Questions Answers Class 11 Chapter 12 Biology Maharashtra Board

Photosynthesis

1. Choose correct option

A. A cell that lacks chloroplast does not

(a) evolve carbon dioxide
(b) liberate oxygen
(c) require water
(d) utilize carbohydrates

Answer: (b) liberate oxygen

B. Energy is transferred from the light reaction step to the dark reaction step by

a. chlorophyll
b. ADP
c. ATP
d. RuBP

Answer: (c) ATP

C. Which one is wrong in photorespiration?

(a) It occurs in chloroplasts
(b) It occurs in day time only
(c) It is characteristic of C4-plants
(d) It is characteristic of C3-plants

Answer: (c) It is characteristic of C4-plants

D. Non-cyclic phosphorylation differs from cyclic photophosphorylation in that former

(a) involves only PS
(b) Include evolution of 02
(c) involves formation of assimilatory power
(d) both (b) and (c)

Answer: (d) both (b) and (c)

E. For fixation of 6 molecules of C02 and formation of one molecule of glucose in Calvin cycle, requires

(a) 3 ATP and 2 MADPPE
(b) 18 ATP and 12 NADPH2
(c) 30 ATP and 18 NADPH2
(d) 6 ATP and 6 NADPIT2

Answer: (b) 18 ATP and 12 NADPH2

F. In maize and wheat, the first stable products formed in bundle sheath cells respectively are

(a) OAA and PEPA
(b) OAA and OAA
(c) OAA and 3PGA
(d) 3PGA and OAA

Answer: (c) OAA and 3PGA

G. The head and tail of chlorophyll are made up of

(a) porphyrin and phytin respectively
(b) pyrrole and tetrapyrrole respectively
(c) porphyrin and phytol respectively
(d) tetrapyrole and pyrrole respectively

Answer: (c) porphyrin and phytol respectively

H. The net result of photo-oxidation of water is release of ……………. .

(a) electron and proton
(b) proton and oxygen
(c) proton, electron and oxygen
(d) electron and oxygen

Answer: (c) proton, electron and oxygen

I. For fixing one molecule of C02 in Calvin cycle are required.

(a) 3ATP + 1NADPFE
(b) 3ATP + 2NADPH2
(c) 2ATP + 3NADPH2
(d) 3ATP + 3NADPFE

Answer: (b) 3ATP + 2NADPH2

J. In presence of high concentration of oxygen, RuBP carboxylase converts RuBP to …………… .

(a) Malic acid and PEP
(b) PGA and PEP
(c) PGA and malic acid
(d) PGA and phosphoglycolate

Answer: (d) PGA and phosphoglycolate

K. The sequential order in electron transport from PSII to PSI of photosynthesis is

(a) FeS, PQ, PC and Cytochrome
(b) FeS, PQ, Cytochrome and PC
(c) PQ, Cytochrome, PC and FeS
(d) PC, Cytochrome, FeS, PQ

Answer: (c) PQ, Cytochrome, PC and FeS

2. Answer the Following Questions

A. Describe the light-dependent steps of photosynthesis. How are they linked to the dark reactions?

Answer: The light-dependent steps of photosynthesis, also known as the light reactions, occur in the thylakoid membranes of chloroplasts and involve the absorption of light by chlorophyll and other pigments to produce ATP, NADPH, and oxygen. The key steps are:

• Photon Absorption and Excitation: Chlorophyll molecules in Photosystem II (PS-II, P680) and Photosystem I (PS-I, P700) absorb light photons, exciting electrons to a higher energy state.
• Photolysis of Water (PS-II): In PS-II, the absorbed light energy splits water molecules into oxygen, protons (H⁺), and electrons. The reaction is:

• Oxygen is released as a byproduct, and electrons replace those lost by PS-II.
• Electron Transport Chain (ETC): Electrons from PS-II are passed through an ETC (including plastoquinone, cytochrome complex, and plastocyanin) to PS-I. This process generates a proton gradient across the thylakoid membrane, driving ATP synthesis via chemiosmosis.
• ATP Synthesis (Photophosphorylation): The proton gradient powers ATP synthase to produce ATP from ADP and inorganic phosphate (Pi).
• NADPH Formation (PS-I): In PS-I, light-excited electrons are transferred to NADP⁺ via ferredoxin, reducing it to NADPH with protons from the stroma.

