4.1.9 Light Dependent Phase (light Reaction)

Introduction

The light-dependent phase, also known as the light reaction, is the first phase of photosynthesis. It occurs in the thylakoid membranes of the chloroplast and requires light to take place. In this phase, light energy is converted into chemical energy in the form of ATP and NADPH. Oxygen is released as a byproduct.

Definition

Light-Dependent Phase (Light Reaction): The light-dependent phase of photosynthesis involves the absorption of light by the photosystems, excitation and flow of electrons through an electron transport chain, chemiosmotic synthesis of ATP, and reduction of NADP to NADPH.

In simple words: The light reaction is the phase of photosynthesis that uses light energy to make ATP and NADPH.

Core Concept

• The light-dependent phase occurs in the thylakoid membranes.
• It involves:
• The flow of excited electrons through an electron transport chain during light reaction is of two different types:
• In non-cyclic electron flow, the excited electrons after leaving a particular photosystem do not come back; these electrons after losing their energy are incorporated into another molecule.
• In cyclic electron flow, the excited electrons after leaving a particular photosystem finally come back to their photosystem again.
• The most important event in light reaction is the production of ATP.
• This production of ATP during light reaction is called photophosphorylation and the mechanism is called chemiosmosis.
• There are two types of photophosphorylation:

Detailed Explanation

Types of Photophosphorylation

(a) Non-Cyclic Photophosphorylation

Definition: It is the predominant pathway of light reaction in higher plants that occurs in routine. In this process both photosystems (PS-I and PS-II) are utilized and two electron transport chains are involved.

• It is the predominant pathway of light reaction in higher plants.
• It occurs in routine.
• Both photosystems (PS-I and PS-II) are utilized.
• Two electron transport chains are involved.
• When PS-II absorbs light, its excited electrons after flowing through an electron transport chain are transferred to PS-I.
• Similarly, the excited electrons which are liberated from PS-I are finally accepted by NADP⁺.
• Therefore, it is called non-cyclic electron flow.
• The events of non-cyclic photophosphorylation are continuous, but here they are discussed in steps for convenience.

Steps of Non-Cyclic Photophosphorylation:

Step 1: Absorption of Light by PS-II and Excitation of Its Electrons

• When just two photons strike the antenna complex of PS-II, the two electrons become excited and begin to move along the atoms of different pigments within photosystem.
• Ultimately, the absorbed energy reaches the reaction centre of PS-II (P680) and causes its two electrons to be excited.
• These excited electrons are captured by the primary electron acceptor of PS-II.
• They leave two "electron holes" in the photosystem behind, making chlorophyll-a a strong oxidizing agent.

Step 2: Photolysis of Water

• The electron holes of photosystem must be filled so that in the presence of water splitting enzyme reactions can proceed.
• When water reacts with oxidized state of chlorophyll in photosystem, it breaks up into 2H⁺ ions, 2e⁻, and ½O₂.
• Since this breakdown occurs in the presence of sunlight, therefore, it is termed as photolysis of water.
• The electrons released from water are used to fill the "electron holes" of PS-II.

Equation: H₂O Photolysis of water→ 2H⁺ + 2e^- + 12O₂

Step 3: Electron Flow from PS-II to PS-I

• The excited/energized electrons which have been released from PS-II and captured by primary electron acceptor now begin to flow to PS-I through an electron transport chain.
• The electrons move from primary electron acceptor to plastoquinone (PQ).
• From PQ, the electrons flow through a complex of the cytochromes (Cyt) which consists of Cyt-b and Cyt-f.
• The cytochrome complex is not only an electron carrier but it also works as a proton pump.
• The electron flow through the cytochrome complex stimulates it to pump the protons from stroma to the thylakoid inner space.
• In this way, the energy of flowing electrons is transformed into a gradient of protons (H⁺) in the thylakoid inner space.
• The proton gradient activates an enzyme in thylakoid membrane called ATP synthase which not only moves the protons back into the stroma but also catalyzes a reaction in which ADP and Pi are combined to form ATP (photophosphorylation).
• This whole mechanism which involves flow of electron, pumping of protons, and generation of ATP by thylakoid membranes is called chemiosmosis.
• This ATP, generated by light reactions, will provide chemical energy for the synthesis of sugar during Calvin cycle.
• The energized electrons after losing their energy move from cytochrome complex to the plastocyanin (PC) and finally incorporated into the PS-I.

Step 4: Absorption of Light by PS-I and Excitation of Its Electrons

• On the other hand, when P700 in the reaction centre of PS-I molecule absorbs two photons of light, electrons are boosted to a higher energy level.
• P700 molecule passes these excited electrons to a primary electron acceptor of PS-I, creating "electron holes".
• The electron holes of P700 are filled by the pair of electrons received from the P680 (photosystem II) via electron transport chain.

