How Do Cyclic and Noncyclic Photophosphorylation Differ in Photosynthesis?
All plants undergo the vital process of photosynthesis. We know it as the process by which plants manufacture food and the light energy gets transformed into chemical energy. Photosynthesis is the basis of life for green plants and all other organisms in the food chains and food webs. Chlorophyll, sunlight, and carbon dioxide are required for this process. Photosynthesis takes place in all green parts of the plant at the cellular level in the chloroplasts.
The process of photosynthesis takes place in a series of steps, as follows.
Absorption of sunlight or light energy by chlorophyll.
Conversion of light energy into chemical energy.
Decomposition of water molecules into hydrogen and oxygen.
Transformation of carbon dioxide to form carbohydrates.
These steps take place in all plants that make their own food but not necessarily in the same order. The entire process can be divided into two major phases, one which is light-dependent and the other, which is light-independent.
What is Photophosphorylation?
The light-dependent reaction phase of photosynthesis is known as photophosphorylation. This process involves producing ATP (three phosphate groups) molecules from the ADP (2 phosphate groups) in the plant cells' chloroplasts in the presence of light. These reactions generate the following two molecules needed for the next stage of photosynthesis by utilizing light energy.
The energy storage molecule ATP
The reduced electron carrier NADPH
This process is carried out in the presence of photosystems. They are the functional units for photosynthesis. They consist of complex pigment organizations that absorb and transfer light energy and assist in transferring electrons. They are of 2 types: photosystem I and photosystem II.
Photophosphorylation is of the Following Two Types
Cyclic Photophosphorylation
Non–cyclic photophosphorylation
We will now study these two types of photophosphorylation, their features, and the difference between cyclic and non-cyclic photophosphorylation.
What is Cyclic Photophosphorylation?
Cyclic photophosphorylation is the process in which organisms, especially prokaryotes, bring about the conversion of ADP to ATP to generate energy for the cells' immediate needs. It takes place in the lamellae of chloroplasts.
In cyclic photophosphorylation, the electrons move in a circular pattern in photosystem I.
Here, the ATP synthesis is brought about by electron transport that is powered by Photosystem I only.
No oxygen or NADPH is produced in this phase.
The movement of electrons is from the primary acceptor to ferredoxin via cytochrome B6F found in mitochondria. The electron then passes to plastocyanin and finally returns to chlorophyll, completing a cycle.
What is Noncyclic Photophosphorylation?
Noncyclic photophosphorylation is the other method of photophosphorylation. It results in the synthesis of ATP molecules from ADP using the energy from excited electrons by photosystem II. In this process, the movement of the electrons happens in a noncyclic manner. The excited electrons do not get back to the chlorophyll.
The noncyclic photophosphorylation occurs in two stages and involves two different chlorophyll photosystems, PS I and PS II.
It is a light reaction and occurs in the thylakoid membrane.
Noncyclic photophosphorylation produces ATP and NADPH.
The generation of ATP is along with a one-way flow of electrons from H₂O to NADP+.
The lost electrons by P680 of PS II are occupied by P700 of PS I.
They are not reverted to the energy center.
The complete movement of the electrons is in a non-cyclic manner or a unidirectional manner.
Now that we know about the two types of phosphorylation, let us study the difference between cyclic and noncyclic photophosphorylation.
The table below explains the difference between cyclic and noncyclic phosphorylation.
Differentiate Between Cyclic and Noncyclic Photophosphorylation
Cyclic and Noncyclic Photophosphorylation Class 11
In the photophosphorylation method, there is the formation of ATP from ADP. ATP consists of three phosphor units while the ADP consists of two phosphor units. When there is an addition of an extra phosphor unit to the ADP it is then called the ATP. The cyclic photophosphorylation takes place under both aerobic and anaerobic conditions which is hence an efficient process than the other process. The ATP that is produced from this process is also highly beneficial for the production of energy for plants and the production of oxygen important for humans. Students can now also Check The Difference Between Cyclic and Noncyclic Photophosphorylation via Vedantu where everything regarding the same is provided in many details.
Significance of noncyclic photophosphorylation
Noncyclic electron transport occurs through a sequence of electron transport where there is the reduction of NADP by PS 1 and the PS 1 is reduced by PS 2 and then finally PS 3 is reduced by electrons that arise from photo-oxidation of water.
This electron transport helps release oxygen and photooxidation of water takes place.
The non-cyclic process is important as it supplies assimilatory power in the form of NADPH and ATP for carbon dioxide assimilation and purifies the atmospheric air.
FAQs on Cyclic vs Noncyclic Photophosphorylation: Complete Comparison
1. What is the main difference between cyclic and non-cyclic photophosphorylation as per the CBSE Class 11 syllabus for 2025-26?
