Cyclic Photophosphorylation

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 a series of cellular processes 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 are 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 required ingredients in order to produce ATP molecules while utilizing energy from sunlight. Chlorophyll molecule acts as a reaction center and the remaining molecules such as pigments within the membrane form an antenna complex.

Function of Reaction Centers and Antenna Complex

Antenna complex, as the name suggests, 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 transfers 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 travels through the electron transport system and then reduces NADP+ to form a molecule of NADPH. In this type, the electron does not travel to complete the whole cycle and does 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 a 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, an electron completes a whole cycle starting from electron activation by energy, leaving chlorophyll, entering into electron transport chain and again back to original position i.e. chlorophyll (a reaction center). Hence, this type of photophosphorylation is called cyclic photophosphorylation.

Steps Involved in Photophosphorylation are Mentioned as Follow

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

2. Due to the gaining of light energy, the electrons (from water molecules) 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 chains are 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 tend 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 the electron transport chain, a 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.)

\[NADP^{+} + H^{+} \rightarrow NADPH\]

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+, re-enters into the reaction center to repeat this cycle.

7. Some light energy is used to break water molecules (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.

\[2H_{2}O \rightarrow 4 H^{+} + O_{2} + 4 e^{-}\]

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 electrons. In the above-mentioned seventh step, two molecules of water break down such that it produces an 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 molecules 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 chains are 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 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 takes place.

3. A continual source of electrons to replenish the lost electrons from chlorophyll is required in light reactions. This electron comes from a 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.

Brief About Cyclic Photophosphorylation

Cyclic photophosphorylation is a part of biology that is studied in class 11 chapter 13 called Photosynthesis In Higher Plants. This chapter is prescribed by the Central Board of secondary education and is taught in schools that follow the curriculum set by the CBSE. Cycling photophosphorylation carries significant weightage in the examination as laid out by the Central board of secondary education and therefore is an extremely important concept in the study of biology. Students who are aspiring to go into the medical field will have to give NEET, which is the national eligibility cum entrance test. The basis for NEET is the study of biology, physics and chemistry of classes 11 and 12 and therefore students should first clear their bases which are taught in class 11 NCERT books.

The study notes on the concept of cycling photophosphorylation are provided on Vedantu‘s website and they act as an extremely important reference guide for students who are preparing for any examination that concerns biology. The study notes provided by Vedantu are made for students who may find it difficult to grasp certain complex concepts that are taught in the classroom. However, they can now get a better understanding of the topic as these notes are written in extremely simplified language. The study material on cycling photophosphorylation is repaired by Vedantu’s expert biology team who have years of experience in this field and are therefore well-versed in the study of biology. They have critically analysed the previous year question papers and the important questions that are asked in the various examinations held by the Central board of secondary education. This type of thorough research helps in making the study material up to date as the CBSE keeps on making minor changes in the curriculum every year. Therefore, students don’t have to worry about any detail that they might miss while studying the material provided by Vedantu.

Key Theories Important in the Study of Cyclic Photophosphorylation-

13.2 Early Experiments

13.3 Where does Photosynthesis take place?

13.4 How many Pigments are involved in Photosynthesis?

13.5 What is Light Reaction?

13.6 The Electron Transport

13.6.1 Splitting of Water

13.6.2 Cyclic and Non-cyclic Photo-phosphorylation

13.6.3 Chemiosmotic Hypothesis

13.7 Where are the ATP and NADPH Used?

13.7.1 The Primary Acceptor of CO2

13.7.2 The Calvin Cycle

13.8 The C4 Pathway

13.9 Photorespiration

13.10 Factors affecting Photosynthesis

13.10.1 Light

13.10.2 Carbon dioxide Concentration

13.10.3 Temperature

13.10.4 Water