, measured at red wavelengths where efficiency peaks. That low ceiling is one reason plants need so much Rubisco and so much leaf surface area to support their growth.
How the Two Reaction Sets Connect
The light reactions and the Calvin cycle are not independent processes that happen to share a compartment. They are tightly coupled through ATP and NADPH. The Calvin cycle consumes these molecules as fast as the light reactions produce them, and the spent forms (ADP and NADP⁺) cycle back to the thylakoid to be recharged. If light suddenly drops, ATP and NADPH production slows, and the Calvin cycle slows with it. If CO₂ drops, the Calvin cycle stalls, NADPH accumulates without being used, and the light reactions back up as well. This feedback loop keeps the two halves of photosynthesis in balance and prevents the buildup of reactive molecules that could damage the chloroplast.