Difference Between Cyclic and Noncyclic Photophosphorylation

Key Difference – Cyclic vs Noncyclic Photophosphorylation
 

Photophosphorylation or photosynthetic phosphorylation is a process in which ATP is produced during the light-dependent reactions of photosynthesis. A phosphate group is added to ADP to form ATP, utilizing the proton motive force generated during the cyclic and noncyclic electron transport chains of photosynthesis. The energy is supplied by the sunlight to initiate the processes and ATP synthesis occurs on ATPase complexes located in the thylakoid membranes of the chloroplasts. ATP synthesis during the cyclic electron flow of anoxygenic photosynthesis is known as cyclic photophosphorylation. ATP production during the noncyclic electron flow of oxygenic photosynthesis is known as noncyclic photophosphorylation. This is the key difference between cyclic and noncyclic photophosphorylation.

CONTENTS
1. Overview and Key Difference
2. What is Cyclic Photophosphorylation
3. What is Noncyclic Photophosphorylation
4. Side by Side Comparison – Cyclic vs Noncyclic Photophosphorylation
5. Summary

What is Cyclic Photophosphorylation?

Cyclic phosphorylation is a process that produces ATP from ADP during the light dependent cyclic electron transport chain of photosynthesis. Photosystem I is involved in this process. When chlorophylls of PS I absorb light energy, high energy electrons are released from the P700 reaction center. These electrons are accepted by a primary electron acceptor and then travel via several electron acceptors such as ferredoxin (Fd), plastoquinone (PQ), cytochrome complex and plastocyanin (PC). Finally, these electrons return to P700 after going through a cyclic movement. When electrons travel downhill through electron carriers, they release potential energy.  This energy is used to produce ATP from ADP by ATP synthase enzyme. Hence, this process is known as cyclic photophosphorylation.

PS II is not involved in cyclic photophosphorylation. Hence, water is not involved in this process; as a result, cyclic photophosphorylation does not generate molecular oxygen as a byproduct. Since the electrons return to the PS I, no reducing power is generated (no NADPH) during the cyclic photophosphorylation.

What is Noncyclic Photophosphorylation?

Noncyclic photophosphorylation is the process of ATP synthesis using light energy by noncyclic electron transport chain of photosynthesis. Two types of photosystems are involved in this process named PS I and PS II. Noncyclic photophosphorylation is initiated by PS II. It absorbs light energy and releases high energy electrons. Water molecules split near the PS II by releasing protons (H+ ions) and molecular oxygen due to the absorbed energy. High energy electrons are accepted by a primary electron acceptor and pass through plastoquinone (PQ), cytochrome complex, and plastocyanin (PC). Then those electrons are taken up by the PS I. The accepted electrons by PS I are passed again through electron acceptors and reach NADP⁺. These electrons combine with H⁺ and NADP⁺ to form NADPH and terminate the electron transport chain.  During the electron transport chain, the released energy is used to produce ATP from ADP. Since the electrons are not returned to PS II, this process is known as noncyclic photophosphorylation.

Compared to the cyclic photophosphorylation, noncyclic photophosphorylation is common and is widely observed in all green plants, algae, and cyanobacteria. It is a viral process for living organisms since this is the only process which liberates molecular oxygen to the environment.

What is the difference between Cyclic and Noncyclic Photophosphorylation?

Summary – Cyclic vs Noncyclic Photophosphorylation

ATP is produced by the light energy absorbed during photosynthesis. This process is known as photophosphorylation. Photophosphorylation can happen via two pathways known as cyclic and noncyclic photophosphorylation. During cyclic photophosphorylation, high energy electrons travel through electron acceptors in cyclic movements and release energy to produce ATP. During noncyclic photophosphorylation, high energy electrons flow through electron acceptors in Z-shaped noncyclic movements. Released electrons do not return to the same photosystems in noncyclic photophosphorylation. However, in both processes, ATP is produced in the same manner using the potential energy released by the electron transport chain. Noncyclic photophosphorylation produces ATP, O₂, and NADPH while cyclic photophosphorylation only produces ATP. Both photosystems are involved in noncyclic photophosphorylation while only one photosystem (PS I) is involved in cyclic photophosphorylation. This is the difference between cyclic and noncyclic photophosphorylation.

Reference:
1. “Photophosphorylation.” Wikipedia. Wikimedia Foundation, 09 Apr. 2017. Web. 16 May 2017. https://en.wikipedia.org/wiki/Photophosphorylation
2. Haneef, Deena T Kochunni Jazir. “Major Differences.” Difference Between Cyclic and Non Cyclic Photophosphorylation. N.p., n.d. Web. 16 May 2017. <http://www.majordifferences.com/2013/02/difference-between-cyclic-and-non.html#.WRtST2grLIU>

Image courtesy:
1. “Cyclic Photophosphorylation” By David Berard – Own work (CC0) via Commons Wikimedia
2. “Thylakoid membrane 3” By Somepics – Own work (CC BY-SA 4.0) via Commons Wikimedia