Cyclic and Non-Cyclic Photophosphorylation:

• Non-cyclic photophosphorylation involves both PS-II and PS-I, producing ATP, NADPH, and O₂.
• Cyclic photophosphorylation involves only PS-I, producing ATP without NADPH or O₂.

B. Distinguish between:
a. respiration and photorespiration

Answer:

b. absorption spectrum and action spectrum

Answer:

c. cyclic photophosphorylation and non cyclic photophosphorylation

Answer:

C. What are the steps that are common to C3 and C4 photosynthesis?

Answer: Both C3 and C4 photosynthesis share the Calvin cycle (C3 pathway) as a common mechanism for carbon fixation, which occurs in the stroma of chloroplasts. The common steps include:

1. Carboxylation: CO₂ is fixed by ribulose-1,5-bisphosphate (RuBP) with the help of the enzyme RuBP carboxylase (Rubisco), forming an unstable 6-carbon intermediate that splits into two molecules of 3-phosphoglyceric acid (3-PGA).
2. Reduction: 3-PGA is reduced to glyceraldehyde-3-phosphate (3-PGAL) using ATP and NADPH from the light reactions.
3. Regeneration: RuBP is regenerated from 3-PGAL through a series of reactions, consuming additional ATP, to continue the cycle.

In C3 plants, these steps occur in mesophyll cells, while in C4 plants, they occur in bundle sheath cells. The C4 pathway adds an initial CO₂ fixation step in mesophyll cells, but the Calvin cycle remains identical in both.

D. Are the enzymes that catalyse the dark reactions of carbon fixation located inside the thylakoids or outside the thylakoids?

Answer: The enzymes that catalyse the dark reactions of carbon fixation, such as RuBP carboxylase (Rubisco) and others involved in the Calvin cycle, are located outside the thylakoids, specifically in the stroma of the chloroplast. The stroma is the fluid-filled region surrounding the thylakoids where the Calvin cycle occurs, utilizing ATP and NADPH produced in the thylakoid membranes during the light reactions.

E. Calvin cycle consists of three phases, what are they? Explain the significance of each of them.

Answer: The Calvin cycle consists of three phases, as described in the document:

1. Carboxylation:
   • Process: CO₂ reacts with RuBP, catalyzed by Rubisco, forming an unstable 6-carbon intermediate that splits into two molecules of 3-phosphoglyceric acid (3-PGA).
   • Significance: This phase initiates carbon fixation by incorporating inorganic CO₂ into an organic molecule, marking the entry of carbon into the biosynthetic pathway of photosynthesis.
2. Reduction (Glycolytic Reversal):
   • Process: 3-PGA is phosphorylated by ATP to form 1,3-diphosphoglyceric acid, which is then reduced by NADPH to glyceraldehyde-3-phosphate (3-PGAL). Two 3-PGAL molecules can combine to form one glucose molecule.
   • Significance: This phase converts fixed carbon into a usable form (3-PGAL), which serves as a precursor for glucose and other carbohydrates, storing energy from the light reactions.
3. Regeneration:
   • Process: Ten molecules of 3-PGAL are used to regenerate six molecules of RuBP through a series of complex reactions, consuming six ATP molecules.
   • Significance: Regeneration ensures the continuity of the Calvin cycle by replenishing RuBP, the CO₂ acceptor, allowing the cycle to fix more CO₂ in subsequent rounds.

These phases collectively transform CO₂ into glucose, utilizing ATP and NADPH from the light reactions, and are critical for producing energy-rich organic compounds.

F. Why are the plants that consume more than the usual 18 ATP to produce 1 molecule of glucose favoured in tropical regions?

Answer: C4 plants, which require 30 ATP to produce one glucose molecule (compared to 18 ATP in C3 plants), are favoured in tropical regions due to their adaptations to high temperatures, bright light, and low CO₂ concentrations. The reasons include:

• Reduced Photorespiration: C4 plants minimize photorespiration, a wasteful process in C3 plants that occurs under high temperatures and low CO₂. The C4 pathway concentrates CO₂ in bundle sheath cells, ensuring Rubisco preferentially performs carboxylation, increasing photosynthetic efficiency.
• Kranz Anatomy: The specialized leaf anatomy (bundle sheath and mesophyll cells) allows efficient CO₂ fixation and transfer, optimizing photosynthesis in hot, dry conditions.
• Higher Productivity: C4 plants can fix CO₂ even at low atmospheric concentrations, making them more productive in tropical environments where stomatal closure (to conserve water) reduces CO₂ availability.

