The dark reaction takes place in the stroma, the fluid-filled space inside the chloroplast but outside the thylakoid membranes. This is where carbon dioxide gets converted into sugar through a process called the Calvin cycle. The stroma contains all the enzymes needed for this conversion, most importantly the carbon-fixing enzyme that kicks off the entire cycle.
The Stroma: Where It All Happens
Chloroplasts have three distinct internal compartments, and the stroma is the largest. Think of it as the “workshop floor” of the chloroplast. It sits inside the chloroplast’s double-membrane envelope but surrounds (rather than being inside) the stacked, disc-like thylakoid membranes where light reactions happen. The stroma contains the chloroplast’s own DNA, various metabolic enzymes, and the raw materials needed to build sugar from carbon dioxide.
The key enzyme in the stroma is commonly called RuBisCO. It grabs carbon dioxide and attaches it to an existing five-carbon molecule, launching the Calvin cycle. RuBisCO is not just important to the dark reaction; it is the most abundant protein on Earth. Plants produce enormous quantities of it because it works remarkably slowly, processing only about three carbon dioxide molecules per second. To compensate for that sluggish pace, the stroma is packed with it. The concentration of RuBisCO in the stroma exceeds the concentration of available carbon dioxide by 100 to 1,000 times.
How the Light Reactions Feed the Dark Reactions
The name “dark reaction” is a bit misleading. It does not require light directly, but it depends entirely on products made by the light reactions, which do need sunlight. Those products, ATP (the cell’s energy currency) and NADPH (an electron carrier), are manufactured in the thylakoid membrane and released into the stroma. ATP is produced by a protein complex whose head sticks out from the thylakoid membrane directly into the stroma, so the energy molecule is made right where it’s needed.
Both ATP and NADPH have extremely short lifespans, surviving only millionths of a second before they’re used up. This means the Calvin cycle effectively stalls without ongoing light reactions to replenish them. That’s why most scientists now prefer the term “light-independent reactions” over “dark reactions.” The cycle doesn’t run in the dark for long. The carbohydrates it produces, on the other hand, can persist for hundreds of millions of years.
The Three Stages of the Calvin Cycle
The Calvin cycle runs through three stages in the stroma, repeating continuously as long as supplies last.
- Carbon fixation: RuBisCO attaches a carbon dioxide molecule to a five-carbon sugar, creating an unstable six-carbon compound that immediately splits into two three-carbon molecules.
- Reduction: ATP and NADPH convert those three-carbon molecules into a small sugar called G3P. This is the step that actually builds the carbon backbone of sugar using the energy captured from sunlight.
- Regeneration: Most of the G3P molecules get recycled to rebuild the original five-carbon sugar so the cycle can grab another carbon dioxide. For every six carbon dioxide molecules that enter the cycle, only one net G3P molecule exits to eventually become glucose.
It takes six full turns of the cycle to produce enough G3P to assemble one glucose molecule. Each turn consumes ATP and NADPH, so the process is energy-intensive and completely reliant on the stroma’s steady supply from the thylakoid membranes.
The Location Shifts in C4 and CAM Plants
In most plants (called C3 plants), the entire Calvin cycle runs in the stroma of mesophyll cells, the general photosynthetic tissue of the leaf. But some plants have evolved workarounds to deal with hot or dry conditions, and this changes where the dark reaction physically occurs.
C4 plants like corn and sugarcane split the work between two cell types. Mesophyll cells capture carbon dioxide first using a different, faster enzyme, then shuttle it as a four-carbon molecule to specialized bundle sheath cells deeper in the leaf. There, the carbon dioxide is released at high concentrations and fed into the Calvin cycle in the bundle sheath stroma. This two-cell system acts as a carbon dioxide pump, saturating RuBisCO and preventing wasteful side reactions.
CAM plants like cacti and pineapples use a time-based separation instead of a spatial one. They open their pores at night to capture carbon dioxide, storing it as an acid in cell vacuoles. During the day, with pores closed to conserve water, that stored acid releases carbon dioxide for the Calvin cycle in the same cell’s stroma. The dark reaction still occurs in the stroma, but the initial carbon capture happens hours earlier.
Why Temperature Matters
Because the dark reaction depends on enzymes dissolved in the stroma’s fluid, temperature has a direct effect on how well it works. Every enzyme has an optimal temperature range, and RuBisCO is no exception. When leaf temperatures climb above the optimum, carbon dioxide absorption drops sharply. Research across wheat, rice, maize, cotton, and cucumber shows that high heat deactivates RuBisCO, and the helper protein responsible for reactivating it is itself sensitive to heat, creating a compounding problem.
Heat stress doesn’t just slow down the first step. Several other Calvin cycle enzymes are also downregulated or physically damaged at elevated temperatures. In Kentucky bluegrass, one of the reduction-stage enzymes drops under heat treatment. In rice, the enzyme responsible for regeneration declines as well. This is one reason why even brief heat waves during the growing season can significantly reduce crop yields. The stroma’s enzyme machinery is productive but fragile.