Plants pull almost all of their carbon dioxide straight from the air around them, absorbing it through microscopic pores on their leaves called stomata. The atmosphere holds roughly 0.04% CO₂ by volume, which sounds negligible, but plants have spent hundreds of millions of years evolving exquisitely tuned machinery to harvest it. The story is more varied than “leaves breathe in CO₂,” though, because not every plant uses the same strategy, and some acquire carbon from surprisingly unconventional places.
Stomata Are the Main Gateway
The surface of most plant leaves is dotted with thousands of tiny openings, each bordered by a pair of specialized cells called guard cells. When these guard cells swell with water, the pore between them opens; when they lose water, the pore closes. This opening-and-closing cycle controls how much CO₂ flows into the leaf and, just as critically, how much water vapor escapes. Stomata serve as the major route for both CO₂ entry and water loss, a trade-off that shapes nearly every aspect of a plant’s life.1PubMed Central. Guard cell signal transduction network: advances in understanding abscisic acid, CO2, and Ca2+ signaling
Guard cells do not just respond to water status. They also sense the CO₂ concentration itself, along with light levels and a stress hormone called abscisic acid (ABA). When the air outside is rich in CO₂, guard cells tend to partially close the stomata because the plant can get enough carbon with a smaller opening, which conserves water. When CO₂ is scarce, the stomata open wider. This feedback loop means a plant is constantly adjusting its intake depending on conditions.1PubMed Central. Guard cell signal transduction network: advances in understanding abscisic acid, CO2, and Ca2+ signaling
Most stomata sit on the underside of leaves, which keeps them somewhat sheltered from direct sun and wind. Some species have stomata on both surfaces. The sheer density can be striking: a single square centimeter of leaf surface can contain hundreds to tens of thousands of stomatal pores, depending on the species and how the plant grew.
The Boundary Layer Problem
Even with stomata wide open, CO₂ doesn’t pour in freely. A thin layer of still air clings to every leaf surface, called the boundary layer. CO₂ molecules have to diffuse through this stagnant zone before they reach the stomatal opening, and in calm conditions the boundary layer can become thick enough to meaningfully slow photosynthesis. Wind strips this layer away, making CO₂ delivery faster.
This effect is large enough to matter in commercial greenhouses, where air movement is often minimal. Modeling work suggests that in such settings, even a very modest increase in wind speed can boost daily photosynthesis by 10 to 20 percent, simply by thinning the boundary layer and letting CO₂ reach the stomata more efficiently.2Oxford University Press. Beyond the boundary: a new road to improve photosynthesis via wind The flip side is that too much wind increases water loss and can physically damage leaves, so there is an optimum range. Leaf size also matters: larger leaves have thicker boundary layers, which is one reason many desert plants have small, narrow leaves that shed still air more readily.
What Happens After CO₂ Gets Inside
Once CO₂ passes through a stoma, it still has to travel through the air spaces inside the leaf and then dissolve into the wet cell walls of the mesophyll, the spongy photosynthetic tissue. This internal journey is its own bottleneck, and plant scientists refer to it as mesophyll conductance. In some species, mesophyll conductance limits photosynthesis just as much as the stomata themselves do.3PubMed. Mesophyll conductance to CO2 and Rubisco as targets for improving intrinsic water use efficiency in C3 plants
The CO₂ that finally reaches the chloroplasts is grabbed by an enzyme called Rubisco, which bolts a carbon atom onto a five-carbon sugar to kick off the chain of reactions that build sugars. This is the basic carbon-fixation pathway used by the majority of plants on Earth, often called the C3 pathway because the first stable product is a three-carbon molecule. Rubisco is famously slow and also a bit clumsy: it sometimes grabs oxygen instead of CO₂, which wastes energy. That imperfection has driven the evolution of alternative strategies.
C4 and CAM Plants Have Workarounds
Some plants, including corn, sugarcane, and many tropical grasses, have evolved what amounts to a carbon-concentrating pump. In these C4 plants, CO₂ is first captured in outer mesophyll cells by a different, faster enzyme and converted into a four-carbon acid. That acid is shuttled to an inner ring of cells surrounding the leaf veins, called the bundle sheath, where it releases concentrated CO₂ directly to Rubisco.4PubMed. Deconstructing Kranz anatomy to understand C4 evolution By flooding Rubisco with CO₂ in a confined compartment, C4 plants dramatically reduce the wasteful oxygen-grabbing reaction. The result is faster photosynthesis under hot, bright, dry conditions, which is why C4 grasses dominate tropical savannas.
