Biology

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Dark Reaction or Cycle or Biosynthetic Phase or Photosynthetic Carbon Reduction (PCR) Cycle

Biosynthetic phase of photosynthesis utilises assimilatory powers (ATP and NADPH + H⁺) produced during light reaction are used to fix and reduce carbon dioxide into carbohydrates. This reaction does not require light. Therefore, it is named Dark reaction. Ribulose 1, 5 bisphosphate (RUBP) act as acceptor molecule of carbon dioxide and fix the CO₂ by RUBISCO enzyme.

The first product of the pathway is a 3 – carbon compound (Phospho Glyceric Acid) and so it is also called as C₃ Cycle. It takes place in the stroma of the chloroplast.

M. Melvin Calvin, A.A. Benson and their co-workers in the year 1957 found this path way of carbon fixation. Melvin Calvin was awarded Nobel Prize for this in 1961 and this pathway named after the discoverers as Calvin-Benson Cycle. Dark reaction is temperature dependent and so it is also called thermo-chemical reaction.

Dark reaction consists of three phases: (Figure 13.16).

1. Carboxylation (fixation)
2. Reduction (Glycolytic Reversal)
3. Regeneration

Phase 1 – Carboxylation (Fixation)

The acceptor molecule Ribulose 1, 5 Bisphosphate (RUBP) a 5 carbon compound with the help of RUBP carboxylase oxygenase (RUBISCO) enzyme accepts one molecule of carbon dioxide to form an unstable 6 carbon compound. This 6C compound is broken down into two molecules of 3-carbon compound phospho glyceric acid (PGA) (Figure 13.17).

Phase 2 – Glycolytic Reversal / Reduction

Phospho glyceric acid is phosphorylated by ATP and produces 1, 3 bis phospho glyceric acid by PGA kinase. 1, 3 bis phospho glyceric acid is reduced to glyceraldehyde 3 Phosphate (G-3-P) by using the reducing power NADPH + H⁺. Glyceraldehyde 3 phosphate is converted into its isomeric form di hydroxy acetone phosphate (DHAP).

Phase 3 – Regeneration

Regeneration of RUBP involves the formation of several intermediate compounds of 6-carbon, 5-carbon,4-carbon and 7- carbon skeleton. Fixation of one carbon dioxide requires 3 ATPs and 2 NADPH + H⁺, and for the fixation of 6 CO₂ requires 18 ATPs and 12 NADPH + H⁺ during C₃ cycle. One 6 carbon compound is the net gain to form hexose sugar.

Overall Equation for Dark Reaction:
6CO₂ + 18ATP + 12NADPH + H⁺ → C₆H₁₂O₆ + 6H₂O + 18Pi + 12NADP⁺

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Photophosphorylation

Phosphorylation taking place during respiration is called as oxidative phosphorylation and ATP produced by the breakdown of substrate is known as substrate level phosphorylation. In this topic, we are going to learn about phosphorylation taking place in chloroplast with the help of light. During the movement of electrons through carrier molecules ATP and NADPH + H⁺ are produced.

Phosphorylation is the process of synthesis of ATP by the addition of inorganic phosphate to ADP. The addition of phosphate here takes place with the help of light generated electron and so it is called as photophosphorylation. It takes place in both cyclic and non-cyclic electron transport.

Cyclic Photophosphorylation

Cyclic photophosphorylation refers to the electrons ejected from the pigment system I (Photosystem I) and again cycled back to the PS I. When the photons activate P700 reaction centre photosystem II is activated. Electrons are raised to the high energy level.

The primary electron acceptor is Ferredoxin Reducing Substance (FRS) which transfers electrons to Ferredoxin (Fd), Plastoquinone (PQ), cytochrome b6-f complex, Plastocyanin (PC) and finally back to chlorophyll P700 (PS I).

During this movement of electrons Adenosine Di Phosphate (ADP) is phosphorylated, by the addition of inorganic phosphate and generates Adenosine Tri Phosphate (ATP). Cyclic electron transport produces only ATP and there is no NADPH + H⁺ formation.

