Cyclic Photophosphorylation, Practice Problems and FAQs

We all know that light is essential for the photosynthesis process to take place. But how is this light helping the plants to make their food? Anyway they are not using the light directly for the preparation of starch or sugar, just like solar cookers do. Then what is the process behind this?

Do you know what light is made of? Yes, the photons. Photons are subatomic particles that carry an amount of energy called quantum and are the building blocks of light. These photons of light have the ability to excite the electrons of the molecules from their current state to higher energy levels. So what do you think, the electrons of which molecules in the plants will get excited by the photons? It will obviously be the electrons of chlorophyll molecules. Right? Photosynthesis takes place in chloroplast. Chlorophyll molecules are present inside the chloroplast. If we study the structure of chloroplast we can understand the arrangement of chlorophyll molecules inside the chloroplast and the reason for that particular arrangement. 

The structure of chloroplast is created in such a way that the photons can fall over it and pass it on all the molecules of the electron transport chain in the chloroplast. So this absorption and transfer of energy is taking place at photosystems present in chloroplast. Hence we can say that the photosystems are structural and operational subunits of protein complexes involved in photosynthesis. 

Fig: Photon striking on photosystem

But the light is not directly generating the sugar, they can help to generate the energy for this process. So now we are going to discuss more about how the light is involved in the generation of the energy for the production of sugar in plants in depth in this article.

Table of contents

• Phases of photosynthesis 
• Photophosphorylation 
• Cyclic photophosphorylation 
• Practice Problems
• FAQs

Phases of photosynthesis

Photosynthesis involves light reaction and dark reaction. By using the raw materials like water and carbon dioxide in the presence of sunlight and pigments, the plants perform the light reactions of photosynthesis. By using the energy generated in the light reactions plants then produce the final products of photosynthesis (sugar) in the dark reactions. Hence we can say that the photosynthesis involves two phases as follows:

• Light reaction or photochemical phase
• Dark reaction or biosynthetic phase

Fig: Phases of photosynthesis

Light reaction

In the light reaction phase of photosynthesis the sequence of reactions are directly driven by light. Hence it is called light dependent phase. Here the light energy is absorbed by the pigments in the antenna complex and passed on to chlorophyll a. The antenna complex is a group of proteins and photosensitive pigments like chlorophyll and carotenoids that is associated with the light-harvesting complex. Hence it is taking place in the thylakoid membranes of chloroplast. The photochemical phase is another name for the light reaction.

The photochemical phase include the following: 

• Trapping of light energy by light harvesting complexes, photosystem I and II. 
• Photolysis of water to compensate for the electron deficiency in the PS II.
• Release of oxygen by the splitting of water molecules.
• Formation of high energy chemical intermediates like ATP and NADPH by photophosphorylation. 
• It involves cyclic and non-cyclic photophosphorylation depending on the conditions.

Dark reaction

Dark reactions are those which are independent of light. These reactions occur in the stroma of the chloroplast. It is also called the biosynthetic phase. The energy like ATP and NADPH formed in the light reaction are utilised to produce the sugar by fixing the carbon dioxide in this phase. It includes the Calvin cycle or C₃ cycle, CAM cycle or Crassulacean Acid Metabolism and C₄ cycle or the Hatch and Slack pathway. 

Photophosphorylation

The addition of the phosphate group to ADP to form ATP, is known as phosphorylation. Since this whole process in photosynthesis is powered by light in the form of the photons, it is known as photophosphorylation. Photophosphorylation is of two types. They are the cyclic and non cyclic photophosphorylation. 

Fig: Types of photophosphorylation

Cyclic photophosphorylation

Cyclic photophosphorylation is the term given to the photophosphorylation procedure that leads to the cyclic movement of electrons during the synthesis of ATP molecules. 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 will happen in both aerobic and anaerobic environments.

This kind of photophosphorylation takes place when plants need more ATP than NADPH. The photophosphorylation continues with only PS I with the reaction centre chlorophyll P 700. It takes place in the lamellar region of the chloroplast. PS II is inactive here. Cyclic photophosphorylation normally happens when the light with only higher wavelengths (700 nm or above) is available.