These adaptations make C4 plants better suited to the intense light and heat of tropical regions, justifying the higher ATP cost for greater photosynthetic output.

G. What is the advantage of having more than one pigment molecule in a photocentre?

Answer: Having multiple pigment molecules (chlorophylls, carotenoids, and xanthophylls) in a photocentre offers several advantages:

• Broader Light Absorption: Different pigments absorb light at various wavelengths (e.g., chlorophyll absorbs red and blue, carotenoids absorb violet to blue). This broadens the spectrum of usable light, maximizing energy capture.
• Energy Transfer Efficiency: Accessory pigments (light-harvesting or antenna molecules) absorb light and transfer energy to the reaction centre (P680 or P700), enhancing the efficiency of the photochemical reaction.
• Photoprotection: Carotenoids protect chlorophyll from photo-oxidation by dissipating excess light energy, preventing damage to the photosynthetic apparatus under high light intensity.

This diversity ensures efficient light harvesting and protection, optimizing photosynthesis under varying light conditions.

H. Why does chlorophyll appear green in reflected light and red transmitted light? Explain the significance of these phenomena in terms of photosynthesis.

Answer:

• Appearance in Reflected Light: Chlorophyll appears green because it absorbs light primarily in the blue (400-500 nm) and red (600-700 nm) regions of the visible spectrum, reflecting green wavelengths (500-570 nm). This reflected light gives leaves their characteristic green color.
• Appearance in Transmitted Light: In transmitted light (e.g., in the spinach leaf experiment), chlorophyll appears red because it transmits red light (around 600-700 nm) that is not absorbed. When illuminated in a dark room, the solution fluoresces red due to the emission of absorbed energy as red light.

Significance in Photosynthesis:

• Selective Absorption: Chlorophyll’s absorption of blue and red light is significant because these wavelengths have the optimal energy for exciting electrons in the photosynthetic process, driving the light reactions.
• Efficient Energy Use: The reflection of green light and transmission of red light indicate that chlorophyll is tuned to use the most energetically favorable wavelengths for photosynthesis, maximizing energy conversion.
• Fluorescence as an Indicator: The red fluorescence in transmitted light shows that absorbed energy is re-emitted when not used for photochemistry, providing insights into chlorophyll’s excitation state and photosynthetic efficiency.

These phenomena highlight chlorophyll’s role in selectively harnessing light energy for photosynthesis.

I. Explain why photosynthesis is considered the most important process in the biosphere.

Answer: Photosynthesis is considered the most important process in the biosphere for the following reasons:

• Food Production: It is the primary process by which green plants synthesize carbohydrates (glucose) from CO₂ and H₂O, serving as the foundation of the food chain for all living organisms, directly or indirectly.
• Oxygen Production: Photosynthesis releases oxygen as a byproduct, which is essential for aerobic respiration in most organisms and maintains atmospheric oxygen levels.
• Energy Transformation: It converts solar energy into chemical energy stored in carbohydrates, providing energy for nearly all life forms.
• Carbon Cycle Regulation: By fixing 70 billion tons of carbon annually, photosynthesis regulates atmospheric CO₂ levels, mitigating climate change.
• Fossil Fuel Formation: Photosynthetic products over millions of years have formed fossil fuels (coal, petroleum, natural gas), which are critical energy resources.
• Ozone Layer Support: The oxygen released supports the formation of the ozone layer, protecting life from harmful UV radiation.

As stated in the document, photosynthesis is “the final light energy trapping process on which all life ultimately depends,” making it indispensable for life on Earth.

J. Why is photolysis of water accompained with non-cyclic photophosphorylation?

Answer: Photolysis of water is accompanied with non-cyclic photophosphorylation because:

• Electron Supply: In non-cyclic photophosphorylation, PS-II (P680) absorbs light, exciting electrons that are transferred through the electron transport chain to PS-I and ultimately to NADP⁺, forming NADPH. The loss of electrons from PS-II creates an electron deficiency, which is replenished by the photolysis of water:

• These electrons restore PS-II to its reduced state.
• Oxygen Evolution: Photolysis produces oxygen as a byproduct, a hallmark of non-cyclic photophosphorylation, unlike cyclic photophosphorylation, which does not involve water splitting.
• Proton Gradient: The protons (H⁺) released during photolysis accumulate in the thylakoid lumen, contributing to the proton gradient that drives ATP synthesis via chemiosmosis.