At each step of electron transport, electron loses potential energy and is used by the transport chain to pump H⁺ ions across the thylakoid membrane. The proton gradient triggers ATP formation in ATP synthase enzyme situated on the thylakoid membrane.

Photosystem I need light of longer wave length (> P700 nm). It operates under low light intensity, less CO₂ and under anaerobic conditions which makes it considered as earlier in evolution (Figure 13.13).

Non-Cyclic Photophosphorylation

When photons are activated reaction centre of pigment system II(P680), electrons moved to the high energy level. Electrons from high energy state passes through series of electron carriers like pheophytin, plastoquinone, cytochrome complex, plastocyanin and finally accepted by PS I (P700). During this movement of electrons from PS II to PS I ATP is generated (Figure 13.16).

PS I (P700) is activated by light, electrons are moved to high energy state and accepted by electron acceptor molecule ferredoxin reducing Substance (FRS). During the downhill movement through ferredoxin, electrons are transferred to NADP⁺ and reduced into NADPH + H⁺ (H⁺ formed from splitting of water by light).

Electrons released from the photosystem II are not cycled back. It is used for the reduction of NADP⁺ into NADPH + H⁺. During the electron transport it generates ATP and hence this type of photophosphorylation is called non-cyclic photophosphorylation. The electron flow looks like the appearance of letter ‘Z’ and so known as Z scheme.

When there is availability of NADP⁺ for reduction and when there is splitting of water molecules both PS I and PS II are activated (Table 13.3). Non-cyclic electron transport PS I and PS II both are involved co operatively to transport electrons from water to NADP⁺ (Figure 13.14).

Bio Energetics of Light Reaction

• To release one electron from pigment system it requires two quanta of light.
• One quantum is used for transport of electron from water to PS I.
• Second quantum is used for transport of electron from PS I to NADP⁺
• Two electrons are required to generate one NADPH + H⁺
• During Non-Cyclic electron transport two NADPH + H⁺ are produced and it requires 4 electrons.
• Transportation of 4 electrons requires 8 quanta of light

Chemiosmotic Theory

Chemiosmosis theory was proposed by P. Mitchell (1966). According to this theory electrons are transported along the membrane through PS I and PS II and connected by Cytochrome b6-f complex. The flow of electrical current is due to difference in electrochemical potential of protons across the membrane.

Splitting of water molecule takes place inside the membrane. Protons or H⁺ ions accumulate within the lumen of the thylakoid (H⁺ increase 1000 to 2000 times). As a result, proton concentration is increased inside the thylakoid lumen. These protons move across the membrane because the primary acceptor of electron is located outside the membrane.

Protons in stroma less in number and creates a proton gradient. This gradient is broken down due to the movement of proton across the membrane to the stroma through CFO of the ATP synthase enzyme. The proton motive force created inside the lumen of thylakoid or chemical gradient of H⁺ ion across the membrane stimulates ATP generation (Figure 13.15).

The evolution of one oxygen molecule (4 electrons required) requires 8 quanta of light. C₃ plants utilise 3 ATPs and 2 NADPH + H⁺ to evolve one Oxygen molecule. To evolve 6 molecules of Oxygen 18 ATPs and 12 NADPH + H⁺ are utilised. C₄ plants utilise 5 ATPs and 2 NADPH + H⁺ to evolve one oxygen molecule. To evolve 6 molecules of Oxygen 30 ATPs and 12 NADPH + H⁺ are utilised.

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Photo Chemical Phase of Light Reaction

In this phase electrons pass through electron carrier molecules and generate assimilatory powers ATP and NADPH + H⁺. Splitting of water molecule generates electrons replacing electrons produced by the light.

Photolysis of Water

The process of Photolysis is associated with Oxygen Evolving Complex (OEC) or water splitting complex in pigment system II and is catalysed by the presence of Mn⁺⁺ and Cl^–. When the pigment system II is active it receives light and the water molecule splits into OH^– ions and H⁺ ions. The OH^– ions unite to form water molecules again and release O₂ and electrons (Figure 13.11).