Step 5: Electron Flow from PS-I to NADP⁺

• The primary electron acceptor of photosystem I passes the photoexcited electrons to a second electron transport chain.
• The electrons are accepted by ferredoxin (Fd). It is an iron-containing protein.
• An enzyme called NADP reductase (flavoprotein enzyme) transfers the electrons from Fd to NADP.
• NADP combines with electrons and hydrogen ions to form NADPH (reduced).
• The NADPH will provide reducing power for the synthesis of sugar in the Calvin cycle.

Equation: NADP⁺ + 2e^- + 2H⁺ → NADPH + H⁺

Z-Scheme

- The path of electron transport through the two photosystems during non-cyclic photophosphorylation is known as Z-Scheme due to its conceptual zigzag shape.

Summary of Non-Cyclic Photophosphorylation:

Overall Equation: 2H₂O + 2NADP⁺ + 3ADP + 3Pi Light→ O₂ + 2NADPH + 3ATP

(b) Cyclic Photophosphorylation

Definition: The rise in NADPH and deficit of ATP may stimulate a temporary shift from a non-cyclic to cyclic electron flow until ATP supply catches up the demand.

• In this mechanism, only PS-I is utilized.
• It absorbs energy in the form of photons.
• When energy reaches the reaction centre of PS-I, the electrons are boosted up to a higher energy level.
• Such excited electrons are first captured by primary electron acceptor of PS-I.
• Then they move through an electron transport chain containing:
• Ferredoxin
• Cytochrome b₆-f complex
• Plastocyanin
• When electrons are passed from cytochrome b₆-f complex, an ATP is generated by chemiosmosis.
• Finally, the electrons after losing the energy return back to P700 chlorophyll in PS-I reaction centre.
• There is no production of NADPH, no occurrence of photolysis of water, and therefore, no release of oxygen.

Steps of Cyclic Photophosphorylation:

Overall Equation: ADP + Pi Light→ ATP

Comparison of Non-Cyclic and Cyclic Photophosphorylation

Intermediate Level

Non-Cyclic Photophosphorylation in Detail

Cyclic Photophosphorylation in Detail

Comparison of Non-Cyclic and Cyclic Photophosphorylation

Z-Scheme in Detail

• The path of electron transport through the two photosystems during non-cyclic photophosphorylation is known as Z-Scheme due to its conceptual zigzag shape.
• The electrons start at water (low energy).
• They are excited by PS-II (P680) to a higher energy level.
• They flow through the electron transport chain (PQ, Cyt, PC).
• They are excited again by PS-I (P700) to an even higher energy level.
• They are finally accepted by NADP⁺ (to form NADPH).
• The overall path looks like a zigzag or Z-shape.

Chemiosmosis in Detail

• The electron flow through the cytochrome complex pumps protons from the stroma to the thylakoid lumen.
• This creates a proton gradient.
• The protons flow back through ATP synthase.
• This drives the synthesis of ATP from ADP and Pi.
• This mechanism is called chemiosmosis.

Equation: ADP + Pi ATP synthase→ ATP

Important Points

• 1. The light-dependent phase of photosynthesis involves:
• - Absorption of light by the photosystems
• - Excitation and flow of electrons through an electron transport chain
• - Chemiosmotic synthesis of ATP
• - Reduction of NADP to NADPH
• 2. The flow of excited electrons through an electron transport chain during light reaction is of two different types:
• - Non-cyclic electron flow
• - Cyclic electron flow
• 3. In non-cyclic electron flow, the excited electrons after leaving a particular photosystem do not come back; these electrons after losing their energy are incorporated into another molecule.
• 4. In cyclic electron flow, the excited electrons after leaving a particular photosystem finally come back to their photosystem again.
• 5. The most important event in light reaction is the production of ATP.
• 6. This production of ATP during light reaction is called photophosphorylation and the mechanism is called chemiosmosis.
• 7. There are two types of photophosphorylation:
• - Non-cyclic photophosphorylation
• - Cyclic photophosphorylation
• 8. Non-Cyclic Photophosphorylation:
• - It is the predominant pathway of light reaction in higher plants that occurs in routine.
• - In this process both photosystems (PS-I and PS-II) are utilized and two electron transport chains are involved.
• - When PS-II absorbs light, its excited electrons after flowing through an electron transport chain are transferred to PS-I.
• - Similarly, the excited electrons which are liberated from PS-I are finally accepted by NADP⁺.
• - Therefore, it is called non-cyclic electron flow.
• - Steps:
• 1. Absorption of light by PS-II and excitation of its electrons
• 2. Photolysis of water
• 3. Electron flow from PS-II to PS-I
• 4. Absorption of light by PS-I and excitation of its electrons
• 5. Electron flow from PS-I to NADP⁺
• - The path of electron transport through the two photosystems during non-cyclic photophosphorylation is known as Z-Scheme due to its conceptual zigzag shape.
• 9. Cyclic Photophosphorylation:
• - The rise in NADPH and deficit of ATP may stimulate a temporary shift from a non-cyclic to cyclic electron flow until ATP supply catches up the demand.
• - In this mechanism only PS-I is utilized.
• - It absorbs energy in the form of photons.
• - When energy reaches the reaction centre of PS-I the electrons are boosted up to higher energy level.
• - Such excited electrons are first captured by primary electron acceptor of PS-I, then they move through an electron transport chain containing ferridoxin, cytochrome b₆-f complex and plastocyanin.
• - When electrons are passed from cytochrome b₆-f complex an ATP is generated by chemiosmosis.
• - Finally, the electrons after losing the energy return back to P700 chlorophyll in PS-I reaction centre.
• - There is no production of NADPH, no occurrence of photolysis of water and therefore, no release of oxygen.