In cyclic photophosphorylation, only PS-I is involved, and electrons cycle back to PS-I without water splitting, so photolysis is not required. Thus, photolysis is specific to non-cyclic photophosphorylation to supply electrons and protons for ATP and NADPH production.

K. In C-4 plants, why is C-3 pathway operated in bundle sheath cells only?

Answer: In C4 plants, the C3 pathway (Calvin cycle) is operated in bundle sheath cells only due to their specialized anatomy and biochemical adaptations:

• Kranz Anatomy: C4 plants have a distinct leaf structure with bundle sheath cells surrounding vascular bundles and mesophyll cells on the periphery. Bundle sheath cells contain large chloroplasts with enzymes for the Calvin cycle (e.g., Rubisco), while mesophyll cells lack these enzymes.
• CO₂ Concentration: The C4 pathway in mesophyll cells fixes CO₂ into a 4-carbon compound (oxaloacetic acid), which is converted to malic acid and transported to bundle sheath cells. Here, malic acid is decarboxylated, releasing CO₂, which increases its concentration around Rubisco in bundle sheath cells.
• Reduced Photorespiration: High CO₂ levels in bundle sheath cells favor Rubisco’s carboxylation activity over oxygenation, minimizing photorespiration, which is inefficient in C3 plants.
• Spatial Separation: The separation of initial CO₂ fixation (C4 pathway in mesophyll cells) and the Calvin cycle (C3 pathway in bundle sheath cells) optimizes photosynthetic efficiency by ensuring Rubisco operates in a CO₂-rich environment.

This spatial organization enhances the efficiency of carbon fixation in C4 plants, particularly in hot and dry conditions.

L. What would have happened if C-4 plants did not have Kranz anatomy?

Answer: If C4 plants lacked Kranz anatomy, the following consequences would occur:

• Loss of CO₂ Concentration Mechanism: Kranz anatomy, with mesophyll cells fixing CO₂ into 4-carbon compounds and bundle sheath cells hosting the Calvin cycle, creates a high CO₂ concentration around Rubisco. Without this, CO₂ levels would be lower, increasing photorespiration and reducing photosynthetic efficiency.
• Increased Photorespiration: Rubisco in mesophyll cells would act as an oxygenase under high O₂ and low CO₂ conditions, leading to the loss of fixed carbon (25% in C3 plants), as seen in photorespiration.
• Reduced Productivity: C4 plants’ high productivity in tropical environments relies on minimizing photorespiration and efficiently fixing CO₂. Without Kranz anatomy, they would resemble C3 plants, with lower efficiency in hot, dry conditions.
• Inefficient Enzyme Distribution: The separation of enzymes (e.g., PEPCO in mesophyll cells, Rubisco in bundle sheath cells) would be disrupted, impairing the C4 pathway’s ability to concentrate CO₂ and regenerate PEP.

C4 plants would lose their adaptive advantage, behaving like C3 plants with reduced carbon fixation and survival in challenging environments.

M. Why does RUBISCO carry out preferentially carboxylation than oxygenation in C4 plants?

Answer: Rubisco carries out preferential carboxylation over oxygenation in C4 plants due to:

• High CO₂ Concentration: The C4 pathway concentrates CO₂ in bundle sheath cells by fixing it into 4-carbon compounds (e.g., malic acid) in mesophyll cells, which are then decarboxylated in bundle sheath cells. This creates a CO₂-rich environment (higher than atmospheric levels), favoring Rubisco’s carboxylase activity (fixing CO₂ with RuBP to form 3-PGA) over its oxygenase activity (reacting with O₂ to form phosphoglycolate).
• Kranz Anatomy: The spatial separation of initial CO₂ fixation (mesophyll cells) and the Calvin cycle (bundle sheath cells) ensures that Rubisco operates in a microenvironment with elevated CO₂ and reduced O₂, minimizing photorespiration.
• Environmental Adaptation: C4 plants are adapted to high temperatures and bright light, where photorespiration is more prevalent in C3 plants. The C4 mechanism suppresses oxygenation, enhancing photosynthetic efficiency.

This preference for carboxylation makes C4 plants more efficient at carbon fixation, especially in tropical and arid conditions.