Electron Transport Chain of Chloroplast
Electron Transport Chain in each photosystem involves four complexes:

Core Complex (CC):
CC I in PS I the reaction centre is P700, CC II in PS II the reaction centre is P680

Light Harvesting Complex or Antenna

Complex (LHC):
Two types: LHC I in PS I and LHC II in PS II.

Cytochrome b6 f Complex:

It is the non-pigmented protein complex connecting PS I and PS II. Plastoquinone (PQ) and Plastocyanin (PC) are intermediate complexes acting as mobile or shuttle electron carriers of Electron Transport Chain. PQ acts as shuttle between PS II and Cytochrome b₆ – f complex and PC connects.

Cytochrome b₆-f and PS I Complex

ATPase complex or Coupling Factor:
It is found in the surface of thylakoid membrane. This complex is made up of CF₁ and CF₀ factors. This complex utilizes energy from ETC and converts ADP and inorganic phosphate (Pi) into ATP (Figure 13.12).

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Photo – Oxidation Phase of Light Reaction

The action of photon plays a vital role in excitation of pigment molecules to release an electron. When the molecules absorb a photon, it is in excited state. When the light source turned off the high energy electrons return to their normal low energy orbitals as the excited molecule goes back to its original stable condition known as ground state.

When molecules absorb or emit light they change their electronic state. Absorption of blue light excites the chlorophyll to higher energy state than absorption of Red light, because the energy of photon is higher when their wavelength is shorter.

When the pigment molecule is in an excited state, this excitation energy is utilised for the phosphorylation. Phosphorylation takes place with the help of light generated electron and hence it is known as photophosphorylation.

Photosystem and Reaction Centre

1. Thlakoid membrane contains Photosystem I (PS I) and Photosystem II (PS II).
2. PS I is in unstacked region of granum facing stroma of chloroplast.
3. PS II is found in stacked region of thylakoid membrane facing lumen of thylakoid.
4. Each Photosystem consists of central core complex (CC) and light harvesting Complex (LHC) or Antenna molecules (Figure 13.10).
5. The core complex consists of respective reaction centre associated with proteins, electron donors and acceptors.
6. PS I – CC I consists of reaction centre P700 and LHC I.
7. PS II – CC II consists of reaction centre P680 and LHC II (Table 13.2).
8. Light Harvesting Complex consists of several chlorophylls, carotenoids and xanthophyll molecules.
9. The main function of LHC is to harvest light energy and transfer it to their respective reaction centre.

The action of photon plays a vital role in excitation of pigment molecules to release an electron. The action of photon plays a vital role in excitation of pigment molecules to release an electron. When the molecules absorb a photon, it is in excited state.

Photo-oxidation is a chain process incorporating a large number of chemical reactions which are subsequent to the outcome of the primary event absorption of a photon, which induces breakdown to free-radical products.

In photosynthesis, the light-dependent reactions take place on the thylakoid membranes. The four photosystems absorb light energy through pigments-primarily the chlorophylls, which are responsible for the green color of leaves. The light-dependent reactions begin in photosystem II.

The light-dependent generation of active oxygen. species is termed photooxidative stress. This can occur in two ways:

1. The donation of energy or electrons directly to oxygen as a result of photosynthetic activity
2. Exposure of tissues to ultraviolet irradiation.

Oxidation is defined as a process in which an electron is removed from a molecule during a chemical reaction. What happens in oxidation? During oxidation, there is a transfer of electrons.

The Light Reactions of Photosynthesis. Light is absorbed and the energy is used to drive electrons from water to generate NADPH and to drive protons across a membrane. These protons return through ATP synthase to make ATP.

This is an intrinsic feature of the regulation of photosynthetic electron transport. Photoinhibition and photooxidation only usually occur when plants are exposed to stress. Active oxygen species are part of the alarm signalling processes in plants.

Light reactions harness energy from the sun to produce chemical bonds, ATP, and NADPH. These energy-carrying molecules are made in the stroma where carbon fixation takes place. The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration.

Photo-oxidation means the release of electrons after absorption of a photon occurs during non-cyclic photophosphorylation. It involves an outsized number of chemical reactions.