Remember

• - Light-dependent phase = thylakoid membranes
• - Photophosphorylation = production of ATP during light reaction
• - Chemiosmosis = mechanism of ATP production
• - Non-cyclic photophosphorylation = PS-I + PS-II; ATP + NADPH + O₂
• - Cyclic photophosphorylation = PS-I only; ATP only
• - Z-Scheme = zigzag path of electrons in non-cyclic photophosphorylation
• - Photolysis of water = splitting of water by light
• - Electron carriers: PQ, Cyt, PC, Fd, NADP reductase

Common Mistakes

• 1. Confusing non-cyclic and cyclic photophosphorylation – Non-cyclic = PS-I + PS-II; Cyclic = PS-I only.
• 2. Thinking cyclic photophosphorylation produces NADPH – Cyclic photophosphorylation produces ATP only.
• 3. Thinking cyclic photophosphorylation releases oxygen – Cyclic photophosphorylation does not release oxygen.
• 4. Confusing the Z-Scheme – The Z-Scheme is for non-cyclic photophosphorylation.
• 5. Forgetting the role of water – Water is the electron source in non-cyclic photophosphorylation.
• 6. Thinking ATP is produced in the stroma – ATP is produced in the thylakoid membrane.
• 7. Confusing photolysis of water – Photolysis of water occurs in PS-II.
• 8. Thinking NADPH is produced in PS-II – NADPH is produced in PS-I.
• 9. Forgetting the role of chemiosmosis – Chemiosmosis is the mechanism of ATP production.
• 10. Thinking cyclic photophosphorylation is the predominant pathway – Non-cyclic photophosphorylation is the predominant pathway.
• 11. Confusing the electron carriers – PQ, Cyt, PC are in non-cyclic; Fd, Cyt, PC are in cyclic.
• 12. Thinking the light reaction only produces ATP – The light reaction also produces NADPH and O₂.

