Cyclic and Non Cyclic Photophosphorylation

Difference between Cyclic Photophosphorylation and Non- Cyclic Photophosphorylation


Photosynthesis is the method in which, the phosphorylation of ADP to generate ATP occur with the help of the energy in form of sunlight is known 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 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.

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. The Gibbs free energy is the energy offered (“free”) to do work. Any reaction that lower 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 a bit 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 are present 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 in the stroma of the chloroplasts.

We will look at the comprehensive process of photophosphorylationi.e. the light reaction

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

Cyclic Photophosphorylation:

Cyclic photophosphorylation is a procedure where the electron is recycled. One of the constituents in the thylakoid membrane is a photosystem, which is packed with chlorophyll. The chlorophyll absorbs the light energy and uses it to stimulate the electron. The electron is then passed towards to an electron acceptor protein, which passes it along with an electron transport channel. As the electron is passed along the transport channel, the electron loses energy, which is then used to make ATP from ATP and Pi. The electron is then recycled and again enters into the photosystem again. Photophosphorylation happens on the stroma lamella or frets. In cyclic photophosphorylation, the high energy electron is free from P700 to ps1 flow down to a cyclic pathway. In cyclic electron flow, the electron starts in a pigment complex called photosystem I, then it passes from the primary acceptor to ferredoxin then to plastoquinone, and then to cytochrome b6f (a similar complex to that is also found in mitochondria), and then to plastocyanin before coming back to chlorophyll. This transfer or shifting channel produces a proton-motive force (PMF), pumping H+ ions across the membrane; this generates a concentration gradient that can be required to power ATP synthase during chemiosmosis process. This pathway is identified as cyclic photophosphorylation, and it produces neither oxygen (O2) nor NADPH. On the other hand, non-cyclic photophosphorylation, NADP+ does not take the electrons; they instead sent back to cytochrome b6f complex.

In bacterial photosynthesis, a single photosystem is needed and therefore is involved in cyclic photophosphorylation. It is ideal in anaerobic conditions and conditions of high irradiance and CO2 compensation points.

Non-Cyclic Photophosphorylation:

The other pathway of light reaction is, 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+ + 1/2 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 1/2O2 are released out for other use. Then a photon is absorbed by chlorophyll pigments which are surrounding 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 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.

The photosystem II 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 1/2O2, 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 noncyclic to cyclic electron flow.

Difference between Cyclic Photophosphorylation and Non- Cyclic Photophosphorylation
Cyclic PhotophosphorylationNon-Cyclic Photophosphorylation
Only Photosystem I involved in the process.Both Photosystem I and II are involved in the process
In Cyclic PhotophosphorylationP700 is the active reaction center.In non-Cyclic PhotophosphorylationP680 is the active reaction center.
Electrons passes in a cyclic manner.Electrons passes in a non – cyclic manner.
Electrons return back to Photosystem IElectrons from Photosystem I am accepted by NADP and it does not return back.
ATP molecules are generated.Both NADPH and ATP molecules are formed.
Water is not required in this process. Water is required ad the Photolysis of water take place.
NADPH is not produced.NADPH is produced.
Oxygen is not developed as the by-productOxygen is developed as a by-product.
This process is ideal only for bacteria.This process is ideal in all green plants.