Cyclic and Non-Cyclic Photophosphorylation, Practice Problems and FAQs

You all know that we all depend on plants directly or indirectly for our food and they prepare their food through the process of photosynthesis. What is the first thing that comes to our mind when we think about photosynthesis? Yes, it will be either sunlight or chloroplast.

Fig: Photosynthesis

But is sunlight really required for plants in the time of making the food? The answer can be no!! Sunlight is not directly required for the preparation of sugar. But where is the sunlight used then? Sunlight is used in the light reaction of photosynthesis where the plants make the energy molecules like ATP and NADPH. These energy molecules are later used for the preparation of sugar molecules in dark reactions.

Hence the process of photosynthesis can be divided into light dependent and light independent reactions. In light dependent reactions the light energy is used for making energy molecules and in light independent reactions, these energy molecules are utilised to produce the sugar molecules. Hence the light dependent reactions are called light reactions or photochemical phase and light independent reactions are called dark reactions or biosynthetic phase.

Fig: Phases of photosynthesis

Do you know what processes actually happen in the light reactions? There are major two processes called cyclic and non cyclic photophosphorylation that happen in this phase to generate the energy molecules. Are these processes complex or simple? Is this process similar for all plants? Now we know that energy molecules are synthesised here. But how are these molecules produced? Do these processes happen in the same site? We have too many questions to solve, right? So here in this article we are going to discuss more about the cyclic and non-cyclic photophosphorylation.

Table of contents

• Light reactions
• Cyclic photophosphorylation
• Non cyclic photophosphorylation
• Cyclic photophosphorylation vs Non-cyclic photophosphorylation
• Practice Problems
• FAQs

Light reaction

The series of reactions in the light reaction are directly powered by light. Here, the pigments in the antenna complex capture the light energy and transfer it to chlorophyll a. The light-harvesting complex is associated with the antenna complex. Antenna complex is a collection of proteins and photosensitive pigments like chlorophyll and carotenoids. Light reaction is happening in the thylakoid membranes of chloroplast and is also known as the photochemical phase.

The photochemical phase involves the capture of light energy by photosystems I and II, photolysis of water to compensate for the electron deficit in PS II, oxygen release from the splitting of water molecules, and photophosphorylation, which produces high-energy chemical intermediates like ATP and NADPH. Let’s understand photophosphorylation now. .

Photophosphorylation

Phosphorylation is the process by which ADP is given a phosphate group in order to create ATP. The name ‘photophosphorylation’ refers to the fact that photons, which are light particles, power the entire process of phosphorylation here. There are two types of photophosphorylation. They are cyclic and non cyclic photophosphorylation. 

Fig: Types of photophosphorylation

If the electrons from PS I return to the same photosystem then it is called cyclic photophosphorylation. If the electrons from PS II are used by PS I, then it is called non-cyclic photophosphorylation. 

First we will discuss cyclic photophosphorylation.

Cyclic photophosphorylation

The photophosphorylation process that results in the cyclic movement of electrons during the creation of ATP molecules is known as cyclic photophosphorylation. It is the process of photophosphorylation in which the electron expelled by the excited photocentre is returned to it after passing through a series of electron carriers. It can occur in both aerobic and anaerobic conditions. The lamellar area of the chloroplast is where it occurs. When only light with longer wavelengths (700 nm or above) is available, cyclic photophosphorylation takes place.

Fig: Reactions of cyclic photophosphorylation

Steps of cyclic photophosphorylation

The following steps are involved in cyclic photophosphorylation: 

I) Light hits PS I

A photon which is a particle with a quantum of light energy, strikes PS I at the initial step of cyclic photophosphorylation. The accessory pigments in the PS I take up the energy.

II) Accessory pigments of PS I → P700 reaction centre

The P700 reaction centre receives the energy from the accessory pigments now. By absorbing energy, the outermost electron of the P700 reaction centre is excited to a higher energy level. There is now a single electron missing from the P700 reaction centre.

III) P700 to ferredoxin (Fd) 

The P700 reaction centre's electron is unstable at the higher energy level. It tends to leave because it wants to avoid instability. Therefore, the electron of P700 reaction centre jumps out of its higher energy orbital and is taken up by chlorophyll A0, a distinct modified chlorophyll molecule. A molecule with a greater potential, A1, receives its electron from A0. Now, an iron-sulphur cluster will get the electron that was previously in molecule A1. The electron is sent away by Fe-S, and it is captured by Fd or ferredoxin. The stroma and thylakoid membrane both include Fd, a tiny, water-soluble movable protein. This protein is movable. The Fd molecule is now the recipient of the electron from P700.

