**3. Calvin-Benson cycle: An overview**

The dark phase of photosynthesis, or the Calvin-Benson cycle, is a series of enzyme-catalyzed reactions that occur in the stroma of chloroplasts [20]. This cycle is responsible for the fixation of CO2 into oces, ultimately producing glucose and other sugars that can be used for various cellular processes [21]. Calvin-Benson cycle consists of three main stages: carboxylation, reduction, and regeneration [22] (**Figure 1**).

The Calvin cycle, also known as the dark phase of photosynthesis, is a series of enzyme-driven reactions that convert carbon dioxide (CO2) into sugars and other organic molecules. The cycle consists of three main phases: carbon fixation, reduction, and regeneration of the CO2 acceptor molecule ribulose-1,5-bisphosphate (RuBP). During these reactions, P plays a vital role in the form of energy transfer molecules (ATP and NADPH) [23].

#### **3.1 Carboxylation**

Carboxylation is the first step of the Calvin-Benson cycle, where CO2 is fixed into an organic molecule through the enzyme ribulose-1,5-bisphosphate carboxylase/ oxygenase (RuBisCO) [24]. RuBisCO catalyzes the reaction between CO2 and the five-carbon sugar ribulose-1,5-bisphosphate (RuBP), resulting in two molecules of 3-phosphoglycerate (3-PGA) [25]. The carbon fixation phase begins with the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), which catalyzes the

#### **Figure 1.**

*Calvin–Benson cycle consists of carbon fixation with carbon dioxide, reduction with NADPH and regeneration of the CO2 acceptor. Image adopted from Schreier and Hibberd [22].*

fixation of CO2 to RuBP, producing two molecules of 3-phosphoglycerate (3-PGA) [25]. P is crucial in this step as 3-PGA contains a phosphate group, which is essential for the subsequent steps of the Calvin cycle.