An even more dramatic workaround belongs to plants with crassulacean acid metabolism, or CAM, found in cacti, agaves, pineapples, and many succulents. CAM plants open their stomata at night, when the air is cooler and humidity is higher, and absorb CO₂ in the dark. They store it as malic acid inside their cells. During the day, with stomata sealed shut to conserve water, they break down that stored acid and release CO₂ internally for photosynthesis. A survey spanning 40 plant families found that this pattern of nocturnal acid accumulation is entirely restricted to CAM species and requires a fundamental reprogramming of the plant’s metabolism compared to ordinary C3 carbon fixation.5PubMed Central. CAM photosynthesis: the acid test
These three strategies, C3, C4, and CAM, all draw their CO₂ from the same source: the atmosphere. The difference is timing and internal architecture. C3 plants grab CO₂ during the day and fix it immediately. C4 plants grab it during the day but shuttle it through an extra biochemical step to concentrate it. CAM plants grab it at night and store it for daytime use.
How Aquatic Plants Get Their Carbon
Submerged aquatic plants face a completely different challenge. CO₂ dissolves in water, but it diffuses about ten thousand times more slowly through water than through air. To make matters worse, the CO₂ in water participates in a chemical equilibrium with bicarbonate ions and carbonate ions. At the slightly alkaline pH of most natural freshwater, very little of the dissolved inorganic carbon is actually in the form of free CO₂; the bulk is bicarbonate.
Many aquatic plants have evolved the ability to use bicarbonate directly as a carbon source. Species like hornwort, Brazilian waterweed, and other common freshwater plants can actively pull bicarbonate from the water and convert it internally to CO₂ for photosynthesis.6Aquatic Botany. Bicarbonate use in three aquatic plants This is not free, though. Running the bicarbonate-extraction machinery costs extra light energy. In one species of waterweed, the estimated energy cost of switching from free CO₂ to bicarbonate use was measurable, though the plants showed no obvious reduction in growth, suggesting the cost is manageable under normal light conditions.7Aquatic Botany. The metabolic cost of bicarbonate use in the submerged plant Elodea nuttallii
Not all aquatic plants have this ability, however. Some rely entirely on dissolved free CO₂ and are restricted to habitats where CO₂ levels stay high, such as fast-flowing streams or acidic waters. This is why aquarium hobbyists often inject CO₂ gas into their tanks: without supplementation, densely planted tanks can run low on dissolved carbon, and plants that lack the bicarbonate trick will struggle.
Non-Leaf Sources of CO₂
Leaves are the obvious CO₂ collectors, but they are not the only plant tissues that capture carbon. Tree bark, green stems, and even fruit sometimes carry out their own photosynthesis. In Scots pine, for example, researchers measured the contribution of bark photosynthesis to the overall carbon balance of the stem. Photosynthesis in the bark accounted for up to about 13% of the stem’s net CO₂ exchange, partially offsetting the carbon the stem was releasing through respiration.8Oxford Academic (Journal of Experimental Botany). Partitioning seasonal stem carbon dioxide efflux into stem respiration, bark photosynthesis, and transport-related flux in Scots pine That is not a trivial contribution when you consider how much woody tissue a mature tree has.
Plants also recycle some of the CO₂ their own cells produce through respiration. Every living cell in a plant burns sugars for energy and releases CO₂ as a byproduct, just as animals do. In trees, isotope-labeling experiments have shown that chlorophyll-containing tissues can recapture up to about 17% of the CO₂ released by respiration before it escapes to the atmosphere.9PubMed Central. Internal recycling of respired CO 2 may be important for plant functioning under changing climate regimes Modeling work suggests that CAM cycling, a mild version of the crassulacean acid metabolism described earlier, is the most efficient native mechanism for reassimilating CO₂ released at night, and could in principle recover all of it.10PubMed. Metabolic design considerations for recycling of respiratory CO(2) in leaves
Roots can absorb dissolved CO₂ from the soil as well, but this is a minor source. The CO₂ concentration in soil air is much higher than in the atmosphere, sometimes ten to a hundred times higher, because soil microbes and roots are constantly respiring. Some of this soil CO₂ dissolves into the water that roots take up and travels through the transpiration stream to the leaves. In theory, this route could supply up to about 5% of a plant’s total carbon, though in practice it usually contributes less than 1%.11PubMed. Plant response to irrigation with water enriched with carbon dioxide The soil CO₂ story is more interesting for its indirect effects: higher soil CO₂ can speed up mineral weathering and influence nutrient availability, which can help plants grow even though the carbon itself is not the main benefit.
Mosses and Other Plants Without Stomata
Not all land plants have stomata. Liverworts and many mosses absorb CO₂ directly through their surfaces, since they lack the waxy cuticle that makes vascular plant leaves relatively gas-tight. In peat-forming Sphagnum mosses, nearly all light absorption and CO₂ fixation happens within the first centimeter of the moss carpet.12Global Change Biology. Interactive effects of water table and precipitation on net CO2 assimilation of three co‐occurring Sphagnum mosses differing in distribution above the water table Below that thin surface layer, the moss is alive but mostly too shaded to photosynthesize. Because these plants rely on passive diffusion rather than stomatal regulation, they are extremely sensitive to how wet or dry they are: a thoroughly soaked Sphagnum carpet has water filling every surface pore, and CO₂ diffuses through water far more slowly than through air. A slightly drier moss surface lets CO₂ in more freely, which is why peatland carbon uptake is so responsive to changes in the water table.