Fig: Step of cyclic photophosphorylation

Steps of cyclic photophosphorylation

The following steps are involved in cyclic photophosphorylation:

I) Light hits PS I

In the first stage of cyclic photophosphorylation a photon of light hits PS I. The energy in it is then absorbed by the accessory pigments.

II) Accessory pigments of PS I → P700 reaction centre

The accessory pigments pass this energy to the P700 reaction centre (RC) of PS I. The outermost electron of P700 RC absorbs energy and gets excited to a higher energy level now. The P700 RC is left with a deficiency of one electron. 

III) P700 to chlorophyll A₀

The electron at the higher energy level of the P700 reaction's centre is unstable now. It does not like instability and has a tendency to leave. So, the electron jumps out of the higher energy orbital of P700 RC and is accepted by another modified chlorophyll molecule, chlorophyll A₀. A₀  has a lower redox potential, and it has the tendency to lose its electrons.

IV) Chlorophyll A0 to A₁

It gives off its electron to a molecule with a higher potential A₁. Once A₁ receives the electron, its redox potential is lowered. Chlorophyll A0, after losing the electron, has electron potential brought back to normal. 

Fig: Electron transfer from A₀ to A₁

V) Chlorophyll A₁ to Fe-S

Now the electron will get transferred to an iron sulphur cluster from A1 molecule. Since chlorophyll A₁ has gained an electron from chlorophyll A₀, and chlorophyll A₁ has a lower potential when compared to the Fe-S and A₀. Fe-S has a higher redox potential than A0 and is able to attract the electron more strongly. After gaining the electron, the redox potential of Fe-S decreases. All further electron transports between the different components of the thylakoid membranes happen in a similar way.

Fig: Transfer of electron from A1 to Fe-S

 VI) Fe-S to ferredoxin 

The Fe-S centre 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. Fd is a small, water-soluble protein that is present in the stroma and thylakoid membrane. It is a mobile protein. So now the electron from P700 has reached the Fd molecule.

Fig: Path of electrons from P700 to Ferredoxin

VII) Ferredoxin → plastoquinone

Since Fd is a mobile-electron carrier. It moves through the stroma from Fe-S to PQ (instead of Fe-S to FNR in non-cyclic photophosphorylation). 

VIII) Plastoquinone → Cyt b₆f

PQ is a mobile transporter that transports two hydrogen molecules. Hence it needs 2e- as well. There are multiple copies of all the molecules like P700, Fd, PQ etc. So, as the second photon hits PS I, one more electron from the P700 RC arrives at PQ with the help of another Fd. 

This means that now there are two P700 RCs with a deficiency of one electron each. Now it will take two H+ ions from the stroma. So, with the two electrons and the two H+ ions, PQ becomes PQH2. It moves through the thylakoid membrane from the stromal side to the lumen side and reaches Cyt b6f complex. At the lumen side, PQ releases the two H+ ions in the lumen and the two electrons to Cyt b₆f.

IX) Cyt b6f → Phycocyanin

From the Cyt b6f complex, two electrons move to the next mobile carrier protein phycocyanin. The electrons move one at a time. 

X) Phycocyanin → P700

The phycocyanin moves through the lumen and delivers the electron to the P700 RC. Since there are two electrons being transported, two electrons are used to compensate for the two-electron deficiencies in the two P700 reaction centres. Thus, both the P700 RCs are back to normal.

XI) 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 phosphorylation. Since this photophosphorylation takes place in a cyclic manner, it is known as cyclic photophosphorylation. 

Fig: Cyclic photophosphorylation

Conditions lead to the occurrence of cyclic photophosphorylation

The following conditions leads to the occurrence of cyclic photophosphorylation: 

• Cyclic photophosphorylation occurs when the non-cyclic photophosphorylation is stopped.
• Non-cyclic photophosphorylation can be stopped normally by illuminating isolated chloroplasts with light of wavelengths greater than 680 nm. In this condition only the photosystem I will be activated and thus the photosystem II which absorbs wavelength at 680 nm remains inactivated. 
• This occurs when no longer NADP is available as an electron acceptor for non-cyclic photophosphorylation. 
• This occurs when the electron flow from water to NADP is stopped in non-cyclic photophosphorylation. 

