The carbon in photosynthesis comes from carbon dioxide in the air. Plants pull CO2 from the atmosphere through tiny pores on their leaves, then use sunlight to convert that inorganic carbon into sugar. Every carbon atom in a plant’s leaves, stems, roots, and fruit was once a molecule of CO2 floating in the atmosphere.
How CO2 Enters the Leaf
Plant leaves are covered in microscopic openings called stomata, each formed by a pair of guard cells that can swell open or pinch shut. When stomata open, CO2 from the surrounding air diffuses inward through the pore and into the spongy tissue inside the leaf. The gas dissolves into a thin film of water coating the interior cells and eventually reaches the chloroplasts, the structures where photosynthesis happens.
This process is passive. CO2 simply moves from where it’s more concentrated (the atmosphere) to where it’s less concentrated (inside the leaf, where it’s constantly being consumed). The tradeoff is water loss: every time stomata open to let CO2 in, moisture escapes. Plants in hot, dry climates have evolved specialized strategies to deal with this problem, which we’ll get to below.
The atmosphere currently contains about 426 parts per million of CO2, according to NOAA’s global monitoring data. That’s a tiny fraction of the air (about 0.04%), yet it supplies virtually all the carbon that land plants need to grow. CO2 concentrations have risen roughly 34% since 1950, and higher levels do stimulate faster photosynthesis in the roughly 90% of plant species that use the most common photosynthetic pathway.
How Carbon Gets Built Into Sugar
Once CO2 reaches the chloroplast, the real chemistry begins in a process called the Calvin cycle. An enzyme called RuBisCO grabs a CO2 molecule and attaches it to a five-carbon sugar already present in the cell. This creates an unstable six-carbon compound that immediately splits into two three-carbon molecules. That splitting is the moment inorganic carbon becomes organic carbon, the step scientists call “carbon fixation.”
Those three-carbon molecules are then converted into a slightly different three-carbon sugar called G3P, using energy (ATP) and chemical reducing power (NADPH) that the light-dependent reactions of photosynthesis have already produced from sunlight. For every three CO2 molecules that enter the cycle, six G3P molecules are made, but five of them get recycled to regenerate the original five-carbon sugar so the cycle can keep running. Only one G3P molecule exits as net product. Two of those G3P molecules combine to form one molecule of glucose.
The overall equation is straightforward: six molecules of CO2 plus six molecules of water, powered by light energy, yield one molecule of glucose and six molecules of oxygen. All six carbon atoms in that glucose came from CO2. The oxygen released as a byproduct comes from splitting water molecules, not from the CO2.
How Scientists Proved It
The definitive proof came from radioactive carbon tracing experiments in the 1940s and 1950s. Starting in 1945, Melvin Calvin and Andrew Benson at the University of California, Berkeley, fed plants CO2 made with radioactive carbon-14 (which has a half-life of about 5,700 years, making it easy to track). They exposed algae to this labeled CO2 for varying lengths of time, then killed the cells and analyzed which molecules contained the radioactive carbon.
When exposure lasted only a few seconds, the radioactive label showed up in just one molecule: a three-carbon compound called 3-phosphoglycerate. At longer exposures, up to about 10 minutes, the label appeared in many different organic molecules as the carbon was shuffled into various sugars, amino acids, and other compounds. This confirmed that CO2 was the starting material and that carbon fixation produced a three-carbon sugar as its first product. Calvin received the Nobel Prize in Chemistry in 1961 for mapping this pathway.
How Aquatic Plants Get Their Carbon
Underwater plants face a different challenge. CO2 dissolves poorly in water and diffuses about 10,000 times more slowly than in air. In most freshwater and marine environments, CO2 makes up only a small fraction of the dissolved inorganic carbon available. The dominant form is bicarbonate, a charged molecule that can’t simply diffuse through cell membranes the way CO2 does.
About 44% of tested aquatic plant species can use bicarbonate as an additional carbon source. They do this in two main ways. Some produce an enzyme on their cell surfaces that converts bicarbonate back into CO2, which then enters the cell normally. Others have specialized transporter proteins in their cell membranes that pull bicarbonate directly inside. Some species use both strategies. Certain submerged plants have even evolved a version of C4 metabolism, concentrating carbon internally to keep photosynthesis running efficiently despite the low CO2 levels underwater.
C4 and CAM Plants Concentrate Carbon Differently
Not all land plants fix carbon the same way. The standard pathway (called C3 photosynthesis) works well in moderate climates, but in hot, dry conditions, stomata stay closed longer to conserve water, which starves the plant of CO2. Two alternative strategies have evolved to cope with this.
C4 plants, which include corn, sugarcane, and many tropical grasses, use a preliminary chemical step to grab CO2 and convert it into a four-carbon compound in the outer cells of the leaf. That compound is then shuttled to inner cells packed tightly around the leaf veins (a layout called Kranz anatomy), where it releases its CO2 directly to RuBisCO. This concentrating mechanism keeps RuBisCO well supplied with CO2 even when stomata are partially closed, reducing water loss and a wasteful side reaction called photorespiration.
CAM plants, including cacti, agaves, and many succulents, take a different approach tied to timing. They open their stomata at night, when temperatures are cooler and less water evaporates, and store CO2 as an organic acid. During the day, with stomata sealed shut, they release that stored CO2 internally and run the Calvin cycle using sunlight. The carbon source is still atmospheric CO2. These plants simply collect it on a different schedule.
Why CO2 Levels Matter for Plant Growth
Because CO2 is the raw material for building plant tissue, its concentration in the atmosphere directly affects how fast plants can photosynthesize. At today’s level of roughly 426 ppm, most C3 plants are not CO2-saturated, meaning they could photosynthesize faster if more CO2 were available. Research on soybeans found that leaf photosynthesis continues to increase as CO2 rises, reaching its maximum rate at around 1,200 ppm, nearly three times current levels.
However, more CO2 does not simply mean better plants. Studies of wild plant species over the past several decades show that as CO2 has risen, the concentration of protein and certain micronutrients in plant tissues has declined. The plants grow larger but become less nutritionally dense, essentially getting diluted by extra carbohydrates. Optimal crop yield in soybean experiments peaked between 1,000 and 1,200 ppm, beyond which additional CO2 provided no further benefit.