Plants need carbon dioxide for the same reason a builder needs bricks: it is the material their bodies are made from. Through photosynthesis, a plant uses the energy in sunlight to turn carbon from the air into sugar, which feeds it and becomes its tissue. The surprising part is how little of a plant comes from the soil. The bulk of a tree’s trunk was once gas in the air. Getting that gas in costs the plant water, and that trade-off shapes how leaves, desert plants and crops work.
What a Plant Builds From Carbon Dioxide
When a plant takes in carbon dioxide, it chemically converts the carbon into sugar, and that sugar stores energy the plant can use later.1Nature Education Knowledge. Effects of Rising Atmospheric Concentrations of Carbon Dioxide on Plants The same carbon also forms the backbone of the molecules that make up the plant’s structure, like the cellulose in stems and wood.2Nature Education Knowledge. Effects of Rising Atmospheric Concentrations of Carbon Dioxide on Plants Carbon, hydrogen and oxygen gathered by photosynthesis make up about 96% of a typical plant’s dry weight.3Nature Education Knowledge. Effects of Rising Atmospheric Concentrations of Carbon Dioxide on Plants Soil minerals and fertilizer supply only around 4%.
The heavy trunk is mostly carbon pulled from the air and combined with water. Water supplies the hydrogen, and sunlight supplies the energy to join everything together. That’s why a plant kept without carbon dioxide stops growing, even in good soil.
Plants make their own sugar from carbon dioxide and water. Some of it is burned right away for energy, just as animals burn food. The rest is stored as starch in roots, seeds and tubers like potatoes, and those stores are what we eat when we eat grains, fruit and vegetables.
Cellulose is made of long chains of sugar units linked end to end, so the sugar a leaf makes is literally the starting material for the stringy fibers in stems. Plants bundle those chains into tough fibers, which is why a stalk can stand upright without a skeleton. The same carbon skeletons can be rearranged into other molecules, such as the oils in seeds, and into the building blocks of proteins once nitrogen from the soil is added.
One enzyme does most of the grabbing. More than 90% of the carbon turned into living matter on Earth is captured by a single enzyme called Rubisco.4PubMed Central. A short history of RubisCO: The rise and fall (?) of Nature’s predominant CO2 fixing enzyme It attaches carbon dioxide to a sugar inside the leaf’s chloroplasts, which starts a recycling loop that builds more sugar.5PubMed Central. A short history of RubisCO: The rise and fall (?) of Nature’s predominant CO2 fixing enzyme
Rubisco makes mistakes, though, and because air holds far more oxygen than carbon dioxide, they add up. It sometimes grabs oxygen instead of carbon dioxide, producing a waste product that would stall photosynthesis if the plant didn’t clean it up.6PubMed. Photorespiration – Rubisco’s repair crew So plants spend extra energy on a repair process to clear that waste, a cost that grows in hot, bright conditions.7PubMed. Photorespiration – Rubisco’s repair crew8International Journal of Plant Sciences. Evolution of CAM and C 4 Carbon‐Concentrating Mechanisms This weakness is why some plants evolved special ways to concentrate carbon dioxide, covered below.
How Carbon Dioxide Gets Into a Leaf
Leaves are dotted with tiny pores called stomata, mostly on their undersides. Two guard cells ring the pore, and they swell to open it and relax to close it.9Pressbooks. 8.5 Stomatal Conductance and Leaves When the guard cells fill with water they bow apart. When the plant is short on water they go limp and the pore closes.
Through these pores, carbon dioxide comes in while oxygen and water vapor go out.10Pressbooks. 8.5 Stomatal Conductance and Leaves The guard cells also sense the carbon dioxide level inside the leaf and adjust the opening in response.11PubMed Central. CO2 sensing and CO2 regulation of stomatal conductance: advances and open questions
The gas moves without any pumping. Carbon dioxide drifts by diffusion, from where it is more concentrated to where it is less. Photosynthesis uses up carbon dioxide inside the leaf, so the level there stays lower than in the outside air, and that difference keeps the gas flowing inward.
