Cyclic Photophosphorylation

Cyclic Photophosphorylation - Definition, Types and Process

Phosphorylation is considered as an important process during photosynthesis. During photosynthesis, the conversion or phosphorylation of adenosine diphosphate (ADP- contains two phosphate group) to adenosine triphosphate (ATP- contains three phosphate groups) using the photon of sunlight is called phosphorylation. The process of photophosphorylation in which an electron expelled by the excited photo-center is returned to it after passing through a series of electron carriers is known as cyclic photophosphorylation is. It takes place under the condition of low light intensity and light of wavelength lower than 680 nm and when CO2 fixation is inhibited. 

Plants are capable of producing energy by utilizing photons from sunlight through photophosphorylation. Plants spread their leaves to the sky in order to carry out light reactions of photosynthesis and by doing this, they absorb some energy. This energy is then converted into chemical energy to provide energy for the normal physiological functioning of plants. In this reaction, light provides the energy and water provides the electron-molecule. This is a major difference between photosynthesis and cellular respiration.

During this reaction, photosynthetic pigments of plants absorb light that activates series of cellular process that ultimately converts light energy into chemical energy and stored in the bonds of the energy molecule ATP. The process of utilizing light energy and electron transport chain to make ATP is known as photophosphorylation. This reaction's name itself suggests the process of gaining a phosphate molecule. ADP molecule gains this phosphate molecule and produces a molecule of ATP. Hence, photophosphorylation is also called as light reactions of photosynthesis. 

Location of Photophosphorylation


The electron transport chains for photosynthesis is carried out in the thylakoid membranes of chloroplasts. This is mainly due to the availability of chlorophyll molecules and accessory pigments to absorb light energy. These both are must required ingredients in order to produce ATP molecule while utilizing energy from sunlight. Chlorophyll molecule acts as a reaction centers and the remaining molecules such as pigments within the membrane form an antenna complex.

The function of Reaction Centers and Antenna Complex


Antenna complex, as the name suggests, it is responsible for the absorption of light energy (also known as photon molecule) and then, it transfers energy into the reaction centers. These reaction centers are key locations where the photon energy is transferred into the electron transport system.

Process of Photophosphorylation


The electrons enter into an excited state i.e. higher energy state when the reaction center chlorophyll receives light energy. This step is causing them to the outer electron orbitals and then to attach a protein in the electron transport chain. This is the step when the plant cell transfer light energy to chemical energy. 

There are two types of photophosphorylation occurs in cells:

  • 1. Noncyclic Photophosphorylation: It is also called as Z-scheme. In this type, an electron from chlorophyll travel through the electron transport system and then, reduce NADP+ to form a molecule of NADPH. In this type, the electron does not travel complete the whole cycle and do not return to the chlorophyll as it is utilized in the reduction of NADP+. It is the only one-way ride for an electron from water molecule to NADPH. Hence, it is called noncyclic photophosphorylation.

  • 2. Cyclic Photophosphorylation: In this type, when an electron gets excited, it leaves chlorophyll, then they travel through the electron transport circuit. Then, they return to chlorophyll again after the energy transfer process to ATP is completed. In this way, electron completes a whole cycle starting from electron activation by energy, leaving chlorophyll, enters into electron transport chain and again back to original position i.e. chlorophyll (a reaction centers). Hence, this type of photophosphorylation is called cyclic photophosphorylation.

  • The steps involved in photophosphorylation are mentioned as follow:

  • 1. Light photon energy is absorbed by antenna complex and followed by its transfer to chlorophyll (reaction centers).

  • 2. Due to the gaining of light energy, the electron (from water molecule) present in reaction centers are excited and move to outer orbitals.

  • 3. In this process, this electron enters into the electron transport chain. (Electron transport chain is collectively made up of membrane-embedded proteins and organic molecules. These are organized into four large complexes known as I to IV.)

  • 4. Proteins present in the electron transport chain tends to pull the electron from chlorophyll and pass them along the chain of proteins.

