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Dark or chemical phase: glucose production


In this phase, the energy contained in the ATP and the NADPH hydrogens2, will be used for the construction of glucose molecules. Glucose synthesis occurs during a complex cycle of reactions (called the pentose cycle or Calvin-Benson cycle), in which several simple compounds participate.

During the cycle, CO molecules2 they join together forming carbon chains that lead to glucose production. The energy required for the establishment of energy-rich chemical bonds comes from ATP and the hydrogen that will promote the reduction of CO2 are provided by NADPH2. See in more detail the Calvin cycle

The Calvin Cycle

The cycle begins with the reaction of a CO molecule.2 with a five carbon sugar known as ribulose diphosphate catalyzed by the enzyme rubisco (ribulose bisphosphate carboxylase / oxygenase, RuBP), one of the most abundant proteins in the plant kingdom.
An unstable six-carbon compound is then formed and soon broken down into two three-carbon molecules (2 molecules of 3-phosphoglyceric acid or 3-phosphoglycerate, known as PGA). The cycle continues until in the end a molecule of glucose and the molecule of ribulose diphosphate.

Note, however, that for the cycle to make logical sense, one must admit the reaction of six CO molecules.2 with six ribulose diphosphate molecules, resulting in one glucose molecule and the regeneration of another six ribulose diphosphate molecules.

CO reduction2 is made from the hydrogen supply by the NADH2 and the power is supplied by the ATP. Remember that these two substances were produced in the clear phase.

The scheme presented is a simplification of the Clavin cycle: in fact, the reactions of this cycle resemble those occurring in glycolysis, but in reverse.

It is also correct to assume that the cycle originates units of type CH2O, which can be channeled into the synthesis of glucose, sucrose, starch and even amino acids, fatty acids and glycerol.