Fig: Path of electrons from P700 to ferredoxin

IV) Ferredoxin → Plastoquinone

Fd or ferredoxin is a carrier of mobile electrons. From the Fe-S complex to PQ, it passes electrons via the stroma.

V) Plastoquinone → Cyt b6f

Two hydrogen molecules are transported by PQ or plastoquinone, a mobile transporter. Therefore, it also requires 2e-. Every molecule, including P700, Fd, PQ, and others, exists in many copies. Thus, one additional electron from the P700 reaction centre travels from PS I to PQ with the aid of another Fd as the second photon strikes PS I. PQ forms PQH2 as a result of the two electrons and the two H + ions (from the stroma). It travels from the stromal side of the thylakoid membrane to the lumen side before arriving at the Cyt b6f complex. PQ releases the two H + ions at the lumen side and transfers two electrons to Cyt b6f on the thylakoid membrane.

VI) Cyt b6f → Phycocyanin → P700

The next mobile carrier protein, phycocyanin, receives two electrons from the Cyt b6f. The electron is delivered to the P700 reaction centre or RC by the phycocyanin as it travels through the lumen. Two electrons are used to make up for the two-electron deficits in the two P700 reaction centres. The two P700 RCs are now functioning normally.

VII) Chemiosmosis

Ions migrate down their electrochemical gradient across a selectively permeable membrane by a process known as chemiosmosis.The influx of H+ ions during the electron transport by PQ results in a higher H+ion concentration in the lumen and lower H+ ion concentration in the stroma. These H + ions undergo facilitated diffusion through the ATP synthase. ATP synthase rotates and forms ATP by the process of phosphorylation. Since in this photophosphorylation the emitted electrons are coming back to the PS I, it is called cyclic photophosphorylation.

Fig: Cyclic photophosphorylation

Non-cyclic photophosphorylation

Non cyclic photophosphorylation is the normal process of photophosphorylation in which the electron expelled by the excited photosystem does not return to it. It occurs when there is bright sunlight. It involves both PS I and PS II, the photosystems. The Z scheme is another name for it. It occurs when the shorter wavelength light like 680 nm and lower is available. It produces both ATP and NADPH. 

First we will discuss the reactions happening at PS I in non-cyclic photophosphorylation.

Reactions of photosystem I

PS I consists of a P700 (chlorophyll a molecule) reaction centre and a bunch of accessory pigments. The following reactions are associated with the PS I or photosystem I.

Fig: Reaction in photosystem I

I) Photon hits PS I

The light reaction of photosynthesis begins when one photon falls on the photosystem I. The energy of the photon is transferred to the accessory pigments in the PS I.

II) Photon → P700

The accessory pigments absorb the energy of the photon and start vibrating. These vibrations are passed on from one accessory pigment molecule to another until the energy in the vibrations is passed on to the P700 reaction centre. The outermost e- of P700 absorbs energy and gets excited to a higher energy level.

III) P700 → Ferredoxin (Fd)

The electron at the higher energy level of the P700 reaction's centre is unstable. So, the electron jumps out of the higher energy orbital of the P700 reaction centre and is accepted by another modified chlorophyll molecule, chlorophyll A0. It gives off its electron to a molecule with a higher potential called A1. Now, an iron-sulphur complex (Fe-S) will get the electron that was previously in molecule A1. The Fe-S complex has a lower reduction potential compared to that of the next protein, ferredoxin (Fd). So, Fe-S gives away the electron and Fd takes it. 

IV) Ferredoxin → Ferredoxin NADP+ oxidoreductase (FNR)

Fd is a small, water-soluble mobile protein. Once Fd or ferredoxin has gained the electron, it moves through the stroma to arrive at the next electron acceptor, FNR. FNR is an enzyme that catalyses the transfer of two electrons from two Fds to one NADP + and it becomes NADPH.

Fig: Activity of FNR (Ferredoxin NADP+ oxidoreductase)

When a second photon hits the PS I, it leads to the excitation of a second electron of a different P700 reaction centre. So, the Fe-S donates the second electron to the second Fd, and this Fd carries the electron to the FNR. At the FNR, two Fd molecules together lose their two electrons and the NADP + gains these two electrons. It becomes NADPH now. NADPH is an electron carrier. It carries two electrons, which it can donate later to an electron acceptor.

Fig: NADPH transferring electron to an electron acceptor

Reason for photosystem II join the light reaction

The two P700 reaction centres, which lost one electron each, are unstable since they lack an electron. They cannot receive energy from photons through the accessory pigments and cannot release more electrons until the originally lost electrons are compensated. Hence photosystem II joins the light reaction. 