The primary differences between cyclic and non-cyclic photophosphorylation relate to the photosystems involved, the path of the electron, and the final products generated. Non-cyclic photophosphorylation uses both Photosystem I (PS I) and Photosystem II (PS II) to produce ATP, NADPH, and oxygen. In contrast, cyclic photophosphorylation only uses PS I and produces only ATP, with no NADPH or oxygen being formed.
2. Can you explain the process of non-cyclic photophosphorylation and list its products?
Non-cyclic photophosphorylation, also known as the Z-scheme, is the primary light-dependent reaction. It involves the following steps:
Light excites an electron in PS II (P680), which is then passed to an electron transport chain (ETC).
To replace the lost electron, a water molecule is split (photolysis), releasing electrons, protons (H+), and oxygen gas.
As the electron moves down the ETC, energy is used to pump protons into the thylakoid lumen, creating a proton gradient that drives ATP synthesis.
The electron reaches PS I (P700), gets re-energised by light, and is passed to another ETC.
Finally, the electron is accepted by NADP+, reducing it to NADPH.
The key products are ATP, NADPH, and O₂.
3. What is cyclic photophosphorylation and where does it occur inside the chloroplast?
Cyclic photophosphorylation is a secondary light-dependent pathway that only involves Photosystem I (PS I). In this process, an electron excited from PS I is passed down an electron transport chain and eventually returns to the same PS I complex, creating a cycle. This electron flow generates a proton gradient across the membrane, which is used to synthesise ATP. Unlike the non-cyclic path, it does not involve PS II, the splitting of water, or the production of NADPH. This process primarily occurs in the stroma lamellae of the chloroplasts.
4. What are the key similarities between the cyclic and non-cyclic pathways of photophosphorylation?
Despite their differences, both pathways share several fundamental features:
Both are driven by light energy absorbed by chlorophyll pigments.
Both involve an electron transport chain (ETC) to move excited electrons.
Both synthesise ATP via chemiosmosis, using the energy from the proton gradient.
Both processes take place within the thylakoid membranes of the chloroplasts.
5. What is the importance of Photosystems I and II in photosynthesis?
Photosystems are functional units within the thylakoid membrane, comprising a reaction centre and light-harvesting pigment molecules. Photosystem II (PS II), with its reaction centre P680, is crucial for initiating non-cyclic flow by absorbing light and splitting water to release oxygen. Photosystem I (PS I), with its reaction centre P700, participates in both non-cyclic flow (to produce NADPH) and cyclic flow (to produce only ATP). Together, they capture light energy and convert it into the chemical energy (ATP and NADPH) needed for carbon fixation.
6. Why do plants need to perform both cyclic and non-cyclic photophosphorylation?
Plants need both pathways to meet the specific energy demands of the Calvin Cycle. The Calvin Cycle requires more ATP than NADPH (a ratio of 3 ATP to 2 NADPH for every CO₂ molecule fixed). Non-cyclic photophosphorylation produces ATP and NADPH in roughly a 1:1 ratio. The cell performs cyclic photophosphorylation to generate the extra ATP required to balance this ratio, ensuring the Calvin Cycle can proceed efficiently without an unnecessary buildup of NADPH.
7. How does photophosphorylation differ from oxidative phosphorylation?
While both processes produce ATP, they differ significantly. Photophosphorylation occurs in the chloroplasts' thylakoid membrane, uses light energy as its power source, and has water as the initial electron donor. In contrast, oxidative phosphorylation occurs in the mitochondria's inner membrane, uses chemical energy from the breakdown of glucose (via NADH and FADH₂), and has oxygen as the final electron acceptor.
8. Why is oxygen released only during non-cyclic photophosphorylation?
Oxygen is a byproduct of splitting water molecules, a process known as photolysis. This process is essential only in non-cyclic photophosphorylation to replace the electrons lost by Photosystem II (PS II) after they are excited by light. Since cyclic photophosphorylation only involves Photosystem I (PS I) and the electron cycles back to its starting point, there is no net loss of electrons from the photosystem and therefore no need to split water. Consequently, oxygen is not produced.
9. Under what environmental or cellular conditions might a plant favour cyclic over non-cyclic photophosphorylation?
A plant may shift towards favouring the cyclic pathway under specific conditions where the demand for ATP is high, but the demand for NADPH is low. This includes:
Low CO₂ availability: When stomata are closed to conserve water, the Calvin Cycle slows down, reducing the need for NADPH.
High ATP demand: The cell may require extra ATP for other metabolic processes besides photosynthesis.
Anaerobic conditions: In some photosynthetic bacteria that only possess PS I, cyclic flow is the only method for ATP synthesis.
