Photosynthesis converts sunlight, water, and carbon dioxide into sugar and oxygen through a chain of reactions that can be grouped into seven main steps: light absorption, water splitting, electron transport, NADPH production, ATP generation, carbon fixation, and the regeneration of the starting molecule that keeps the cycle running. These steps span two linked stages, one powered directly by light and one that uses the chemical energy light produced, all happening inside a single organelle called the chloroplast.
Where It All Happens
Before walking through the steps, it helps to know the layout. Inside every leaf cell sit chloroplasts, and inside each chloroplast is a system of folded internal membranes called thylakoids. In higher plants, many of these thylakoids are stacked into columns called grana. The two major protein complexes that capture light, Photosystem II and Photosystem I, sit in different parts of these membranes: Photosystem II concentrates in the stacked grana regions, while Photosystem I sits mainly in the unstacked membranes that connect the stacks. Electron carriers must navigate between these two locations, threading through a somewhat crowded, winding path.1PubMed Central. Granal thylakoid structure and function: explaining an enduring mystery of higher plants The fluid-filled space surrounding the thylakoids, called the stroma, is where the sugar-building reactions of the Calvin cycle take place. This physical separation matters because it creates compartments that let the plant build up a concentration of protons on one side of the membrane, which drives ATP production.
Step 1: Light Absorption
Everything starts when pigment molecules, primarily chlorophyll, absorb photons of light. Chlorophyll absorbs red and blue wavelengths strongly and reflects green, which is why leaves look green to us. But chlorophyll does not work alone. Surrounding each photosystem is an antenna complex made of hundreds of pigment molecules, including accessory pigments like carotenoids that capture wavelengths chlorophyll misses. When any pigment in the antenna absorbs a photon, the energy hops rapidly from molecule to molecule until it reaches a special pair of chlorophyll molecules at the reaction center. In Photosystem II membranes, this energy transfer and the initial charge separation happen remarkably fast, on the order of about 150 picoseconds.2PubMed Central. Excitation energy transfer and charge separation in photosystem II membranes revisited That speed matters because it means the captured energy is funneled into useful chemistry before it can be lost as heat or fluorescence.
Step 2: Water Splitting
Once the reaction center of Photosystem II is energized, it ejects a high-energy electron. That electron needs to be replaced, and this is where water comes in. A cluster of manganese, calcium, and oxygen atoms at the heart of Photosystem II pulls electrons from water molecules, splitting them into protons, electrons, and molecular oxygen. This is the source of nearly all the oxygen in Earth’s atmosphere. What makes this reaction remarkable is that it extracts four electrons from two water molecules cleanly, producing oxygen gas without releasing dangerous intermediates like hydrogen peroxide or free radicals in significant amounts.3PubMed Central. Water oxidation chemistry of photosystem II The protons released during water splitting are dumped into the thylakoid interior, contributing to the proton gradient that will later power ATP synthesis.
Step 3: The Electron Transport Chain
The high-energy electron that left Photosystem II does not go directly to the sugar-building machinery. Instead, it passes through a series of protein carriers embedded in the thylakoid membrane. The central player here is the cytochrome b6f complex, which sits between Photosystem II and Photosystem I and acts as a relay station. It accepts electrons from a mobile carrier called plastoquinol and hands them off to another carrier called plastocyanin, which ferries them to Photosystem I.4Biochimica et Biophysica Acta (BBA) – Bioenergetics. Cytochrome b6f – Orchestrator of photosynthetic electron transfer
This relay is not just a passive handoff. As electrons move through the cytochrome b6f complex, the complex actively pumps protons from the stroma into the thylakoid interior.5PubMed. The Cytochrome b6f Complex: Biophysical Aspects of Its Functioning in Chloroplasts A mechanism called the Q-cycle doubles the number of protons moved per electron, strengthening the proton gradient.4Biochimica et Biophysica Acta (BBA) – Bioenergetics. Cytochrome b6f – Orchestrator of photosynthetic electron transfer Think of the electron transport chain as simultaneously doing two jobs: passing electrons toward their final destination and stockpiling the energy needed to make ATP.
Step 4: NADPH Production
When the electron arrives at Photosystem I, it gets a second energy boost from another photon of light, pushing it to an even higher energy level. From there, it is passed to a small iron-sulfur protein called ferredoxin, which carries it to the enzyme ferredoxin-NADP+ reductase. This enzyme uses the electrons, along with a proton from the stroma, to reduce NADP+ into NADPH.6PubMed. Photo-induced electron transfer from photosystem I to NADP(+): characterization and tentative simulation of the in vivo environment NADPH is essentially a biological reducing agent, a molecule loaded with electrons and ready to donate them to carbon dioxide during sugar construction. Together with ATP, NADPH is one of the two chemical currencies the light reactions produce for the Calvin cycle to spend.
