**5. Phosphorus as a component of ATP and NADPH**

ATP and NADPH are two energy-rich molecules that play a central role in the dark phase of photosynthesis (**Figure 2**). ATP is often referred to as the "energy currency" of the cell, as it can store and release energy in the form of phosphate bonds. In the CBB cycle, ATP provides the energy required for the fixation of CO2 into organic molecules, such as glucose and other sugars. NADPH is an electron carrier that provides the reducing power needed for the synthesis of these organic molecules. Both ATP and NADPH contain phosphate groups, which are essential for their function as energy carriers. In the case of ATP, the molecule consists of three phosphate groups linked to an adenosine molecule. The energy stored in ATP is released when one of the phosphate groups is removed through a process called hydrolysis, resulting in the formation of adenosine diphosphate (ADP) and inorganic phosphate (Pi). This energy can then be used to drive various cellular processes, including the reactions of the CBB cycle [37]. Similarly, NADPH contains a phosphate group as part of its structure. The molecule is derived from its oxidized form, NADP<sup>+</sup> , by the addition of two electrons and a proton, resulting in the formation of NADPH and the release of a second proton. The reducing power of NADPH is then used in the CBB cycle to convert the fixed CO2 into organic molecules, such as glucose and other sugars [48].

Given the central role of ATP and NADPH in the dark phase of photosynthesis, the availability of P can have a significant impact on the efficiency of the CBB cycle and, consequently, on plant growth and productivity. Indeed, several studies have reported a strong correlation between P-availability and the rate of photosynthesis, with P-deficiency leading to reduced rates of CO2 assimilation and decreased production of ATP and NADPH [49].