Reactions of photosystem II

Photosystem I, which was discovered first, was responsible for the formation of NADPH. The new photosystem, which was discovered later, was known as PS II. 

Fig: Reactions in photosystem II in non-cyclic photophosphorylation

I) Photon hits PS II

When one photon falls on the photosystem II, the accessory pigments absorb it.

II) Photon → P680

The accessory pigments pass the energy from the photon to the P680 reaction centre. The outermost electron of the P680 reaction centre absorbs the energy and gets excited to a higher energy level.

III) P680 → Pheophytin (Pheo)

P680, with the excited electron, has a higher redox potential when compared to the next protein in the cascade called pheophytin. So, the P680 reaction centre loses the electron and Pheophytin gains the electron.

IV) Pheophytin → Plastoquinone (PQ)

The next electron acceptor is plastoquinone or PQ. It is present inside the thylakoid membrane. It is a mobile electron carrier. It carries two electrons. A second photon is used to trigger the release of a second electron from a second P680 reaction centre, and this second electron is transported to the PQ protein. PQ or plastoquinone takes two electrons from the two pheophytins and 2H + from the stroma to become PQH2.

VI) Plastoquinone → Cytochrome b6f

PQH2 moves across the thylakoid membrane from the outer side to the inner side, where it meets the next protein complex, cytochrome b6f (Cyt b6f). Cyt b6f spans throughout the thylakoid membrane. Once PQH2 arrives at the Cyt b6f, two H + are released into the lumen and two electrons donated to Cyt B6f.

VII) Cytochrome b6f → Phycocyanin

The electrons, again one at a time now, are transferred to the next protein. Cyt b6f has a higher potential as compared to phycocyanin (PC). Cyt b6f loses an electron and PC gains an electron. PC or phycocyanin is a mobile electron carrier present inside the lumen. The PC finally gives off its electron to the unstable P700. The P700 is stable now. Two electrons from the Cyt b6f are used to satisfy the deficiency of two electrons in the two P700s. 

The two P680 reaction centres are left with the deficiency of electrons now. The two P680 reaction centres get their electrons from the splitting of water molecules caused by light through a process known as photolysis. We will check out what is called the photolysis of water now.

Photolysis of water

Photolysis is the splitting of water in the presence of sunlight. It yields four H+ ions or protons, four electrons, and one oxygen molecule into the thylakoid lumen. Photolysis occurs in the M complex. Its type, structure, number of proteins and associated electron donors and acceptors are poorly known. Manganese is the most important metal in this complex.

Fig: Photolysis of water

The photolysis or splitting of two molecules of water results in the release of four electrons. Two of these electrons are used to compensate for the loss of two electrons by two P680 reaction centres and now there are two extra electrons. 

If we double the photons at each photosystem, then we would have four photons at PS II, four photons at PS I. So there will be four electron deficiencies at the four P680 reaction centres. At this point, all the four electrons are utilised to compensate for the four electron deficiencies at the P680 reaction centre.

Fig: Z Scheme or non cyclic photophosphorylation

Number of electrons and photons

The electrons released from the splitting of water are used to compensate for the loss of electrons by two P680 reaction centres. Since there are 2 more free electrons, these are used to compensate for other excitations of the P680. So 4 photons were used instead of 2 on both photosystems. Thus all 4 electrons are used up. Hence for the creation of 1 oxygen molecule, 4 photons are used at each photosystem and 4 electrons travel through both photosystems.

Cyclic photophosphorylation vs non-cyclic photophosphorylation

The following are the major differences between cyclic and non-cyclic photophosphorylation: 

Fig: The entire process of light reaction

Practice Problems

1. The illustration shown below represents:

1. Non-cyclic photophosphorylation taking place in the membrane system of chloroplast
2. Cyclic photophosphorylation occurring in the stroma
3. Cyclic photophosphorylation occurring in the stroma thylakoid membrane
4. Non-cyclic photophosphorylation occurring in the thylakoid

Solution: The cyclic photophosphorylation that takes place in the membrane of the stromal lamellae of chloroplast is depicted in the image above. Only PS-I and not PS-II are engaged in cyclic photophosphorylation. When light with a wavelength of 700 nm is absorbed by the PS-I reaction centre, it releases electrons that are then cycled back to PS I the reaction centre itself by the electron transport system. In contrast to non-cyclic photophosphorylation, which produces both ATP and NADPH, cyclic photophosphorylation produces only ATP. When light with a wavelength longer than 680 nm is available, this process typically takes place. Hence the correct option is c.

