What Are the Products of Dark Reactions?

The dark reactions of photosynthesis produce sugar phosphate molecules with three carbon atoms each. These reactions, known as the Calvin cycle, are called dark reactions because they do not use light energy directly, but they still depend on the energy captured from light and only occur during the day.

Plants use the energy from sunlight to convert carbon dioxide and water into sugar. The energy from the sun does not go directly into breaking down the carbon dioxide and water and forming sugars, however. Instead, there is an intermediate step where the energy is stored in a different molecule called ATP. This molecule is then transported to a different part of the chloroplast where it is used to power the Calvin cycle.

The first step of the dark reaction involves a molecule with five carbon atoms reacting with a carbon dioxide molecule to form an intermediate molecule with six carbon atoms. This molecule splits immediately, and each half goes through two more reactions before the Calvin cycle is through with them. Each time through the carbon cycle produces six of these molecules, but five of them are consumed afterward to regenerate the molecules used to perform the Calvin cycle again, meaning only one of these three-carbon molecules remains after each completed Calvin cycle.