Cyclic and Non Cyclic Photophosphorylation

Photosynthesis is the method in which the phosphorylation of ADP to generate ATP occurs with the help of the energy in the form of sunlight is known as photophosphorylation. Only two sources of energy are accessible to living organisms: sunlight and reduction-oxidation (redox) reactions. All organisms produce ATP, which is the common energy currency of life. Generally, in photosynthesis this involves photolysis, or photodissociation, of water and a constant unidirectional flow of electrons from water to photosystem II.

In the photophosphorylation process, light energy is used to make a high-energy electron donor and a lower-energy electron acceptor. Electrons then move suddenly from donor to acceptor through an electron transport chain.

In simple words, Photophosphorylation is the use of sunlight energy to phosphorylate ADP to produce ATP during photosynthesis.

ATP and Reaction 

ATP is produced by an enzyme called ATP synthase. Both the structure of this enzyme and its primary gene are remarkably alike in all known forms of life.

ATP synthase is run by a transmembrane electrochemical potential channel, usually in the form of a proton channel. The role of the electron transport chain is to generate this gradient. In all living organisms, a sequence of redox reactions is used to create a transmembrane electrochemical potential gradient or known as proton motive force (pmf).

Redox reactions are chemical reactions in which electrons are shifted from a donor molecule to an acceptor molecule. The principal force driving these reactions is the Gibbs free energy of the reactants and products. Gibbs free energy is the energy offered (“free”) to do work. Any reaction that lowers the overall Gibbs free energy of a system will proceed spontaneously (given that the structure is isobaric and also adiabatic), although the reaction may progress slowly if it is kinetically inhibited.

The shifting of electrons from a high-energy molecule (the donor) to a lower-energy molecule (the acceptor) can be systematically separated into a series of in-between redox reactions. This is an electron transport chain.

The fact that a reaction is thermodynamically possible does not mean that it will truly occur. A combination of hydrogen gas and oxygen gas does not spontaneously burn. It is necessary either to provide activation energy or to minimize the intrinsic activation energy of the system, in order to do that most biochemical reactions continue at a useful rate. Living systems use complex macromolecular structures to lower the activation energies of biochemical reactions.

A thermodynamic reaction which travels from a higher-energy state to a lower-energy state such as separation of charges, or the creation of an osmotic gradient, in such a way that the total free energy of the system decreases and making it thermodynamically possible, while useful work is completed at the same time.

Electron transport chains (most commonly known as ETC) generate energy in the form of a transmembrane electrochemical potential channel. This energy is used to do valuable work. The channel can be used to transport molecules across membranes. It can be used to do mechanical work, such as revolving bacterial flagella. It can be used to produce ATP and NADPH, high-energy molecules that are required for growth.

Inside a chloroplast, there are thylakoid disks that have their own phospholipid bilayer membrane, which includes embedded proteins, which allow the process of cyclic and non-cyclic photophosphorylation to take place.

We all are well mindful of the complete process of photosynthesis.  It is the biological procedure of converting light energy into chemical energy. In this method, light energy is captured and used for converting carbon dioxide and water into glucose and oxygen gas. The complete process of photosynthesis is carried out into two ways:

Light Reaction 

The light reaction process occurs in the grana of the chloroplast. Where light energy gets transformed into chemical energy as ATP and NADPH. In this very light reaction, the adding of phosphate in the presence of light or the making of ATP by cells is known as photophosphorylation.

Dark Reaction

In the dark reaction, the energy produced earlier in the light reaction is used to fix carbon dioxide into carbohydrates. The location where this occurs is in the stroma of the chloroplasts.

We will look at the comprehensive process of photophosphorylation i.e. the light reaction:

Photophosphorylation is the process of creating energy-rich ATP molecules by shifting the phosphate group into ADP molecule in the presence of light. This process may be either a cyclic process or a non-cyclic process.

Non-Cyclic Photophosphorylation

The other pathway of light reaction, non-cyclic photophosphorylation, is a two-stage process comprising two different chlorophyll photosystems. Being a light reaction, non-cyclic photophosphorylation happens in the thylakoid membrane. Where first, a water molecule is broken down into 2H⁺ + ½ O₂ + 2e^- by a procedure called photolysis (light-splitting). Then the two electrons from the water molecule are preserved in photosystem II, while the 2H⁺ and ½ O₂ are released for other use. Then a photon is absorbed by chlorophyll pigments which surround the reaction core center of the photosystem. The light stimulates the electrons of each pigment, producing a chain reaction that finally transfers energy to the core of photosystem II, stimulating the two electrons that are transferred to the primary electron acceptor, pheophytin. The shortage of electrons is replenished by taking electrons from another water molecule. 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 PQ, the 2 electrons are released to Cytochrome b6f complex and the two hydrogen ions are left out into thylakoid lumen. The electrons then travel through the Cyt b6 and Cyt f. Then they are passed along with plastocyanin, providing the energy for hydrogen ions (H⁺) to be forced into the thylakoid space. This produces a gradient, making hydrogen ions flow back into the stroma of the chloroplast, by providing the energy for the regeneration of ATP.

Photosystem II was difficult and it replaced its lost electrons from an exterior source; however, the two other electrons are not returned to photosystem II as they would do in the cyclic pathway. Instead, the still-stimulated electrons are relocated to a photosystem I complex, which increases their energy level to a higher level using a second solar photon. The extremely stimulated electrons are transferred to the acceptor molecule, but this time they are passed on to an enzyme known as Ferredoxin-NADP+ reductase which uses them to catalyze the reaction (as shown below):

NADP+ + 2H⁺ + 2e^- → NADPH + H⁺

This consumes the hydrogen ions created by the splitting of water, which give rise to net manufacture of ½ O₂, ATP, and NADPH + H⁺ with the use of solar photons and water. The concentration of NADPH in the chloroplast may help to regulate which pathway electrons take along the light reactions. When the chloroplast runs low on ATP level for the Calvin cycle, NADPH will collect, and the plant may shift from non cyclic to cyclic electron flow.

Difference between Cyclic Photophosphorylation and Non- Cyclic Photophosphorylation

Similarities

• Photophosphorylation, both cyclic and noncyclic, is a light-dependent photosynthetic process.

• Photophosphorylation mechanisms, both cyclic and noncyclic, are components of ETS that carry out phosphorylation (phosphate group addition) or ATP synthesis.

Conclusion

As a result, we can deduce that light-dependent photosynthetic processes that carry out phosphorylation to make ATP are cyclic and noncyclic photophosphorylation. The photosynthetic cells then employ the ATP to undertake numerous actions necessary for their development and survival.