Dark phase

The dark, biosynthetic or assimilative phase of photosynthesis is a set of reactions that occurs at night and day, which They convert carbon dioxide and other compounds into glucose. These reactions, unlike light reactions (light phase or light phase), do not require light to occur, hence the name dark reactions. These reactions take the products of the light phase (mainly ATP and NADPH) and perform more chemical processes on them. There are two dark reactions: carbon fixation and the Calvin Cycle.

Carbon fixation

It has now been discovered that both phases of photosynthesis require light energy to carry out their functions. The dark phase or carbon fixation or assimilation phase cannot be carried out without solar energy, because it is indirectly regulated by it, given that some enzymes involved in the carbon assimilation process are dependent on light, and the process in its absence. This second phase is also temperature dependent and slower. Carbon fixation is the first step of dark reactions. The carbon from CO₂ is "fixed" inside a big carbohydrate. There can be three paths (processes) that exist for this type of reaction to occur: Fixation of the C₃ carbon (the most common among these), fixation of the C₄ carbon., and CAM.

• C3 plants set directly CO₂ in the Calvin cycle, without previous fixations; the RuBisCO enzyme catalyzes the reaction between ribulose-1,5-bisphosphate (a pentosa, that is, a 5C monosaccharide) with CO₂, to create a six-carbon molecule, which is unstable and will be separated into two phosphoglycerate molecules, each of which has three carbon atoms.

• In plants C4, carbon dioxide, instead of immediately entering the Calvin cycle, reacts with phosphoenolpiruvate by action of the phosphoenolpiruvate carboxylase enzyme originating oxalacetate, which is later converted into mallet, 4-carbon molecules. Malate is taken to cells, where it is descarboxylated, producing CO₂ necessary for the cycle of Calvin, in addition to pyruvate.

• The CAM plants perform a process; it is given in the crasulaceae that, as adaptation to desert environments, these plants close their stomas by day and therefore could not capture CO₂ to perform photosynthesis; they absorb it at night, when the stomas open and incorporate it, as in C plants₄ to the phosphoenolpiruvato that ends up turning into malatus. The malate supplies, during the day, the CO₂ necessary for the cycle of Calvin.

Calvin Cycle

The Calvin cycle is the process by which carbon dioxide is incorporated into ribulose 1,5-bisphosphate, which ends up yielding a net molecule of glucose, which the plant uses as energy (mitochondrial respiration) and as a source of carbon, and on which most of life on Earth depends. This is the second phase of photosynthesis, where energy is stored in organic molecules such as glucose. Calvin Cycle reactions are also known as light-independent reactions.