Step 5: ATP Generation
By this point, the thylakoid interior has accumulated a heavy concentration of protons from two sources: the water-splitting reaction and the proton-pumping activity of the cytochrome b6f complex. This creates a steep electrochemical gradient across the thylakoid membrane, with protons eager to flow back out into the stroma. They can only escape through a specialized protein channel called ATP synthase. As protons stream through ATP synthase, they spin a rotor-like subunit that physically forces ADP and phosphate together to form ATP.7PubMed Central. Proton gradient across the chloroplast thylakoid membrane governs the redox regulatory function of ATP synthase The whole process is driven entirely by the energy stored in the proton gradient.8Physiologia Plantarum. New results about structure, function and regulation of the chloroplast ATP synthase (CF0CF1)
With Steps 1 through 5 complete, the light-dependent reactions have produced their outputs: oxygen released into the air, and ATP and NADPH ready for use. The remaining steps belong to the Calvin cycle, which runs in the stroma and does not require light directly, though it depends entirely on the ATP and NADPH that light provides.
Step 6: Carbon Fixation
The Calvin cycle begins when an enzyme called RuBisCO grabs a molecule of carbon dioxide from the air and attaches it to a five-carbon sugar called ribulose-1,5-bisphosphate, or RuBP. This reaction produces two molecules of a three-carbon compound. RuBisCO is one of the most abundant proteins on Earth, present in massive quantities in every photosynthetic organism, partly because it works slowly and the plant compensates by making a lot of it.9PubMed Central. A short history of RubisCO: the rise and fall (?) of Nature’s predominant CO2 fixing enzyme This carbon-fixation step is where inorganic carbon dioxide is first incorporated into an organic molecule. It is, in a sense, the whole point of the process: every carbon atom in every sugar, fat, and protein in a plant traces back to a CO2 molecule that RuBisCO grabbed.
Step 7: Reduction and RuBP Regeneration
After carbon fixation, the three-carbon molecules are converted into a simple sugar (glyceraldehyde-3-phosphate, or G3P) using the ATP and NADPH produced by the light reactions. Some of this G3P exits the cycle to be assembled into glucose and other carbohydrates the plant needs. But most of it, roughly five out of every six molecules, stays in the cycle and is rearranged through a series of enzymatic steps to regenerate RuBP, the five-carbon molecule that RuBisCO needs to fix another CO2. The last three enzymes in this regeneration sequence convert five-carbon sugars back into RuBP, priming the cycle for another turn.10Frontiers in Plant Science. Ribulose-1,5-bisphosphate regeneration in the Calvin-Benson-Bassham cycle: Focus on the last three enzymatic steps that allow the formation of Rubisco substrate Without this regeneration, the cycle would stall after a single round, and the plant could not continue fixing carbon.
This regeneration step is often the bottleneck when conditions change. If ATP or NADPH supply dips because light intensity drops, RuBP regeneration slows, limiting the rate of the entire Calvin cycle. The balance between supply and demand at this stage is one of the key regulators of how fast a plant can photosynthesize.
RuBisCO’s Costly Mistake
RuBisCO has a well-known flaw: it sometimes grabs oxygen instead of carbon dioxide. When that happens, the plant enters a pathway called photorespiration that releases previously fixed carbon as CO2 without producing useful sugar. This side reaction can reduce a plant’s overall carbon gain by more than 25%.11PubMed Central. Photorespiration: The Futile Cycle? Because the oxygenation reaction wastes energy and releases carbon, researchers have long viewed photorespiration as a drag on productivity and have invested significant effort into understanding whether it can be reduced.12PubMed. Photorespiration in the context of Rubisco biochemistry, CO2 diffusion and metabolism
That said, recent work suggests photorespiration is not purely wasteful. It may help protect the plant from damage caused by excess light energy and play roles in nitrogen metabolism. Still, for crop yields, it remains one of the biggest inefficiencies in photosynthesis, and it gets worse as temperatures rise because RuBisCO’s preference for oxygen over carbon dioxide increases with heat.
How Some Plants Sidestep Photorespiration
Not all plants accept this inefficiency passively. Corn, sugarcane, and many tropical grasses use a strategy called C4 photosynthesis. They pre-fix carbon dioxide in an outer ring of cells using a different enzyme that does not react with oxygen, then concentrate the CO2 around RuBisCO in an inner set of cells called bundle sheath cells. The high CO2 concentration in these cells allows fast carbon fixation while keeping photorespiration to a minimum.13Australian Journal of Plant Physiology. Analysis of Bundle Sheath Conductance and C4 Photosynthesis using a PEP-Carboxylase Inhibitor This is why C4 crops tend to thrive in hot, sunny conditions where C3 plants (like wheat and rice) struggle with photorespiratory losses.