Getting through a pore is only part of the trip. Carbon dioxide still has to cross air spaces, cell walls and the watery inside of cells to reach the chloroplasts.12Plant, Cell and Environment. Mesophyll conductance to CO2: current knowledge and future prospects This inner route slows the gas enough to limit how fast the plant can capture carbon.13PubMed. Mesophyll conductance: walls, membranes and spatial complexity Leaves with more chloroplast surface facing the inner air spaces, and thinner cell walls, pass carbon dioxide along more easily.14PubMed. Mesophyll conductance: walls, membranes and spatial complexity A slow inner route forces the plant to spend more water and nitrogen for each bit of carbon it gains.15PubMed. Mesophyll conductance: walls, membranes and spatial complexity
That helps explain why leaves are flat and thin. A thin leaf keeps the inner path short, and a wide surface catches more sunlight and holds more pores.
Plants also adjust how many pores they build. Light and the carbon dioxide level in the air help set how many stomata a growing leaf develops.16PubMed. Long-distance CO(2) signalling in plants These signals act early in leaf growth, steering how many surface cells become pores.17PubMed. Long-distance CO(2) signalling in plants
The Water Cost of Breathing In Carbon Dioxide
Stomata must open for photosynthesis to work efficiently.18LibreTexts. Why Does a Plant Need Carbon Dioxide From the Air? But open pores also let water evaporate from the leaf.19LibreTexts. Why Does a Plant Need Carbon Dioxide From the Air? How much carbon a plant gains for each drop of water lost is the central trade-off of plant life.20PubMed. Leaf stomatal configuration and photosynthetic traits jointly affect leaf water use efficiency in forests along climate gradients In dry soil, that means a choice: close up to save water, or stay open to keep growing.
The exchange is lopsided. The air outside holds only a tiny fraction of carbon dioxide, so the plant must move a great deal of air past its cells to gather a little carbon. Meanwhile the moist inner leaf is saturated with water vapor, so vapor escapes much faster than carbon dioxide enters. A plant typically loses hundreds of water molecules for each carbon dioxide molecule it captures, which is why water is the real price of growing.
In a garden, this shows up in familiar ways. Wilting on a hot day is partly the plant losing water faster than roots can replace it. A plant that closes its pores to survive drought also stops building sugar, which is why drought-stressed crops stay small even when they recover. Watering in the morning helps plants keep their pores open through the day.
As soil dries, roots send chemical signals that tell guard cells to start closing the pores, even before the leaves visibly wilt. A potted plant left dry often stalls its growth well before it looks unhealthy.
This water loss isn’t only a loss. Evaporating water cools the leaf the way sweat cools skin, and the pull of evaporation helps draw water and minerals up from the roots.
How Corn, Cacti and Water Plants Get Enough Carbon Dioxide
Plants such as corn, sugarcane and many tropical grasses use a system called C4 photosynthesis. Cacti, pineapples, jade plants and many succulents use a system called CAM. Each found its own way around the limits above.
Corn-type plants chemically pump carbon dioxide from outer leaf cells into inner cells where Rubisco is kept.21PubMed. Deconstructing Kranz anatomy to understand C4 evolution The first capture step happens in the outer cells, using an enzyme that grabs carbon dioxide without confusing it with oxygen. The carbon is passed inward as a small acid to special cells that surround the veins, and there it is released right beside the carbon-grabbing enzyme. Splitting the job between two cell types lets these plants build up a high carbon dioxide level around it.
With so much carbon dioxide around, the oxygen mix-up rarely happens.22PubMed. Deconstructing Kranz anatomy to understand C4 evolution These plants outgrow ordinary plants in hot, bright places where the mix-up is worst.23PubMed. Deconstructing Kranz anatomy to understand C4 evolution24International Journal of Plant Sciences. Evolution of CAM and C 4 Carbon‐Concentrating Mechanisms
The pump isn’t free. Running it takes extra energy, so in cool, shady conditions corn-type plants have no advantage over ordinary ones. It pays off when leaves are hot and bright, which explains why such plants are common in tropical grasslands and warm summers but rarer in cool climates.
Desert plants take another route. They open their pores at night, store carbon dioxide as an acid, and close their pores during the hot day.25Wikipedia. Crassulacean acid metabolism In daylight, the stored acid releases carbon dioxide inside the leaf for photosynthesis.26Wikipedia. Crassulacean acid metabolism These plants lose far less water and can ride out dry spells.27PubMed Central. CAM photosynthesis: the acid test For houseplant owners, that means succulents and some orchids do much of their gas exchange at night. It’s one reason they tolerate infrequent watering, and overwatering them is a more common mistake than underwatering.
