Question

How many ATP are produced during non-cyclic photophosphorylation?

Answer 1

Non-cyclic photophosphorylation:

1. Non-cyclic photophosphorylation is a two-stage process comprising two different chlorophyll photosystems i.e. PS I and PS II.
2. In the process of photophosphorylation, energy from sunlight is utilized to yield a high-energy electron donor and a lower-energy electron acceptor.
3. In the electron transport chain, the electrons move from donor to acceptor.
4. Being a light reaction, non-cyclic photophosphorylation happens in the thylakoid membrane.
   Here first, a water molecule is broken down into 2H+ + ½ O2 + 2e- by a procedure called photolysis (light-splitting).
5. Then the two electrons from the water molecules are preserved in photosystem II while the 2 H+and ½ O2 are released for other use.
6. Then a photon is absorbed by chlorophyll pigments which surround the reaction core centre of the photosystem.
7. The light stimulates the electrons of each pigment, producing a chain reaction that finally transfers energy to the core of PS II, stimulating the two electrons that are transferred to the primary electron acceptor, pheophytin.
8. The shortage of electrons is replenished by taking electrons from another water molecule.
9. The electrons transfer from pheophytin to plastoquinone, which takes the 2 electrons from pheophytin, and two hydrogen ions from the stroma and forms PQH2, which later is broken into PA the 2 electrons are released to cytochrome b6f complex and the two hydrogen ions are left out into thylakoid lumen.
10. The electrons then travel through the Cyt b6 and Cyt f.
11. Then they are passed along with plastocyanin, providing the energy for hydrogen ions to be forced into the thylakoid space.
12. This produces a gradient, making hydrogen ions flow back into the stroma of the chloroplast, by providing the energy for the regeneration of ATP.
13. 1 ATP and 2 NADPH₂ molecules are produced in a single step of non-cyclic photophosphorylation.
14. Since each NADPH₂ produced is equivalent to 3 ATP molecules.
15. Therefore, 2 NADPH₂ will yield 6 ATP molecules.
16. Hence, the addition of 1 ATP and 2 NADPH2, it will give 7 ATP molecules.
17. Overall 7 ATP molecules are produced during non-cyclic photophosphorylation.