During light reactions,where oes the hydrogen atom in NADPH comes from?

Question

During light reactions,where oes the hydrogen atom in NADPH comes from?

Answer 1

Noncyclic photophosphorylation. There are 2 pathways for energy production in chloroplasts, cyclic and non-cyclic.

Noncyclic photophosphorylation is a two-stage process involving two different chlorophyll photosystems. First, a water molecule is broken down into 2H+ + 1/2O2 + 2e- by a process called photolysis (or light-splitting). The two electrons from the water molecule are kept in photosystem II, while the 2H+ and 1/2O2 are left out for further use. Then a photon is absorbed by the chlorophyll core of photosystem II, exciting the two electrons which are transferred to the acceptor molecule. The deficit of electrons is replenished by taking electrons from another molecule of water. The electrons transfer from the primary acceptor to plastoquinone, then to plastocyanin, producing proton-motive force as with cyclic electron flow and driving ATP synthesis.

The photosystem II complex replaced its lost electrons from an external source, however, the two other electrons are not returned to photosystem II as they would in the analogous cyclic pathway. Instead, the still-excited electrons are transferred to a photosystem I complex, which boosts their energy level to a higher level using a second solar photon. The highly excited electrons are transferred to the acceptor molecule, but this time are passed on to an enzyme called Ferredoxin- NADP reductase|NADP+ reductase, for short FNR, which uses them to catalyst the reaction (as shown):

NADP+ + 2H+ + 2e- → NADPH + H+

This consumes the H+ ions produced by the splitting of water, leading to a net production of 1/2O2, ATP, and NADPH+H+ with the consumption of solar photons and water.

The concentration of NADPH in the chloroplast may help regulate which pathway electrons take through the light reactions. When the chloroplast runs low on ATP for the Calvin cycle, NADPH will accumulate and the plant may shift from noncyclic to cyclic electron flow.

Briant Burke, MD

www.naturopathix.com