Here's why:
* NADPH is primarily used in anabolic reactions (building up molecules) like photosynthesis and fatty acid synthesis. It carries electrons with high reducing power, allowing for the reduction of molecules.
* Cellular respiration is a catabolic process (breaking down molecules) that primarily uses NADH and FADH2 as electron carriers. These molecules carry electrons with lower reducing power than NADPH, which is suitable for the electron transport chain and oxidative phosphorylation, where energy is released.
While NADPH can be produced in the pentose phosphate pathway, which is a part of cellular metabolism, it's not directly involved in the main steps of cellular respiration.
