Plants build their food out of thin air. They combine carbon dioxide from the air with water, use sunlight as the energy source, and end up with sugar. The oxygen we breathe is a leftover of that process, and because animals eat plants or eat animals that did, the same process feeds nearly every food chain on Earth. It isn’t perfectly efficient, though, and that weak spot shapes which plants thrive in hot, dry places and how scientists hope to grow more food.
Most of a Plant Comes From Air and Water, Not Soil
For a long time, people assumed plants ate soil. In the 1600s, a willow grew from about 2.3 kg to nearly 68 kg in five years while its soil lost only 57 grams.1Biology LibreTexts. How Plants Make Food: The Process of Photosynthesis Then in 1772, a sealed jar showed that a mouse or a candle “used up” the air, but adding a mint sprig “revived” it.2Biology LibreTexts. How Plants Make Food: The Process of Photosynthesis Later work found that plants revive air only in daylight, take in carbon dioxide, and give off a separate gas, oxygen.3Biology LibreTexts. How Plants Make Food: The Process of Photosynthesis
Soil does matter, but it supplies water and small amounts of minerals like nitrogen, not bulk food. Fertilizer is more like vitamins than meals.
The recipe itself is simple: plants combine carbon dioxide and water, powered by light, to make sugar and release oxygen. The work happens in chloroplasts, small green compartments packed into leaf cells. Chloroplasts descend from free-living bacteria called cyanobacteria that were taken in by an ancient cell.4PubMed Central. Genomics and chloroplast evolution: what did cyanobacteria do for plants? Like cyanobacteria today, they make food from light while releasing oxygen.5PubMed Central. Are Cyanobacteria an Ancestor of Chloroplasts or Just One of the Gene Donors for Plants and Algae
Once the sugar is made, the plant puts it to work. It burns some for energy, day and night, so part of what photosynthesis produces pays the plant’s own running costs. Leftover sugar is linked into starch and stored, which is why potatoes and grains are packed with it. Sugar is also built into cellulose, the tough material of stems, wood and cell walls, so most of a tree’s bulk is rebuilt air.
Why Leaves Are Green and How They Catch Light
Light is a mix of colors, and the leaf’s pigments pick among them. The green pigment, chlorophyll, comes in two forms that together absorb mainly red and blue-violet light.6Biology LibreTexts. 8.2: Light-Dependent Reactions of Photosynthesis Neither absorbs green, so green light is reflected back to your eyes and leaves look green.7Biology LibreTexts. 8.2: Light-Dependent Reactions of Photosynthesis Orange and yellow pigments absorb blue-green and violet light and reflect yellow, orange and red.8Biology LibreTexts. 8.2: Light-Dependent Reactions of Photosynthesis When the green pigment breaks down in autumn, those yellows and oranges show through.
Leaves are also built to trap light. Inside the leaf’s food-making compartments, light-catching membranes are folded and stacked like piles of coins.9PubMed. Granal stacking of thylakoid membranes in higher plant chloroplasts: the physicochemical forces at work and the functional consequences that ensue The stacks pack a huge membrane area into a tiny space, which boosts light capture, and they help plants work well in both full sun and canopy shade.10PubMed. Granal stacking of thylakoid membranes in higher plant chloroplasts: the physicochemical forces at work and the functional consequences that ensue
You can read these rules in leaves around the house. Houseplants lean toward a window because they grow toward the light, bringing more leaf area into the brightest spot. Leaves grown in shade are often larger and darker green, a way of catching more of the scarce light that reaches them. Red- or purple-leaved plants still contain the green pigment, but other pigments mask it, so they aren’t exceptions to the rule.
Grow lights follow the same logic. Plants absorb red and blue light most strongly, so many grow lights supply mostly those colors. With little green included, the light looks pink or purple to us, even though the plants are using it efficiently.
Step One, Light Splits Water and Makes Oxygen
The first stage runs on light. Light energy drives a protein machine that splits water into oxygen, hydrogen particles and electrons.11PubMed Central. Water oxidation chemistry of photosystem II At its heart sits a tiny cluster of manganese and calcium atoms that pulls the water apart.12PubMed. Electronic structure of the oxygen-evolving complex in photosystem II prior to O-O bond formation That means the oxygen plants release comes from water, not from carbon dioxide. The splitting machine does all this without making much of the harmful reactive oxygen chemicals such reactions usually produce.13PubMed Central. Water oxidation chemistry of photosystem II
The freed electrons then travel along a chain of proteins, a bit like current through a wire.14PubMed. Regulation of photosynthetic electron transport As they move, hydrogen particles are pumped into a compartment, building up pressure like water behind a dam.15PubMed. Regulation of photosynthetic electron transport Flowing back out, those particles spin a tiny motor that makes ATP, the cell’s energy currency.16PubMed. Balancing the central roles of the thylakoid proton gradient The electrons finally load a second carrier, NADPH, which carries building power for making sugar.17Wikipedia. Calvin cycle When light stops, ATP and NADPH aren’t replenished and the supply runs down, so sugar-building slows soon after.
Step Two, Building Sugar From Carbon Dioxide
In the Calvin cycle, the plant spends the ATP and NADPH from the light stage to turn carbon dioxide into sugar.18Wikipedia. Calvin cycle An enzyme called Rubisco grabs carbon dioxide and attaches it to a carbon-carrying molecule.19Annual Review of Plant Biology. The Costs of Photorespiration to Food Production Now and in the Future
The cycle rebuilds its own starting molecule, so it keeps going without being restocked. Each turn attaches one carbon atom from carbon dioxide, so a single turn doesn’t make a finished sugar. Several turns are needed to build one sugar molecule, which is why the cycle runs continuously in daylight.