Practice Problems

Q1. Which statement about cyclic photophosphorylation is true?

A. Both PS l and PS ll are involved
B. The electrons that PS I releases are once again cycled back to PS I
C. External electron donor is necessary
D. Both NADPH and ATP are produced

Solution: Only PS I, not PS II, participates in cyclic photophosphorylation. When light with a wavelength of 700 nm enters the PS I reaction centre, it releases electrons that are then cycled back to the reaction centre via the electron transport system. Therefore, it does not need an external electron donor. In contrast to non-cyclic photophosphorylation, which produces both ATP and NADPH, cyclic photophosphorylation solely produces ATP. This process takes place in the stromal lamella membrane and often takes place when light with a wavelength greater than 680 nm is accessible. The electrons from PSI are taken up by NADP+ and NADPH is synthesised in non-cyclic photophosphorylation. Hence the correct option is b.

Q2. Which of the following does not result from light-dependent reactions of photosynthesis?

A. NADPH
B. Glucose
C. ATP
D. O2

Solution: With the aid of chlorophyll and sunlight, green plants use a special process called photosynthesis to create carbohydrates from water and CO₂. Assimilatory powers, ATP, and NADPH are created during the light-dependent reaction. The generation of oxygen is caused by the splitting of water during the light reaction in non-cyclic photophosphorylation. Utilising the assimilatory abilities generated during the light reaction, glucose is created in the dark reaction. In stroma, glucose is synthesised without the need for light. As a result, the process that creates glucose is not dependent on light. Hence the correct option is b.

Q3. How ATP is synthesised in cyclic photophosphorylation?
Answer: When the electrons move through the electron carriers present in the cyclic photophosphorylation, there will be an influx of H⁺ ions by PQ. So the concentration of H⁺ ions will be more in lumen than the stroma. As a result a process called chemiosmosis happens. It is the process of migration of ions down their electrochemical gradient across a selectively permeable membrane. These H+ ions undergo facilitated diffusion through the ATP synthase. ATP synthase rotates and forms ATP by the process of phosphorylation.

Q4. From where does the PQ get the two electrons to function?
Answer: PQ is a mobile transporter that transports two hydrogen molecules. Hence it needs 2e- as well. There are multiple copies of all the molecules like P700, Fd, PQ etc. So, as the second photon hits PS I, one more electron from the P700 RC arrives at PQ with the help of another Fd. Now PQ will take two H⁺ ions from the stroma and with the two electrons and the two H+ ions, PQ becomes PQH₂.

FAQs

Question 1. Why is there no NADPH produced by cyclic photophosphorylation?
Answer: Cyclic photophosphorylation occurs at the stromal lamellae membrane, which lacks PS II and NADP reductase. It thus just employs PS I. As a result, it has no impact on the creation of NADPH + H+ and instead only generates ATP.

Question 2. How can the cyclic photophosphorylation in chloroplasts be distinguished experimentally from other photophosphorylation processes?
Answer: Cyclic phosphorylation can be scientifically distinguished from other types of chloroplast photophosphorylation by two features as follows: 

• The resistance to orthophenanthroline, DCMU, and CMU is one step. Photosystem II specific inhibitors like CMU, DCMU, and orthophenanthroline have no effect on cyclic photophosphorylation. 
• The next step is to make sure that the reaction can be started by light that photosystem I absorbs at the far-red end of the chlorophyll absorption spectrum.

Question 3. What separates photophosphorylation from oxidative phosphorylation?
Answer: While photophosphorylation occurs in the chloroplast during photosynthesis, oxidative phosphorylation takes place in the mitochondria during cellular respiration.

Question 4. How does inhibiting plastoquinone affect the plant body?
Answer: When the herbicide prevents the reduction of plastoquinone, light-induced charge separation causes recombination processes that, through a chlorophyll triplet state, produce reactive oxygen species. These will seriously hurt the photosystem II and ultimately kill the plant.

Related Topics