Underwater plants tap a different source. In water, carbon dioxide also exists as dissolved bicarbonate, a form related to baking soda. Some water plants, including common aquarium plants like hornwort and Brazilian waterweed, can use bicarbonate as a carbon source.28Aquatic Botany. Bicarbonate use in three aquatic plants Not all can. In one group of high-altitude water plants, 10 of 17 species could, and water acidity affected the ability.29PubMed. Inorganic carbon utilization strategies of plateau aquatic plants in response to native habitats
Still water holds little carbon dioxide, so aquarium plants can run short. Those that can use bicarbonate cope better in hard, alkaline water. This is why some aquarium keepers add carbon dioxide to planted tanks.
Does More Carbon Dioxide Help Plants
Generally, yes, up to a point. In open-field trials where carbon dioxide was raised around crops and trees, ordinary plants photosynthesized about 31% faster in full light.30Plant, Cell and Environment. The response of photosynthesis and stomatal conductance to rising [CO2]: Mechanisms and environmental interactions Their leaf pores opened about 22% less, so they lost less water.31Plant, Cell and Environment. The response of photosynthesis and stomatal conductance to rising [CO2]: Mechanisms and environmental interactions With more carbon dioxide outside, the plant can get the same amount of carbon through smaller openings, and narrower openings let out less water vapor. That is why plants grown in higher carbon dioxide can do better under mild drought. On a larger scale, plants on land have absorbed about a quarter of the carbon dioxide humans have added to the air, thanks partly to this growth boost.32NASA Science. A Global Garden: Plants Storing Carbon – NASA Science
Faster growth has a catch. Vegetables grown in higher carbon dioxide had about 9.5% less protein, 16% less iron and 9.4% less zinc.33PubMed Central. Effects of Elevated CO2 on Nutritional Quality of Vegetables: A Review They had more sugars, vitamin C and some antioxidants.34PubMed Central. Effects of Elevated CO2 on Nutritional Quality of Vegetables: A Review Plants pack in extra carbon faster than they take up minerals from soil.35PubMed. Elevated CO(2), nutrition dilution, and shifts in Earth’s insect abundance The plant builds more carbon-rich material, such as sugars and starch, while its mineral content stays about the same, so the concentration per bite falls, like adding extra water to a cup of juice.
The heat and drought that come with climate change can cancel out the growth boost. Corn-type plants gain less, because they already concentrate carbon dioxide.
Some commercial greenhouses add carbon dioxide to speed growth. It works because the gas is often the limiting ingredient on bright days. Home growers rarely need this, since outdoor air and ventilation usually suffice.
Where the Carbon Goes After a Plant Takes It In
A large share goes underground. Wheat and barley send 20% to 30% of the carbon they capture below ground.36Journal of Plant Nutrition and Soil Science. Carbon input by plants into the soil. Review About half of that stays in roots, about a third returns to the air through root and microbe breathing, and the rest feeds soil life and organic matter.37Journal of Plant Nutrition and Soil Science. Carbon input by plants into the soil. Review Pasture plants send 30% to 50% below ground.38Journal of Plant Nutrition and Soil Science. Carbon input by plants into the soil. Review Grass roots die and regrow constantly, adding carbon to the soil each time, which helps explain why old grasslands tend to have deep, dark, carbon-rich soil.
Roots receive sugar from the leaves and burn part of it to power growth and to take up minerals from the soil. The rest builds new root tissue, and some leaks into the soil as small compounds that feed microbes near the root. This steady feeding is why the soil right around roots teems with life compared with soil further away.
Root-partner fungi carry fresh sugar from plants out to soil microbes, while helping roots gather nutrients.39PubMed Central. Exploring the transfer of recent plant photosynthates to soil microbes: mycorrhizal pathway vs direct root exudation Their threads grow into the roots and out into the soil, reaching far more ground than roots alone, and they hand water and nutrients such as phosphorus to the plant. In return the plant pays with sugar made from air carbon, so it is literally feeding its helpers with carbon dioxide it captured.
Carbon also leaves the plant again. Plants burn some sugar for energy, releasing carbon dioxide back out. When leaves fall and rot, microbes return much of their carbon to the air. Wood that lasts, in trunks or timber, keeps carbon out of the air for decades or longer.
Over a much longer span, plants reshaped the planet’s air. When land plants spread hundreds of millions of years ago, they pulled carbon dioxide from the air, cooling the climate and raising oxygen.40Chemical Geology. The impacts of land plant evolution on Earth’s climate and oxygenation state – An interdisciplinary review Feedbacks between plants and the atmosphere sped that fall in carbon dioxide.41PubMed Central. Feedbacks and the coevolution of plants and atmospheric CO2