Question

What happens during non-cyclic photophosphorylation?

Answer 1

Photophosphorylation:

1. Photophosphorylation is the use of solar energy to phosphorylate ADP and produce ATP during photosynthesis.
2. Photosynthesis converts light energy into chemical energy.
3. In this process, light energy is captured and used to convert carbon dioxide and water into glucose and oxygen gas.
4. Photophosphorylation can follow the cyclic or non-cyclic cycle.
5. The complete process of photosynthesis is carried out in two ways: light reaction and dark reaction.

Non-Cyclic Photophosphorylation:

1. In the first case, the water molecule is decomposed to $2\mathrm{H}+\frac{1}{2}{\mathrm{O}}_2+2{\mathrm{e}}^{-}$ by a process called photolysis (light splitting).
2. The two electrons of the water molecule are then kept in photosystem II, while the 2H+ and ½O2 are released for other uses.
3. A photon is then absorbed by the chlorophyll pigments surrounding the reaction center of the optical system.
4. Light excites the electrons in each pigment, setting off a chain reaction that ultimately transfers the energy to the center of photosystem II, and excites the two electrons to be transferred to the primary electron acceptor, pheophytin.
5. The electron shortage is compensated by taking electrons from another water molecule.
6. Electrons are transferred from pheophytin to plastoquinone, taking 2 electrons from pheophytin, and two hydrogen ions from the stroma and forming PQH2, which is then cleaved to PQ, 2 electrons are released into the cytochrome b6f complex and two hydrogen ions are left in the lumen of the thylakoid.
7. Then the electrons pass through Cyt b6 and Cyt f. They are then infused with plastocyanin, which powers the hydrogen (H⁺) ions bound into the thylakoid space.
8. This creates a gradient, where hydrogen ions flow back to the chloroplast's buffer, providing energy for ATP regeneration.
9. Photosystem II is very picky and replaces its lost electrons from an external source; however, the other two electrons are not returned to photosystem II as they would be in the cyclic pathway.
10. Instead, the still excited electrons are transferred to a photosystem I complex, which raises their energy level to a higher level using a second solar photon.
11. The extremely excited electrons are transferred to the acceptor molecule, but this time they are transferred to an enzyme called Ferredoxin-NADP + reductase that uses them to catalyze the reaction (as shown below):
    ${\mathrm{NADP}}^{+}+2\mathrm{H}+2{\mathrm{e}}^{-}\to \mathrm{NADPH}+{\mathrm{H}}^{+}$
12. This consumes the hydrogen ions produced by water splitting, resulting in the production of $\frac{1}{2}{O}_2$ , ATP , and ${\mathrm{NADPH}}^{+}{\mathrm{H}}^{+}$ using solar and water photons.
13. NADPH concentrations in chloroplasts can help regulate the paths of electrons in light reactions.
14. When chloroplasts lack ATP for the Calvin cycle, NADPH accumulates and the plant can switch from acyclic electron flow to a cyclic electron flow.