The dark reaction is the second stage of photosynthesis, where plants use energy captured from sunlight to build sugar from carbon dioxide. Despite the name, it doesn’t happen in the dark. It’s called the “dark reaction” simply because it doesn’t directly require light the way the first stage does. Instead, it runs on the chemical energy (ATP and NADPH) that the light-dependent reactions produce. This process takes place in the stroma, the fluid-filled space inside chloroplasts, and is more formally known as the Calvin cycle.
Why “Dark Reaction” Is Misleading
The term “dark reaction” has stuck around in textbooks for decades, but it gives a wrong impression. These reactions don’t need darkness, and they don’t prefer it. They happen during the day, right alongside the light reactions, because they depend on a continuous supply of ATP and NADPH that the light reactions generate. Without sunlight powering those light reactions, the supply of energy carriers dries up and the Calvin cycle grinds to a halt. The more accurate name, “light-independent reactions,” reflects that sunlight isn’t a direct ingredient in this stage, even though it’s still essential indirectly.
How the Calvin Cycle Works
The Calvin cycle converts carbon dioxide from the air into a simple sugar that the plant can use for energy and growth. It does this in three stages: carbon fixation, reduction, and regeneration.
Carbon Fixation
The cycle begins when CO₂ from the atmosphere is attached to a five-carbon molecule already present in the stroma. The enzyme responsible for this step is sometimes called the most important protein on Earth. It makes up 30 to 50 percent of the soluble protein in a leaf, and by one estimate, there are roughly 5 kilograms of it for every person alive. More than 90% of all inorganic carbon converted into living matter passes through this single enzyme. Its job is straightforward: grab a CO₂ molecule and stitch it onto the five-carbon acceptor, which immediately splits into two three-carbon molecules.
Reduction
Those three-carbon molecules are then converted into a higher-energy form using the ATP and NADPH delivered from the light reactions. This is where the energy captured from sunlight actually gets locked into chemical bonds the plant can use. The product of this stage is a small three-carbon sugar called G3P (glyceraldehyde-3-phosphate), the building block for glucose and other carbohydrates.
Regeneration
Most of the G3P molecules don’t leave the cycle. They get rearranged and recombined through a series of steps that rebuild the original five-carbon molecule, so it’s ready to grab another CO₂ and start the process again. This regeneration phase also consumes ATP. Only one out of every six G3P molecules produced actually exits the cycle to be used by the plant. The rest are recycled to keep things running.
The Energy Cost of Making Glucose
Because glucose is a six-carbon sugar and each turn of the Calvin cycle fixes just one carbon, the cycle must run six times to produce a single glucose molecule. That adds up to 18 molecules of ATP and 12 molecules of NADPH per glucose. All of that energy comes from the light reactions, which is why photosynthesis as a whole still depends entirely on sunlight even though this stage doesn’t use light directly. Once ATP and NADPH are spent in the Calvin cycle, they’re converted back into their “empty” forms (ADP and NADP+) and shuttled back to the thylakoid membrane, where the light reactions recharge them.
Not All Plants Fix Carbon the Same Way
The basic Calvin cycle described above is how C3 plants handle carbon fixation. Most plants on Earth, including wheat, rice, and soybeans, are C3 plants. But this system has a flaw: the carbon-fixing enzyme sometimes grabs oxygen instead of CO₂, roughly 20% of the time. This triggers a wasteful process called photorespiration that costs the plant energy without producing sugar.
C4 plants like corn, sugarcane, and sorghum evolved a workaround. They use a different enzyme in their outer leaf cells to capture CO₂ first, locking it into a four-carbon molecule (hence the name C4). That molecule is then transported to specialized inner cells called bundle sheath cells, where it releases concentrated CO₂ directly to the Calvin cycle. By flooding the area around the carbon-fixing enzyme with CO₂ and keeping oxygen away, C4 plants virtually eliminate photorespiration. This also lets them keep their pores partially closed in hot, dry conditions, conserving water while still fixing carbon efficiently.
CAM plants, like cacti and succulents, take a different approach to the same problem. They open their pores at night to collect CO₂, store it as an acid, and then release it internally during the day for the Calvin cycle. This is an extreme water-saving strategy for desert environments.
How Temperature Affects the Dark Reaction
Because the Calvin cycle is driven by enzymes, temperature has a significant effect on how fast it runs. Moderate warmth speeds up the reactions, just as it does for most enzyme-driven processes. But excessive heat reduces the activity of the key carbon-fixing enzyme and can impair the overall photosynthetic machinery. Prolonged heat stress, beyond 12 hours of high temperatures in studies on poplar trees, caused damage severe enough that photosynthetic capacity didn’t fully recover. Changes in temperature also affect how wide a plant opens its stomata (leaf pores), which controls how much CO₂ gets in. So on very hot days, plants may close their stomata to conserve water, starving the Calvin cycle of its carbon supply even if the enzyme itself is still functional.