N. What would have happened if plants did not have accessoy pigments?

Answer: If plants lacked accessory pigments (e.g., carotenoids, xanthophylls), the following would occur:

• Limited Light Absorption: Accessory pigments absorb light in regions of the spectrum (e.g., violet to blue) that chlorophyll does not efficiently capture. Without them, plants would rely solely on chlorophyll’s absorption of red and blue light, reducing the range of usable wavelengths and lowering photosynthetic efficiency.
• Reduced Energy Transfer: Accessory pigments act as light-harvesting molecules, transferring absorbed energy to reaction centres (P680, P700). Their absence would decrease the efficiency of energy delivery to photosystems, slowing the light reactions.
• Increased Photo-oxidation: Carotenoids protect chlorophyll from damage by dissipating excess light energy and neutralizing reactive oxygen species. Without them, chlorophyll would be more susceptible to photo-oxidation, leading to damage to the photosynthetic apparatus under high light intensity.
• Lower Photosynthetic Yield: The combined effect of reduced light absorption, inefficient energy transfer, and increased damage would significantly lower the rate of photosynthesis, impacting plant growth and survival.

Accessory pigments are essential for optimizing light use and protecting the photosynthetic machinery.

O. How can you identify whether the plant is C3 or C4 ? Explain / Justify.

Answer: To identify whether a plant is C3 or C4, the following characteristics can be examined:

1. Leaf Anatomy (Kranz Anatomy):
   • C4 Plants: Exhibit Kranz anatomy, with large bundle sheath cells containing chloroplasts (often without well-developed grana) surrounding vascular bundles, and mesophyll cells with smaller chloroplasts. This can be observed under a microscope.
   • C3 Plants: Lack Kranz anatomy; chloroplasts are uniformly distributed in mesophyll cells, with no distinct bundle sheath chloroplasts.
   • Justification: Kranz anatomy is a hallmark of C4 plants, enabling the spatial separation of C4 and C3 pathways for efficient CO₂ fixation.
2. First Stable Product of CO₂ Fixation:
   • C4 Plants: The first stable product is a 4-carbon compound (oxaloacetic acid, OAA), formed in mesophyll cells.
   • C3 Plants: The first stable product is a 3-carbon compound (3-phosphoglyceric acid, 3-PGA).
   • Justification: Biochemical analysis (e.g., using radioactive 14CO₂) can trace the initial fixation product, distinguishing C4 from C3 pathways.
3. Enzyme Activity:
   • C4 Plants: High activity of phosphoenolpyruvate carboxylase (PEPCO) in mesophyll cells for initial CO₂ fixation.
   • C3 Plants: Rely solely on Rubisco for CO₂ fixation.
   • Justification: Enzyme assays can confirm the presence of PEPCO, a key C4 enzyme.
4. Photorespiration Levels:
   • C4 Plants: Low or no photorespiration due to CO₂ concentration in bundle sheath cells.
   • C3 Plants: High photorespiration, especially in high temperatures and low CO₂.
   • Justification: Measuring CO₂ release under high O₂ conditions can indicate photorespiration levels.
5. Environmental Adaptation:
   • C4 Plants: Thrive in hot, dry, and high-light environments (e.g., sugarcane, maize).
   • C3 Plants: Common in temperate regions (e.g., wheat, rice).
   • Justification: Habitat and growth conditions can provide clues, as C4 plants are adapted to tropical climates.

By combining anatomical, biochemical, and ecological observations, C3 and C4 plants can be accurately identified.

P. In C4 plants, bundle sheath cells carrying out Calvin cycle are very few in number, even then, C4 plants are highly productive. Explain.

Answer: C4 plants are highly productive despite having fewer bundle sheath cells carrying out the Calvin cycle due to the following reasons:

• CO₂ Concentration Mechanism: The C4 pathway in mesophyll cells fixes CO₂ into 4-carbon compounds (e.g., malic acid), which are transported to bundle sheath cells and decarboxylated, releasing CO₂. This creates a high CO₂ concentration around Rubisco, enhancing the efficiency of the Calvin cycle and reducing photorespiration.
• Kranz Anatomy: The specialized arrangement of mesophyll and bundle sheath cells ensures efficient transfer of 4-carbon compounds, maximizing CO₂ delivery to the Calvin cycle in bundle sheath cells.
• Reduced Photorespiration: Unlike C3 plants, C4 plants minimize photorespiration, which wastes 25% of fixed carbon in C3 plants. This conservation of carbon increases net photosynthetic output.
• High Enzyme Efficiency: Rubisco in bundle sheath cells operates at near-optimal conditions due to elevated CO₂, allowing fewer cells to achieve high rates of carbon fixation.
• Adaptation to Harsh Conditions: C4 plants thrive in high temperatures and low CO₂, maintaining high photosynthetic rates even when stomata partially close to conserve water, unlike C3 plants.

