Photosynthesis is how a plant turns sunlight, water and carbon dioxide from the air into sugar, giving off oxygen as a by-product. It runs as two linked stages: one captures light energy, and the other spends that energy building sugar. Land plants and ocean algae together produce roughly equal shares of the new plant material made on Earth each year.1PubMed. Primary production of the biosphere: integrating terrestrial and oceanic components The same process feeds almost every food chain, including the crops on your plate.
Where Photosynthesis Happens Inside a Leaf
Inside leaf cells sit chloroplasts, small green compartments where photosynthesis takes place. They are packed with chlorophyll, the green pigment that absorbs red and blue light and reflects green. Grow lights often glow pink-purple because they supply the red and blue light the plant actually uses. A single leaf cell can hold dozens of chloroplasts, and they crowd near the upper surface of the leaf, where light is strongest. Leaves are thin and flat to catch light over a large surface, and the crowding of chloroplasts near the top is why many leaves look greener on top.
Chloroplasts began as free-living, light-using bacteria that an ancient single cell swallowed but did not digest.2Nature Education. The Origin of Plastids Over generations the captured bacterium shrank into a working part of the cell, wrapped in two membranes.3Nature Education. The Origin of Plastids This capture happened once, in the shared ancestor of red algae, green algae and land plants.4PubMed Central. The endosymbiotic origin, diversification and fate of plastids
Inside each chloroplast are stacks of flat membrane sacs where light is captured. The fluid around those stacks is where sugar is built.
Stage One, Turning Light Into Chemical Energy
Chlorophyll is arranged in antenna-like clusters that pass absorbed light energy from pigment to pigment toward a central spot.5The Journal of Physical Chemistry B. Understanding the Energy Transfer Function of LHCII, the Major Light-Harvesting Complex of Green Plants Each hand-off between chlorophylls happens in under a trillionth of a second.6PubMed. How the molecular structure determines the flow of excitation energy in plant light-harvesting complex II Helper clusters make the hand-off about twice as fast, which wastes less energy.7PubMed Central. The role of light-harvesting complex I in excitation energy transfer from LHCII to photosystem I in Arabidopsis
The energy reaches a reaction center, a special chlorophyll pair that releases an electron to a neighbouring molecule.8PubMed Central. Charge separation in the photosystem II reaction center resolved by multispectral two-dimensional electronic spectroscopy The separation of charge takes only about 3 trillionths of a second,9PubMed Central. Determination of the primary charge separation rate in isolated photosystem II reaction centers with 500-fs time resolution so light is turned into stored electrical energy almost instantly.
The reaction center must then replace its lost electron, and it takes one from water. A cluster of manganese and calcium atoms holds water and builds up four charges before splitting it.10PubMed. Quantum mechanics/molecular mechanics study of the catalytic cycle of water splitting in photosystem II Splitting water releases oxygen, the oxygen we breathe.
The energy ends up in two carriers. Electrons move through a chain linking two photosystems, a route often drawn as a Z.11PubMed Central. Photosynthesis: basics, history and modelling Each photosystem gives the electrons a boost, so the Z is really two lifts in a row. Between the two photosystems the electrons run downhill through a chain of carriers, and that downhill run pays for pumping protons into the membrane sacs.