Rubisco makes a costly mistake, though. It sometimes grabs oxygen instead of carbon dioxide.20Annual Review of Plant Biology. The Costs of Photorespiration to Food Production Now and in the Future That makes a toxic by-product the plant must clean up, a process called photorespiration that you’ll see named in crop research.21Annual Review of Plant Biology. The Costs of Photorespiration to Food Production Now and in the Future The cleanup recovers most of the carbon but releases some as carbon dioxide and uses up energy.22Annual Review of Plant Biology. The Costs of Photorespiration to Food Production Now and in the Future Photorespiration is widely seen as wasteful.23PubMed. Photorespiration in the context of Rubisco biochemistry, CO(2) diffusion and metabolism The mistake gets more common in heat and when leaf pores close, which lets oxygen build up inside the leaf.
In a garden, many familiar crops, including wheat, rice, beans and tomatoes, lose some of their output this way. The loss grows during hot, dry spells. Regular watering helps leaf pores stay open so carbon dioxide keeps flowing in, which gives the carbon-grabbing enzyme more of the right target.
How Corn and Cacti Work Around the Problem
Leaves take in carbon dioxide through tiny pores, but water escapes through them too.24PubMed. Predicting stomatal responses to the environment from the optimization of photosynthetic gain and hydraulic cost Plants constantly balance opening the pores to feed against closing them to save water.25PubMed. Predicting stomatal responses to the environment from the optimization of photosynthetic gain and hydraulic cost
Some plants have found ways around the trade-off. C4 plants split the work between two cell types arranged in rings around leaf veins.26PubMed Central. Using evolution as a guide to engineer kranz-type c4 photosynthesis The outer cells capture carbon dioxide into a four-carbon compound and pass it inward.27PubMed Central. Using evolution as a guide to engineer kranz-type c4 photosynthesis Carbon dioxide piles up around the carbon-grabbing enzyme while oxygen is kept out, nearly eliminating photorespiration.28PubMed Central. Using evolution as a guide to engineer kranz-type c4 photosynthesis Familiar C4 plants include corn, sugarcane and many tropical grasses.
CAM plants take another route. They open their leaf pores at night, when it is cooler, and store the carbon as malic acid.29PubMed. Crassulacean acid metabolism photosynthesis: working the night shift By day, with pores tightly shut, they release the carbon dioxide for the Calvin cycle.30PubMed. Crassulacean acid metabolism photosynthesis: working the night shift They lose far less water and keep the enzyme flooded with carbon dioxide.31PubMed Central. CAM photosynthesis: the acid test CAM plants include cacti, pineapple, aloe and many succulents.
You can spot these plants in a yard. Crabgrass is a C4 weed, which helps explain why it surges in midsummer lawns when heat stresses other grasses. Cool-season lawn grasses use the ordinary process and tend to slow down in summer heat. Succulents tolerate a missed watering because CAM lets them lose very little water through their pores.
What Happens When Light, Cold or Heat Push Too Hard
Light beyond what a plant can use can damage it.32PubMed Central. Nonphotochemical quenching in plants: Mechanisms and mysteries So plants switch on a safety valve that releases the extra energy as heat.33PubMed Central. Nonphotochemical quenching in plants: Mechanisms and mysteries The valve is triggered by the hydrogen buildup from the light stage.34PubMed. Balancing the central roles of the thylakoid proton gradient It costs the plant some captured energy that could have gone into sugar, a price it accepts to avoid damage. Cold makes things worse: chilled tomato seedlings treated even moderate light as too much.35PubMed Central. Exogenous Melatonin Mitigates Photoinhibition by Accelerating Non-photochemical Quenching in Tomato Seedlings Exposed to Moderate Light during Chilling
Temperature and air matter too. Warmer temperatures up to about 35°C and more carbon dioxide help leaves ramp photosynthesis up faster when light changes.36PubMed. Dynamic photosynthesis in different environmental conditions
In the garden, seedlings raised indoors or in shade and moved straight into strong sun can bleach or scorch, because their leaves can’t yet shed the extra energy. Hardening off means setting plants outside for a few hours at first and lengthening the time over several days, which lets them adjust. Very hot spells stress many plants, so shade cloth or watering can help in a heat wave.
Why Photosynthesis Matters for Food and the Planet
Plant photosynthesis is the largest flow of carbon on land, pulling carbon dioxide from the air into living matter.37Agricultural and Forest Meteorology. Constraining global terrestrial gross primary productivity in a global carbon assimilation system with OCO-2 chlorophyll fluorescence data Forests and fields therefore act as a major brake on rising carbon dioxide.
It is also the target of efforts to grow more food. Most efforts to raise crop yields focus on improving photosynthesis in ordinary crops like wheat and rice.38PubMed. Strategies for manipulating Rubisco and creating photorespiratory bypass to boost C(3) photosynthesis: Prospects on modern crop improvement Scientists are redesigning Rubisco and building shortcuts around the photorespiration cleanup.39PubMed. Strategies for manipulating Rubisco and creating photorespiratory bypass to boost C(3) photosynthesis: Prospects on modern crop improvement A long-standing goal is giving rice the C4 system.40PubMed. Engineering C4 photosynthesis into C3 chassis in the synthetic biology age Less photorespiration could mean more grain per field, because less captured carbon would be lost, and the benefit would matter most in hot climates, where the mistake is most common. Such crops are still mostly in research and not yet on farms.
Other researchers try to copy the leaf itself. Artificial photosynthesis aims to use sunlight to split water into hydrogen fuel and oxygen.41PubMed. Solar fuels via artificial photosynthesis Current systems are inefficient and need outside electricity.42PubMed. Solar fuels via artificial photosynthesis The goal is an artificial leaf more efficient than plants that doesn’t compete for farmland.43PubMed Central. From natural to artificial photosynthesis