2. Which of the following statements about non-cyclic photophosphorylation is accurate?

1. The excited electron does not return to PS-II after leaving PS-II; instead, it travels to PS-I
2. It involves PS-I as well as PS-II
3. In the process of non-cyclic photophosphorylation, ATP and NADPH are generated
4. All of the above 

Solution: Non cyclic photophosphorylation is the normal process of photophosphorylation in which the electron expelled by the excited photosystem does not return to it. It occurs when there is bright sunlight. It involves both PS I and PS II, the photosystems. The Z scheme is another name for it. It occurs when the shorter wavelength light (680 nm and lower) is available. It produces both ATP and NADPH. Hence the correct option is d.

3. How are the electrons lost by PS-II compensated?

1. Electrons released during photolysis of water
2. Electrons released by PS-l 
3. Electrons released from FNR
4. Electrons released from plastocyanin

Solution: The reaction centre of PS-II absorbs a particular wavelength (680 nm and low) of red light during non-cyclic photophosphorylation, which results in the release of electrons with high energy. After travelling via an electron transport chain, these electrons arrive at PS-I. The splitting of water, which releases electrons, protons, and oxygen, fills the gap left by the loss of electrons in PS-II. The enzyme FNR, also known as ferredoxin-NADP + oxidoreductase, catalyses the process by which electrons are transferred from ferredoxin (Fd) to NADP + during non-cyclic photophosphorylation. NADPH is created as a result of this. Thus, the production of NADPH is aided by the electrons released by FNR. The reaction centre of photosystem I receives electrons released from plastocyanin. Hence the correct option is a. 

4. Which of the following is related to the water splitting complex?

1. PS-I, located on the inner side of thylakoid membrane
2. PS-II, located on the inner side of thylakoid membrane
3. PS-Il, located on the outer side of thylakoid membrane
4. PS-I, located on the outer side of thylakoid membrane

Solution: The water splitting complex is situated on the inner side of the thylakoid membrane and is connected to PS-II (facing the lumen of thylakoids). The splitting of water in the presence of sunlight is referred to as photolysis. It releases one oxygen molecule, four H+ ions or protons, and four electrons into the thylakoid lumen. Photolysis occurs in the M complex. Its type, structure, number of proteins and associated electron donors and acceptors are poorly known. Manganese is the most important metal in this complex. The electrons are used to fill the hole left by the loss of electrons in PS-II. Protons accumulate inside the thylakoid lumen and create a proton gradient, which leads to the production of ATP. Water splitting generates oxygen, which diffuses out through the stomata. Hence the correct option is b.

FAQs

1. Who discovered the light reaction, first?

Answer: In 1937, Robin Hill made the discovery of light reaction. He discovered the reactions occurring in the chloroplast. Light reactions are the series of reactions in photosynthesis where the light is directly involved. The light reaction is also known as the photochemical phase. It is also known as the Hill reaction, because the light reaction was discovered by Robert Hill in 1937. He discovered that oxygen is released from chloroplasts in the presence of sunlight and electron acceptors. 

2. How does photosynthesis become the reverse of cellular respiration?

Answer: Photosynthesis is the process by which carbon dioxide is fixed in the form of sugar in the presence of sunlight and chlorophyll. Cellular respiration is the process by which the sugar molecules are broken down to release energy and carbon dioxide by utilising oxygen. In photosynthesis oxygen is released, sugars are formed and carbon dioxide is fixed. In cellular respiration oxygen is utilised, sugars are broken down and carbon dioxide is released. Hence we can consider that photosynthesis is the reverse of cellular respiration. Energy yielding molecules like ATP and NADPH are produced by photosynthesis. Cellular respiration produces ATP and NADH. Both sets of processes are carried out by plants and other photosynthetic organisms. Most plants except CAM (Crassulacean Acid Metabolism) plants absorb carbon dioxide only during the day. Plants use oxygen during the day and night to release carbon dioxide. 

3. What colour of light promotes photosynthesis the most?

Answer: The best visible light wavelengths for photosynthesis are in the blue (425–450 nm) and red (600–700 nm) regions. In order to promote photosynthesis, light sources should ideally emit light in the blue and red regions.

4. What prevents plants from using green light?

Answer: Plant leaves are normally green as the green light is less efficiently absorbed by chlorophylls a and b rather than the blue or red light regions. Green light has a higher probability to diffusely reflected from the cell walls than the blue or red light. Light is not reflected by chlorophyll. Absorption of light by chlorophyll can be measured using the extracted and purified chlorophyll in a test tuberather than using an intact leaf.

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