Photolysis is an oxidative process that means the splitting of water to form oxygen, protons and electrons in the presence of light. As a result, oxygen evolution happens and the electron travels to PS-IIvia the Mn-protein.

The light-dependent reactions use light energy to make two molecules needed for the next stage of photosynthesis: the energy storage molecule ATP and the reduced electron carrier NADPH. In plants, the light reactions take place in the thylakoid membranes of organelles called chloroplasts.

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Modern Concept of Photosynthesis

Photosynthesis is an Oxidation and Reduction process. Water is oxidised to release O₂ and CO₂ is reduced to form sugars. The first phase requires light and is called light reaction or Hill’s reaction.

1. Light Reaction:

It is a photochemical reaction whereas dark reaction is a thermochemical reaction. Solar energy is trapped by chlorophyll and stored in the form of chemical energy (assimilatory power) as ATP and reducing power NADPH + H⁺, NADPH + H⁺ alone are known as reducing powers. This reaction takes place in thylakoid membrane of the chloroplast. Oxygen is evolved as a result of splitting of water molecules by light.

Light reaction is discussed in two phases:

(i) Photo-Oxidation Phase:

• Absorption of light energy.
• Transfer of energy from accessory pigments to reaction centre.
• Activation of Chlorophyll ‘a’ molecule.

(ii) Photo Chemical Phase:

• Photolysis of water and oxygen evolution
• Electron transport and synthesis of assimilatory power.

2. Dark reaction (Biosynthetic Phase):

Fixation and reduction of CO₂ into carbohydrates with the help of assimilatory power produced during light reaction. This reaction does not require light and is not directly light driven. Hence, it is called as Dark reaction or CalvinBenson cycle (Figure 13.9).

photosynthesis, the process by which green plants and certain other organisms transform light energy into chemical energy. During photosynthesis in green plants, light energy is captured and used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds.

During photosynthesis, plants take in carbon dioxide (CO₂) and water (H₂O) from the air and soil. This transforms the water into oxygen and the carbon dioxide into glucose. The plant then releases the oxygen back into the air, and stores energy within the glucose molecules.

Recent advances include combining photosystem complexes with hydrogenases for hydrogen production, using isolated thylakoids, photosystems on nanostructured electrodes such as gold nanoparticles, carbon nanotubes, ZnO nanoparticles for electricity generation.

The process by which green plants make their own food (like glucose) from carbon dioxide and water by using sunlight energy (in the presence of chlorophyll) is called photosynthesis.

Green plants are the main producers of food in the ecosystem. All other organisms directly or indirectly depend on green plants for food. The process of photosynthesis also helps in maintaining the balance of carbon dioxide and oxygen in the air.

The three events that occur during the process of photosynthesis are:

1. Absorption of light energy by chlorophyll
2. Conversion of light energy to chemical energy and splitting of water molecules into hydrogen and oxygen
3. Reduction of carbon dioxide to carbohydrates.

Photosynthesis and respiration, both using electron flow coupled with phosphorylation, have a common origin (‘conversion hypothesis’), but photosynthesis came first. Anaerobic (nitrate or sulphate) respiration cannot have preceded photosynthesis as neither nitrate nor sulphate existed on the early earth.

It fixes and balances the amount of carbon dioxide and oxygen in the atmosphere by plants exhaling oxygen and animals and humans exhaling carbon dioxide. It helps in the synthesis of organic compounds from inorganic compounds. It provides nutrition to the plants and thus helps in their growth and development.

The first stage of photosynthesis is called the light reactions. During this stage, light is absorbed and transformed to chemical energy in the bonds of NADPH and ATP.

The Light Reactions of Photosynthesis. Light is absorbed and the energy is used to drive electrons from water to generate NADPH and to drive protons across a membrane. These protons return through ATP synthase to make ATP.

Scientists think that glycolysis evolved before the other stages of cellular respiration. This is because the other stages need oxygen, whereas glycolysis does not, and there was no oxygen in Earth’s atmosphere when life first evolved about 3.5 to 4 billion years ago.