  • 5. During this movement of an electron through different proteins of electron transport chain, chemiosmosis reaction takes place and as a result, ATP is formed.


  • The energy from the movement of electrons is used to transport hydrogen ions (H+) across the thylakoid membrane. Every single movement of electron transport is coupled with the movement of hydrogen ions. The energy associated with the movement of hydrogen ions is used to make ATP from ADP and inorganic phosphate. For this reaction to take place, enzyme ATP synthase is required.

  • 6. After passing through the proteins of electron transport chain, this electron is accepted by NADP+ molecule, and in turn, it is reduced and produce its reduced form i.e. NADPH. (NADP+: nicotinamide adenine dinucleotide phosphate and NADPH, a reduced form of NADP+. NADP+ molecule acts as an electron carrier.)


  • NADP+ + H+ → NADPH

    The above-mentioned sixth step takes place only during noncyclic photophosphorylation. In cyclic photophosphorylation, the electron, after passing through the electron transport chain, instead of reacting with NADP+, reenter into reaction center to repeat this cycle.

  • 7. Some light energy is used to break water molecule (H2O) by photolysis and produces protons (H+), electrons (e-), and oxygen gas (O2). These electrons are now transferred to chlorophyll. This is particularly important in order to replace the lost electron. This step is also shown only in noncyclic photophosphorylation. The proton ions released by this reaction are released into the plant cell. The liberated oxygen by this reaction is released into the cell and ultimately, released in the atmosphere as a waste product photosynthesis.


  • 2H2O → 4 H+ + O2 + 4 e-

    Note- Oxygen molecule (O2) released as a part of photosynthesis does not come from carbon dioxide (CO2). As mentioned in the above step, it is produced when the water molecule is split to provide electron. In the above-mentioned seventh step, two molecules of water break down such that it produces oxygen molecule, not an oxygen atom. 
     
    The electron from the water molecule does not enter into the ATP molecule during the light reaction. The overall electron transport flow can be mentioned as follow:

  • 1. Electrons from water molecule enter into the chlorophyll to replace the lost electron.

  • 2. Then they enter the electron transport chain.

  • 3. Finally, it is accepted by NADP+ to form reduced NADP+.

  • The pathway of energy flow is different from that of electron transport flow. The energy transfer pathway is mentioned as follow:

  • 1. First, light energy is absorbed by the antenna complex.

  • 2. Then, it transfers to the reaction center chlorophyll.

  • 3. After that, it is transferred to the electron transport chain.

  • 4. Then it is transferred to proton motive force (movement of H+ during electron transport chain) and finally, it transfers to ATP.

  • Details of Electron Transport Chain


    Electron transport chain is collectively made up of membrane-embedded proteins and organic molecules. The electronic transport chain components are found in the plasma membrane of prokaryotes, whereas in eukaryotes, many copies of these molecules are found in the inner mitochondrial membrane. The electron transport chain contains proteins such as Fd (ferredoxin), PQ (plastoquinone), Cyt C (cytochrome C), Q (ubiquinone), and PC (plastocyanin). The enzyme NADP reductase is also present.

    While traveling of an electron through the chain, it enters into a lower energy level from a higher energy level. It means it moves from less electron-hungry molecules to more electron-hungry molecules. Hence, this type of transfer of electron is an example of downhill electron transfer. The above-mentioned different protein complexes use the released energy (released during electron transfer) and that turn out into pumping of a proton from the mitochondrial matrix to the intermembrane space. 

    Summary


  • 1. In the light reaction of photosynthesis, a plant converts energy from one form to another" from solar energy to potential energy to chemical energy.

  • 2. The location of light reaction is in the thylakoid membranes. It starts with absorbing energy from sunlight, followed by a series of events and ultimately, the generation of ATP molecules take place.

  • 3. A continual source of electrons to replenish the lost electrons from chlorophyll is required in light reactions. This electron comes from water molecule which breaks down and releases oxygen gas as a byproduct.

  • 4. At the end of this process, the cell is full of high energy molecules like NADPH and ATP- those can be used in the Calvin cycle for the production of carbohydrates.