Parasitic Plants and Stolen Carbon
Some plants skip the atmospheric route entirely, or at least supplement it heavily, by tapping into other plants. Hemiparasites like yellow rattle and eyebright are green and photosynthetically active, but they also push specialized roots called haustoria into the vascular tissue of neighboring plants and siphon off water, nutrients, and organic carbon.13PubMed Central. Interactions between hemiparasitic plants and their hosts: the importance of organic carbon transfer Isotope-mixing studies have estimated that yellow rattle derives roughly half of its carbon from its host, while eyebright gets about a quarter.14PubMed. Heterotrophic carbon gain by the root hemiparasites, Rhinanthus minor and Euphrasia rostkoviana (Orobanchaceae) Full parasites like dodder and broomrape have lost functional chlorophyll altogether and depend entirely on their host for carbon. For these species, CO₂ from the air is irrelevant; their carbon has been pre-fixed by another plant.
How Rising CO₂ Is Reshaping Plant Anatomy
The CO₂ concentration in Earth’s atmosphere has risen substantially since the Industrial Revolution and continues to climb. Plants are responding, and one of the clearest anatomical changes is a reduction in stomatal density. A survey of 100 species found that, on average, plants grown under elevated CO₂ had about 14% fewer stomata per unit of leaf area, with roughly three-quarters of the species studied showing some reduction.15New Phytologist. The influence of CO2 concentration on stomatal density The logic is straightforward: if each breath of air contains more CO₂, the plant can afford fewer openings and still fix enough carbon, while losing less water.
This trend is visible not only in greenhouse experiments but also in preserved leaf specimens collected over the past century and a half. A study of nine common species from Florida reconstructed a roughly 34% reduction in maximum stomatal conductance for every 100 parts per million increase in atmospheric CO₂ over the past 150 years. Both flowering plants and conifers showed this response, though they achieved it through different combinations of fewer pores versus smaller pores.16PubMed Central. Global CO2 rise leads to reduced maximum stomatal conductance in Florida vegetation
The downstream effects are significant. Fewer or smaller stomata mean less water vapor leaving the leaf, which changes how much water the plant pulls from the soil. Scaled up across entire ecosystems, this can affect streamflow, soil moisture, and regional humidity. It also means that the simple relationship many people assume, more CO₂ equals more plant growth, is complicated by the fact that plants are simultaneously adjusting their plumbing.
The Evolutionary Backstory
Stomata are ancient structures. Fossil evidence and genetic studies suggest that the ability to adjust stomatal density in response to CO₂ levels dates back to the earliest land plants, over 400 million years ago.17PubMed Central. Origins and Evolution of Stomatal Development Early land plants were small, stem-like organisms with few stomata and no true leaves. Broad, flat leaves did not appear until much later, and their emergence appears to have been linked to a dramatic drop in atmospheric CO₂ during the late Paleozoic era.
The connection is elegant. When CO₂ was very high, a flat leaf with few stomata would have overheated. It intercepts far more solar energy than a narrow stem, and without enough stomatal openings to drive cooling transpiration, the leaf temperature would have become lethal. Analyses of fossil leaves show that when broad leaves finally appeared in the Late Devonian and Carboniferous periods, they had substantially higher stomatal densities than their ancestors, consistent with the decline in atmospheric CO₂ that had occurred by that time. In other words, the low-CO₂ atmosphere forced plants to open more pores, which enabled enough water loss to keep flat leaves cool, which in turn allowed the evolution of the leaf shapes we now take for granted.18PubMed. Evolution of leaf-form in land plants linked to atmospheric CO2 decline in the Late Palaeozoic era
Carbon Isotopes and How Scientists Track the Source
If you want to know where a plant actually got its carbon, you can read the answer in its chemistry. Carbon comes in two stable forms: the common carbon-12 and the slightly heavier carbon-13. Rubisco preferentially grabs the lighter isotope, so plant tissue ends up with a characteristic depletion in carbon-13 compared to the CO₂ in the air. The degree of this depletion differs predictably between C3, C4, and CAM plants because each pathway handles carbon differently before Rubisco sees it.19PubMed. Environmental and physiological determinants of carbon isotope discrimination in terrestrial plants
This isotope signature has become one of the most widely used tools in plant ecology. It can reveal whether a parasitic plant is getting its carbon from a C3 or a C4 host, whether a CAM plant is relying more on nighttime or daytime CO₂ fixation, or how tightly a plant’s stomata were clamped during a drought. It can even be read from ancient plant fossils, providing a window into photosynthetic strategies and atmospheric conditions millions of years in the past. In a sense, every plant carries a chemical record of where and how it acquired its carbon, written into its tissues at the molecular level.