NADPH is not produced in cyclic photophosphorylation because Photosystem II (PSII) is not involved, and the electron flow does not ultimately reduce NADP+ to NADPH.
Understanding the Key Differences
To understand why NADPH isn't produced, let's break down the key differences between cyclic and non-cyclic photophosphorylation:
Non-Cyclic Photophosphorylation (Z-scheme)
- Involves both Photosystem II (PSII) and Photosystem I (PSI).
- Water is split at PSII to replenish electrons, releasing oxygen as a byproduct.
- Electrons flow from PSII to PSI via the electron transport chain.
- At the end of the electron transport chain, electrons and protons (H+) combine with NADP+ to form NADPH.
- ATP and NADPH are produced.
Cyclic Photophosphorylation
- Involves only Photosystem I (PSI).
- Electrons excited at PSI are cycled back to the cytochrome b6f complex in the electron transport chain.
- Water is not split, so no oxygen is produced.
- The cyclic flow of electrons generates a proton gradient, which drives ATP synthesis (photophosphorylation).
- Crucially, the electrons do not reach NADP+ reductase, and therefore NADP+ is not reduced to NADPH.
Detailed Explanation
The absence of NADPH production in cyclic photophosphorylation stems directly from the electron pathway. After being excited by light in PSI, electrons are passed to ferredoxin (Fd), a mobile electron carrier. Instead of proceeding to NADP+ reductase to reduce NADP+, these electrons are shunted back to plastoquinone (PQ), which is part of the electron transport chain connecting PSII and PSI in non-cyclic photophosphorylation. This creates a cycle.
Because the electrons return to the electron transport chain, they contribute to the proton gradient used for ATP production via chemiosmosis. However, since the electrons do not reach the enzyme NADP+ reductase, which catalyzes the reduction of NADP+ to NADPH, this crucial step for NADPH synthesis is bypassed. No combination of electrons and protons with NADP+ occurs in cyclic photophosphorylation.
In summary, cyclic photophosphorylation prioritizes ATP production when NADPH levels are already sufficient or when the plant needs more ATP for processes like carbon fixation.
