**4. Phosphorus in ATP synthesis and NADPH production**

In the dark phase of photosynthesis, P is mainly involved in the synthesis of ATP and the production of NADPH. Both ATP and NADPH are essential energy carriers in the Calvin-Benson cycle, providing the energy and reducing power required for the fixation of CO2 into organic molecules [1].

### **4.1 ATP synthesis**

Adenosine triphosphate (ATP) is a high-energy molecule that serves as the primary energy currency for cells [33]. It is synthesized in the light phase of photosynthesis through a process called photophosphorylation, which occurs in the thylakoid membrane of chloroplasts (**Figure 2**). During this process, the energy derived from absorbed light is used to pump protons across the thylakoid membrane, creating a proton gradient [35]. The resulting proton motive force drives the synthesis of ATP from ADP and inorganic phosphate (Pi) through the enzyme ATP synthase [36]. P is a critical component of ATP, as it forms the phosphate groups that store and release energy during ATP hydrolysis. High-energy phosphate, held as a part of the chemical *Role of Phosphorus in the Photosynthetic Dark Phase Biochemical Pathways DOI: http://dx.doi.org/10.5772/intechopen.112573*

**Figure 2.**

*Light reactions harness energy from the sun to produce chemical bonds, ATP, and NADPH. Image adopted from [34].*

structures of adenosine diphosphate (ADP) and ATP, is the source of energy that drives the multitude of chemical reactions within the plant [33]. When ADP and ATP transfer the high-energy phosphate to other molecules (termed phosphorylation), the stage is set for many essential processes to occur.

Several enzymes are involved in the process of ATP synthesis including ATP synthase [37], which according to several studies is sited in the thylakoid membrane of chloroplasts [38–40]. The activity of ATP synthase, the enzyme responsible for ATP synthesis, is regulated by several factors including the availability of phosphorus [41, 42]. Studies have revealed that P-availability can affect ATP synthesis and cellular energy metabolism. P-deficiency in plants can decrease ATP synthesis and lead to reduced growth and development [9]. This is because P-deficiency inhibits the catalytic activity of ATP synthase, which catalyzes ATP synthesis. Additionally, P-limitation in marine phytoplankton was reported to reduce ATP synthesis [43]. In the dark phase of photosynthesis, ATP is used as an energy source for various enzymatic reactions, such as the phosphorylation of 3-PGA by PGK [44].

#### **4.2 NADPH production**

Nicotinamide adenine dinucleotide phosphate (NADPH) is another essential energy carrier in the Calvin-Benson cycle, where it provides reducing power for the conversion of 1,3-BPG to G3P by GAPDH [29]. NADPH is produced during the light phase of photosynthesis in a process called linear electron flow (LEF), which occurs in the thylakoid membrane of chloroplasts [45]. During LEF, electrons are transferred from water molecules to NADP<sup>+</sup> through a series of protein complexes and electron carriers, including photosystem II (PSII), the cytochrome b6f complex, photosystem I (PSI), and ferredoxin-NADP+ reductase (FNR) [46]. The reduction of NADP+ to NADPH involves the transfer of two electrons and one proton, with the latter being derived from the hydrolysis of water molecules. P is not directly involved in the production of NADPH, but it is essential for the stability and function of NADP<sup>+</sup> and NADPH, as they both contain a phosphate group [47].