Desert and drought-adapted plants, like cacti and agaves, use yet another workaround called CAM (Crassulacean Acid Metabolism). These plants open their pores at night to take in CO2, storing it as an organic acid, and then release it internally during the day when the pores are closed. In obligate CAM plants, up to 99% of carbon dioxide uptake happens at night, dramatically reducing water loss because nighttime air is cooler and less drying.14Annals of Botany. Crassulacean acid metabolism and fitness under water deficit stress: if not for carbon gain, what is facultative CAM good for? Some plants can switch between normal C3 photosynthesis and CAM depending on how dry conditions get, though in these facultative species, the nighttime carbon fixation typically accounts for only about 11% of what the plant would fix under well-watered conditions.14Annals of Botany. Crassulacean acid metabolism and fitness under water deficit stress: if not for carbon gain, what is facultative CAM good for?
Too Much Light Is Also a Problem
You might assume that more sunlight always means more photosynthesis, but plants saturate. Beyond a certain light intensity, the photosystems cannot use all the incoming energy, and the excess can damage proteins and membranes. To prevent this, plants have a safety valve called nonphotochemical quenching, or NPQ. When excess light hits the antenna complexes around Photosystem II, the plant can harmlessly dump the extra energy as heat rather than letting it generate destructive reactive oxygen species.15PubMed Central. Manipulation of Photoprotection to Improve Plant Photosynthesis – Section: NONPHOTOCHEMICAL QUENCHING Excited chlorophyll molecules return to their ground state through one of three routes: driving photochemistry, emitting fluorescence, or dissipating heat through NPQ.16aBIOTECH. Review Strategies for adaptation to high light in plants – Section: Thermal energy dissipation
When light conditions shift suddenly, say a cloud passes and then the sun hits full strength again, the plant ramps up NPQ and triggers expression of protective genes to prevent damage.17PubMed Central. Transcriptional regulation of photoprotection in dark-to-light transition-More than just a matter of excess light energy The downside is that NPQ can be slow to turn off once the light dims again. While the plant is still dissipating energy as heat after a cloud returns, it is wasting perfectly usable photons. This sluggish recovery has attracted attention from crop scientists because speeding up NPQ relaxation could improve the total amount of carbon a field crop fixes over a fluctuating day.
Where Chloroplasts Came From
The photosynthetic machinery inside a plant cell was not invented by plants. Chloroplasts descend from ancient cyanobacteria that were engulfed by a larger cell more than a billion years ago in an event called primary endosymbiosis.18Acta Societatis Botanicorum Poloniae. Primary endosymbiosis: have cyanobacteria and Chlamydiae ever been roommates? Over evolutionary time, the engulfed cyanobacterium lost most of its independence, transferring the majority of its genes to the host cell’s nucleus, but it retained its own small genome and its double membrane, both of which are relics of that ancient swallowing event. The evidence for this origin is strong: chloroplasts share key proteins involved in oxygenic photosynthesis and gene expression with modern cyanobacteria.19PubMed Central. Are Cyanobacteria an Ancestor of Chloroplasts or Just One of the Gene Donors for Plants and Algae? This single event gave rise to all green algae, land plants, red algae, and glaucophytes, a group collectively known as the Archaeplastida.20PubMed Central. Origin and evolution of plastids and photosynthesis in eukaryotes
Other photosynthetic organisms, like brown algae and many single-celled plankton, got their chloroplasts secondhand by swallowing a cell that already had one, an event called secondary endosymbiosis. This layered history explains why some algal chloroplasts have three or even four surrounding membranes instead of the two found in plant chloroplasts.
Artificial Photosynthesis and Solar Fuels
Engineers have been trying to mimic nature’s trick for decades. Artificial photosynthesis systems aim to use sunlight to split water and reduce carbon dioxide into fuels, borrowing the same basic logic of light harvesting, charge separation, and catalytic chemistry.21Inorganic Chemistry Communications. Artificial photosynthesis: Emerging strategies for solar-to-fuel conversion One hybrid approach combined inorganic water-splitting catalysts with the bacterium Ralstonia eutropha, which consumed the hydrogen produced by the catalysts and used it, along with atmospheric CO2, to build biomass and liquid fuels. The system achieved a CO2 reduction energy efficiency of roughly 50% when making biomass, scrubbing about 180 grams of CO2 per kilowatt-hour of electricity.22PubMed. Water splitting-biosynthetic system with COâ‚‚ reduction efficiencies exceeding photosynthesis That efficiency actually exceeds what natural photosynthesis achieves, though scaling such systems up from a laboratory bench to an industrial process remains a major challenge.
The appeal of artificial photosynthesis is not just theoretical elegance. If you could produce hydrogen or liquid carbon-based fuels using only sunlight, water, and CO2, you would have a carbon-neutral energy source that sidesteps the intermittency problem of solar panels by storing energy chemically rather than electrically. Current research focuses on finding catalysts that are cheap, durable, and efficient enough to make this practical.23Bulletin of the Chemical Society of Japan. Recent Improvements in the Production of Solar Fuels: From CO2 Reduction to Water Splitting and Artificial Photosynthesis Most prototype systems still rely on rare or expensive materials, and durability under real-world conditions is an ongoing struggle.