The proton flow back out drives a turbine-like enzyme that makes ATP.12PubMed Central. Proton gradient across the chloroplast thylakoid membrane governs the redox regulatory function of ATP synthase The main flow makes both ATP and NADPH, the plant’s two energy carriers.13PubMed. Physiological Functions of Cyclic Electron Transport Around Photosystem I in Sustaining Photosynthesis and Plant Growth A side loop makes only ATP, letting the plant top up whichever carrier runs short.14PubMed. Physiological Functions of Cyclic Electron Transport Around Photosystem I in Sustaining Photosynthesis and Plant Growth
Stage Two, Building Sugar From Air
The sugar-building stage, the Calvin cycle, has three steps: capturing carbon dioxide, converting it to sugar pieces, and rebuilding the starting molecule.15PubMed Central. Regulation of Calvin-Benson cycle enzymes under high temperature stress The capture step is done by Rubisco, the carbon-grabbing enzyme.16PubMed Central. Regulation of Calvin-Benson cycle enzymes under high temperature stress Once Rubisco attaches carbon dioxide, the reaction is essentially one-way and does not run backward.17ACS Catalysis. Carbon Dioxide Fixation in RuBisCO Is Protonation-State-Dependent and Irreversible
Rubisco attaches each carbon dioxide molecule to a five-carbon carrier, producing a six-carbon intermediate that immediately splits into two three-carbon pieces. Those pieces are loaded with energy from ATP and electrons from NADPH, which converts them into three-carbon sugar units.18PubMed Central. Regulation of Calvin-Benson cycle enzymes under high temperature stress Because each turn adds only one carbon from the air, the plant needs three turns to bring in enough carbon for one net three-carbon unit that can leave the cycle. Most of the units are rearranged to rebuild the five-carbon carrier, which lets the cycle restart.19PubMed Central. Regulation of Calvin-Benson cycle enzymes under high temperature stress
This stage is often called the “dark” stage, which is misleading. It doesn’t need light directly, but it depends on the energy carriers made in the light, so in most plants it largely stops at night.
The spare sugar units are made into glucose, sucrose, starch and cellulose. Sucrose travels to roots, fruits and growing tips. Starch is stored in leaves for nighttime use, and cellulose builds cell walls and wood. Plants also burn some sugar for their own energy. The wood, fruit and grain we use are built from carbon pulled from the air.
Photosynthesis’s Built-In Flaw
Rubisco sometimes reacts with oxygen instead of carbon dioxide, making a toxic compound.20PubMed. The Costs of Photorespiration to Food Production Now and in the Future The plant must recycle it, which uses energy and releases some already-captured carbon as carbon dioxide.21PubMed Central. Photorespiration In ordinary crops like wheat and rice, this can cut yields by as much as 50% under severe conditions.22PubMed. Optimizing photorespiration for improved crop productivity
Carbon dioxide and oxygen compete for the same site on Rubisco. The ratio of the two gases around it decides how often each reaction occurs, and more carbon dioxide inside the leaf means fewer mistakes. The error occurs even in cool, well-watered plants.
Heat makes it worse, since hot weather favours the oxygen reaction. Closed leaf pores let oxygen build up inside the leaf too, so hot, dry spells hit most crops twice.
How Corn and Cacti Do It Differently
In C4 plants such as corn and sugarcane, outer leaf cells first grab carbon dioxide with a different enzyme. They pack it into a four-carbon molecule that moves to inner cells around the leaf veins.23Wikipedia. C4 carbon fixation There it is released, flooding Rubisco with carbon dioxide and suppressing the oxygen mistake.24Wikipedia. C4 carbon fixation This relies on a special ring-shaped leaf layout around the veins.25PubMed. Deconstructing Kranz anatomy to understand C4 evolution
CAM plants, common among desert succulents, tropical air plants and some water plants, take in carbon dioxide at night.26International Journal of Plant Sciences. Evolution of CAM and C 4 Carbon‐Concentrating Mechanisms They can keep their pores shut in the heat of the day and lose far less water. CAM uses similar chemistry to C4 but separates steps in time rather than in space.27International Journal of Plant Sciences. Evolution of CAM and C 4 Carbon‐Concentrating Mechanisms
You can spot these plants at home. Corn, sugarcane and many lawn grasses are C4, while cacti, jade plant, aloe and pineapple use CAM. Most vegetables, trees and wheat use the ordinary route. C4 plants thrive in hot sun, while plants using the ordinary route generally do better in cooler seasons.