These adaptations enable C4 plants to achieve high productivity with fewer bundle sheath cells, making them efficient photosynthesizers in challenging environments.

Q. What is functional significance of Kranz anatomy?

Answer: The functional significance of Kranz anatomy in C4 plants includes:

• Spatial Separation of Pathways: Kranz anatomy, with mesophyll cells surrounding bundle sheath cells, separates the initial CO₂ fixation (C4 pathway) in mesophyll cells from the Calvin cycle (C3 pathway) in bundle sheath cells. This division optimizes the photosynthetic process.
• CO₂ Concentration: Mesophyll cells fix CO₂ into 4-carbon compounds (e.g., oxaloacetic acid, malic acid), which are transported to bundle sheath cells and decarboxylated, releasing CO₂. This creates a high CO₂ concentration around Rubisco, enhancing carboxylation efficiency.
• Reduced Photorespiration: The elevated CO₂ in bundle sheath cells minimizes Rubisco’s oxygenase activity, reducing photorespiration and conserving fixed carbon, unlike C3 plants.
• Efficient Enzyme Distribution: Mesophyll cells contain PEPCO for initial CO₂ fixation, while bundle sheath cells house Rubisco and Calvin cycle enzymes, ensuring specialized roles for each cell type.
• Adaptation to Harsh Environments: Kranz anatomy supports C4 plants’ ability to fix CO₂ efficiently in high temperatures, bright light, and low CO₂ conditions, making them well-suited to tropical and arid regions.

3. Correct the pathway and name it

Answer:

1. The pathway shown is C4 pathway.
2. M. D. Hatch and C. R. Slack while working on sugarcane found four carbon compounds (dicarboxylic acid) as the first stable product of photosynthesis.
3. It occurs in tropical and sub-tropical grasses and some dicotyledons.
4. The first product of this cycle is a 4-carbon compound oxaloacetic acid. Hence it is also called as C4 pathway and plants are called C4 plants.

Mechanism:

1. C02 taken from atmosphere is accepted by a 3-carbon compound, phosphoenolpyruvic acid in the chloroplasts of mesophyll cells, leading to the formation of 4-C compound, oxaloacetic acid with the help of enzyme phosphoenolpyruvate carboxylase.
2. It is converted to another 4-C compound, malic acid.
3. It is transported to the chloroplasts of bundle sheath cells.
4. Malic acid (4-C) is converted to pyruvic acid (3-C) with the release of C02 in the cytoplasm.
5. Thus, concentration of C02 increases in the bundle sheath cells.
6. Chloroplasts of these cells contain enzymes of Calvin cycle.
7. Because of high concentration of C02, RuBP carboxylase participates in Calvin cycle and not photorespiration.
8. Sugar formed in Calvin cycle is transported into the phloem.
9. Pyruvic acid generated in the bundle sheath cells re-enter mesophyll cells and regenerates phosphoenolpyruvic acid by consuming one ATP.
10. Since this conversion results in the formation of AMP (not ADP), two ATP are required to regenerate ATP from AMP.
11. Thus, C4 pathway needs 12 additional ATP.
12. The C3 pathway requires 18 ATP for the synthesis of one glucose molecule, whereas C4 pathway requires 30 ATP.
13. Thus, C4 plants are better photosynthesizers as compared to C3 plants as there is no photorespiration in these plants.

4. Is there something wrong in following schematic presentation? If yes, correct it so that photosynthesis will be operated.

Answer: Non-cyclic photophosphorylation:

• It involves both photosystems- PS-I and PS-II.
• In this case, electron transport chain starts with the release of electrons from PS-II.
• In this chain high energy electrons released from PS-II do not return to PS-II but, after passing through an electron transport chain, reach PS-I, which in turn donates it to reduce NADP to NADPH.
• The reduced NADP+ (NADPH) is utilized for the reduction of CO2 in the dark reaction.
• Electron-deficient PS-II brings about oxidation of water-molecule. Due to this, protons, electrons and oxygen atom are released.
• Electrons are taken up by PS-II itself to return to reduced state, protons are accepted by NADP+ whereas oxygen is released.
• As in this process, high energy electrons released from PS-II do not return to PS-II and it is accompanied with ATP formation, this is called Non-cyclic photophosphorylation.