Quick Revision

• The light-dependent phase of photosynthesis involves the absorption of light by the photosystems, excitation and flow of electrons through an electron transport chain, chemiosmotic synthesis of ATP, and reduction of NADP to NADPH.
• The flow of excited electrons through an electron transport chain during light reaction is of two different types i.e., non-cyclic and cyclic.
• In non-cyclic electron flow, the excited electrons after leaving a particular photosystem do not comeback; these electrons after losing their energy are incorporated into another molecule.
• On the other hand, in cyclic electron flow, the excited electrons after leaving a particular photosystem finally come back to their photosystem again.
• The most important event in light reaction is the production of ATP.
• This production of ATP during light reaction is called photophosphorylation and the mechanism is called chemiosmosis.
• There are two types of photophosphorylation:
• (a) Non-cyclic photophosphorylation
• (b) Cyclic photophosphorylation
• (a) Non-cyclic photophosphorylation:
• It is predominant pathway of light reaction in higher plants that occurs in routine.
• In this process both photosystems i.e., PS-I and PS-II are utilized and two electron transport chains are involved.
• When PS-II absorbs light, its excited electrons after flowing through an electron transport chain are transferred to PS-I.
• Similarly, the excited electrons which are liberated from PS-I are finally accepted by NADP⁺.
• Therefore it is called non-cyclic electron flow.
• The events of non-cyclic photophosphorylation are continuous but here they are discussed in steps for convenience.
• Absorption of light by PS-II and excitation of its electrons:
• When just two photons strike the antenna complex of PS-II, the two electrons become excited and begin to move along the atoms of different pigments within photosystem.
• Ultimately, the absorbed energy reaches the reaction centre of PS-II (P680) and causes its two electrons to be excited.
• These excited electrons are captured by the primary electron acceptor of PS-II and leave two "electron holes" in the photosystem behind making chlorophyll a strong oxidizing agent.
• Photolysis of water:
• The electron holes of photosystem must be filled so that in the presence of water splitting enzyme reactions can proceed.
• When water reacts with oxidized state of chlorophyll in photosystem, it breaks up into 2H⁺ ions, 2e⁻ and ½O₂.
• Since this breakdown occurs in the presence of sunlight therefore, it is termed as photolysis of water.
• The electrons released from water are used to fill the "electron holes" of PS-II.
• H₂O → (Photolysis of water) → 2H⁺ + 2e⁻ + ½O₂
• Electron flow from PS-II to PS-I:
• The excited/energized electrons which have been released from PS-II and captured by primary electron acceptor now begin to flow to PS-I through an electron transport chain.
• The electrons move from primary electron acceptor to the plastoquinone (PQ).
• From PQ the electrons flow through a complex of the cytochromes (Cyt) which consist of Cyt-b and Cyt-f.
• The cytochrome complex is not only an electron carrier but it also works as proton pump.
• The electron flow through the cytochrome complex stimulates it to pump the protons from stroma to the thylakoid inner space.
• In this way the energy of flowing electrons is transformed into a gradient of protons (H⁺) in the thylakoid inner space.
• The proton gradient activates an enzyme in thylakoid membrane called ATP synthase which not only moves the protons back into the stroma but also catalyzes a reaction in which ADP and Pi are combined to form ATP (photophosphorylation).
• This whole mechanism which involves flow of electron, pumping of protons and generation of ATP by thylakoid membranes is called chemiosmosis.
• This ATP, generated by light reactions will provide chemical energy for the synthesis of sugar during Calvin cycle.
• The energized electrons after losing their energy, move from cytochrome complex to the plastocyanin (PC) and finally incorporated into the PS-I.
• Absorption of light by PS-I and excitation of its electrons:
• On the other hand, when P700 in the reaction centre of PS-I molecule absorbs two photon of light, electrons are boosted to a higher energy level.
• P700 molecule passes these excited electrons to a primary electron acceptor of PS-I, creating "electron holes".
• The electron holes of P700 are filled by the pair of electrons received from the P680 (photosystem II) via electron transport chain.
• Electron flow from PS-I to NADP⁺:
• The primary electron acceptor of photosystem I passes the photoexcited electrons to a second electron transport chain.
• The electrons are accepted by ferredoxin (Fd). It is an iron containing protein.
• An enzyme called NADP reductase (flavoprotein enzyme) transfers the electrons from Fd to NADP.
• NADP combines with electrons and hydrogen ions to form NADPH (reduced).
• The NADPH will provide reducing power for the synthesis of sugar in the Calvin cycle.
• NADP⁺ + 2e⁻ + 2H⁺ → NADPH + H⁺
• The path of electron transport through the two photosystems during non-cyclic photophosphorylation is known as Z-Scheme due to its conceptual zigzag shape.
• (b) Cyclic photophosphorylation:
• The rise in NADPH and deficit of ATP may stimulate a temporary shift from a non-cyclic to cyclic electron flow until ATP supply catches up the demand.
• In this mechanism only PS-I is utilized.
• It absorbs energy in the form of photons.
• When energy reaches the reaction centre of PS-I the electrons are boosted up to higher energy level.
• Such excited electrons are first captured by primary electron acceptor of PS-I, then they move through an electron transport chain containing ferridoxin, cytochrome b₆-f complex and plastocyanin.
• When electrons are passed from cytochrome b₆-f complex an ATP is generated by chemiosmosis.
• Finally, the electrons after losing the energy return back to P700 chlorophyll in PS-I reaction centre.
• There is no production of NADPH, no occurrence of photolysis of water and therefore, no release of oxygen.

60-Second Revision

• Light-dependent phase = thylakoid membranes
• Photophosphorylation = production of ATP during light reaction
• Chemiosmosis = mechanism of ATP production
• Non-cyclic photophosphorylation = PS-I + PS-II; ATP + NADPH + O₂
• Cyclic photophosphorylation = PS-I only; ATP only
• Z-Scheme = zigzag path of electrons in non-cyclic photophosphorylation
• Photolysis of water = splitting of water by light
• Electron carriers: PQ, Cyt, PC, Fd, NADP reductase
• Warning: Non-cyclic photophosphorylation uses both PS-I and PS-II; cyclic uses only PS-I. Cyclic produces ATP only; non-cyclic produces ATP, NADPH, and O₂. The Z-Scheme is for non-cyclic photophosphorylation. Photolysis of water occurs in PS-II. NADPH is produced in PS-I. Chemiosmosis is the mechanism of ATP production.