What Slows Photosynthesis, Drought and Harsh Light
Leaves take in carbon dioxide through tiny pores called stomata. Each pore is flanked by two guard cells that swell with water to open it and lose water to close it. Light normally signals the pores to open at daybreak, so carbon dioxide enters when energy is available to use it. In drought, a stress hormone signals the pores to close, saving water but blocking carbon dioxide.28PubMed Central. Impacts of Drought on Photosynthesis in Major Food Crops and the Related Mechanisms of Plant Responses to Drought
Pore closure is the first and main brake on photosynthesis in mild to moderate drought.29PubMed Central. Drought‐inhibition of Photosynthesis in C3 Plants: Stomatal and Non‐stomatal Limitations Revisited In severe drought the sugar-building machinery itself breaks down, which becomes the bigger limit.30PubMed Central. Drought‐inhibition of Photosynthesis in C3 Plants: Stomatal and Non‐stomatal Limitations Revisited In cotton under severe drought, pore opening fell by about half and photosynthesis by about a quarter.31PubMed Central. Leaf hydraulic decline coordinates stomatal and photosynthetic limitations through anatomical adjustments under drought stress in cotton
Light is a hazard too. Light beyond what a plant can use can damage its machinery,32PubMed Central. Nonphotochemical quenching in plants: Mechanisms and mysteries so plants switch on a fast safety system that sheds extra energy as heat.33PubMed Central. Nonphotochemical quenching in plants: Mechanisms and mysteries
Nutrients matter as well. On poor soil, photosynthesis is held back most by whichever nutrient is scarcest.34PubMed. Nutrient limitations on photosynthesis: from individual to combinational stresses
In the garden, midday wilting can mean the pores have closed. Yellowing older leaves often hint at a nitrogen shortage, and bleached patches after moving a plant into sun signal light stress. Watering deeply in the morning gets water in before heat closes the pores. Move shade plants into sun gradually over a week or two, and test soil before fertilizing to find the limiting nutrient.
How Shade Plants Make the Most of Dim Light
Deep-shade plants absorb about as much usable light as sun plants.35American Journal of Botany. CORRELATES OF LEAF OPTICAL PROPERTIES IN TROPICAL FOREST SUN AND EXTREME‐SHADE PLANTS They do it with thinner leaves and less chlorophyll.36American Journal of Botany. CORRELATES OF LEAF OPTICAL PROPERTIES IN TROPICAL FOREST SUN AND EXTREME‐SHADE PLANTS Their chloroplasts line the cells in a dense layer that catches light more efficiently.37American Journal of Botany. CORRELATES OF LEAF OPTICAL PROPERTIES IN TROPICAL FOREST SUN AND EXTREME‐SHADE PLANTS Some forest-floor climbers succeed by catching dim light efficiently rather than racing toward the sun.38PubMed Central. Climbing plants in a temperate rainforest understorey: searching for high light or coping with deep shade?
Plants That Gave Up Photosynthesis
Some parasitic flowering plants have lost the ability to photosynthesize, and their chloroplast genes have shrunk dramatically.39PubMed. Photosynthetic evolution in parasitic plants: insight from the chloroplast genome In one fully parasitic plant, all genes for both photosystems and for Rubisco are gone.40PubMed Central. The loss of photosynthesis pathway and genomic locations of the lost plastid genes in a holoparasitic plant Aeginetia indica Such plants live entirely on sugar taken from a host’s roots.
They are easy to spot because they often look pale, brown, orange or pink. Dodder twines over garden plants like orange string.
Can Scientists Improve Photosynthesis
Scientists have added gene shortcuts to rice that recycle the oxygen mistake inside the chloroplast.41PubMed. A Synthetic Photorespiratory Shortcut Enhances Photosynthesis to Boost Biomass and Grain Yield in Rice These raise carbon dioxide around Rubisco and boost photosynthesis.42PubMed. A Synthetic Photorespiratory Shortcut Enhances Photosynthesis to Boost Biomass and Grain Yield in Rice One such shortcut raised rice yields by 22% to 35% in paddy fields.43Crop Journal. A synthetic glycolate metabolism bypass in rice chloroplasts increases photosynthesis and yield Gains from another version varied by planting season and were largest in strong light.44PubMed. Engineering a New Chloroplastic Photorespiratory Bypass to Increase Photosynthetic Efficiency and Productivity in Rice
Artificial systems are advancing too. A lab system pairs water-splitting catalysts with bacteria to turn carbon dioxide into fuel.45PubMed. Water splitting-biosynthetic system with CO₂ reduction efficiencies exceeding photosynthesis Run on solar panels, it would convert about 10% of sunlight’s energy, more than natural plants.46PubMed. Water splitting-biosynthetic system with CO₂ reduction efficiencies exceeding photosynthesis