Electron Transport Chain in Photosynthesis: Definition, Photosystem II, Photosystem I and Light Reaction

Electron Transport Chain in Photosynthesis: Electrons travel through photosystem \({\rm{II}}\) and then pass through photosystem \({\rm{I}}\). That appears to be extremely strange. Photosystem \({\rm{I}}\) and \({\rm{II}}\) don’t line up with the path electrons traverse via the electron transport chain because they weren’t discovered in that sequence. Photosystem \({\rm{II}}\) was the first to be discovered. Photosystem \({\rm{II}}\) was identified later, and it was determined to be earlier in the electron transport chain. But it was too late; the name got stuck. When Water molecules are broken down to liberate electrons, then the electrons proceed down a gradient, accumulating energy in the form of \({\rm{ATP}}\) in the process. Let’s deep drive into the Electron Transport Chain in Photosynthesis. Read on more about Photosystem \({\rm{II}}\) \(\left( {{\rm{PS II}}} \right)\), Photosystem \({\rm{I}}\) \(\left( {{\rm{PS I}}} \right)\), the Light Reaction (Hill Reaction).

Know Everything About Photosynthesis Here 

In the electron transport chain, a group of proteins carry electrons through a membrane within the thylakoid to build a proton gradient that drives the synthesis of adenosine triphosphate \(\left( {{\rm{ATP}}} \right)\). The cell uses \({\rm{ATP}}\) as an energy source for metabolic processes and cellular functions.

The electrons move from a higher potential to a lower potential as they travel through the electron transport chain, shifting from less electron density to more electron density molecules. These “downhill” electron exchanges release energy, which is used by various protein complexes to push protons from the thylakoid to the intermembrane space, creating a proton gradient.

The Calvin cycle and light-dependent reactions are the two stages of photosynthesis. Light energy is used in light-dependent processes in the thylakoid membrane to produce \({\rm{ATP}}\) and \({\rm{NADPH}}\).

Discovery

1. Robert Hill proposed the light-driven photosynthesis reaction, commonly known as the light reaction (Hill reaction), in \(1939\).
2. Isolated chloroplasts can reduce pyridine nucleotides in light, as proved in \((1951)\).
3. Arnon and his colleagues \((1954)\) established that isolated chloroplasts might create \({\rm{ATP}}\).
4. In \(1957\), it was discovered that the photochemical process and an enzyme system are capable of utilising the reduced pyridine nucleotide as soon as it is generated.
5. According to Arnon \((1967)\), \({\rm{NAD}}{{\rm{H}}_2}\) is used in bacterial photosynthesis instead of \({\rm{NADPH}}\).

The Light Reaction (Hill Reaction)

As a result of light energy, the light reaction is assumed to be responsible for the formation of oxygen from water. This is how it goes: absorption of light by chlorophyll and light energy splitting \({{\rm{H}}_2}{\rm{O}}\).

\({{\rm{H}}_2}{\rm{O}} + {\rm{Light}} \to {{\rm{H}}^ + } + {\rm{O}}{{\rm{H}}^ – }\)

1. The \(\left( {\rm{H}} \right)\) binds to \({\rm{NADP}}\) molecules and is converted to \({\rm{NADPH}}\).
2. \({\rm{NADPH}}\) can trigger phosphoglyceric acid reduction- Phosphoglyceraldehyde.
3. \({{\rm{H}}_2}{\rm{O}}\) and oxygen are formed by the \(\left( {{\rm{OH}}} \right)\) group:
   \(4{\rm{O}}{{\rm{H}}^ – } \to 2{{\rm{H}}_2}{\rm{O}} + {{\rm{O}}_2} + 4{{\rm{e}}^ – }\) 

The light reaction gives rise to two very important productions:

(i) A reducing agent \({\rm{NADPH}}\)
(ii) An energy-rich compound \({\rm{ATP}}\).

In the dark phase of photosynthesis, these two products of the light reaction are used.

The energy transformations in photosynthesis are as follow:

1. An absorbed quantum’s radiant energy is converted into the energy of an activated pigment molecule. Here \(‘{\rm{P’}}\) is pigment.
   \({\rm{P}} + {\rm{Light energy}} \to {\rm{P}} + {{\rm{e}}^ – }\)
2. The activated pigment now takes an electron from the water molecule’s hydroxyl ion. The \({\rm{O}}{{\rm{H}}^ – }\) stands for ‘radical.’ These aren’t charged, but they’re extremely reactive.
3. The \({{\rm{H}}^ + }\) ions from water are transported to particular molecules, along with the electron bound to the pigment, which subsequently conveys the reducing power to further processes.
   \({\rm{NADP}} + {{\rm{H}}^ + } + {{\rm{e}}^ – }\; \to {\rm{NADPH}}\)
4. The reintegration of the split products of water into the water molecules themselves is another reaction.
   \(({{\rm{H}}^ + }) + ({\rm{O}}{{\rm{H}}^ – }) \to {{\rm{H}}_2}{\rm{O}}\)

The light reaction is basically carried out by two types of the photosystem, i.e. photosystem I and photosystem II.

Photosystem I (PS I)

1. Photosystem \({\rm{I}}\) \(\left( {{\rm{P}}700} \right)\), like photosystem \({\rm{II}}\) \(\left( {{\rm{P}}680} \right)\), is stimulated by light absorption and becomes oxidised before transferring its electrons to the primary electron acceptor, which is then reduced.
2. While the reduced electron acceptor of photosystem \({\rm{I}}\) transfer electrons to convert \({\rm{NADP}}\) to \({\rm{NAD}}\), the oxidised \({\rm{P}}700\) receives electrons from photosystem \({\rm{II}}\).
3. \({\rm{NADPH}}\) is a strong reducing agent that is used in the photosynthetic carbon process to reduce \({\rm{C}}{{\rm{O}}_2}\) to carbohydrates.
4. The conversion of \({\rm{C}}{{\rm{O}}_2}\) to carbohydrates necessitates the use of energy in the form of ATP, which is generated via the electron transport chain.
5. Photophosphorylation is the process of producing \({\rm{ATP}}\) from \({\rm{ADP}}\) in the presence of light in chloroplasts.

Photosystem II (PS II)

The electrons travel through the chloroplast electron transport chain \(\left( {{\rm{ETC}}} \right)\) to photosystem \({\rm{II}}\), which reduces \({\rm{NAD}}{{\rm{P}}^ + }\) to \({\rm{NADPH}}\). The \({\rm{ETC}}\) moves protons from the stroma into the thylakoid lumen. \({\rm{ATP}}\) synthase uses the resulting electrochemical gradient to make \({\rm{ATP}}\).

1. The absorption of light by photosystem \({\rm{II}}\) starts the electron transport chain of photosynthesis \(\left( {{\rm{P}}680} \right)\).
2. \({\rm{P}}680\) is activated, and its electrons are transferred to an electron acceptor molecule when it absorbs light.
3. As a result, \({\rm{P}}680\) becomes a potent oxidising agent, splitting a water molecule to liberate oxygen. Photolysis is the term for the light-induced splitting of water molecules.
4. Manganese, calcium, and chloride ions, on the other hand, play key roles in water photolysis. Electrons are produced once water is photolysed, and they are then passed to the oxidised \({\rm{P}}680\).
5. The electron-deficient \({\rm{P}}680\) can now replenish its electrons from the water molecule (because of the fact that it has already transmitted its electrons to an acceptor molecule).
6. The principal electron acceptor is reduced after accepting electrons from the excited \({\rm{P}}680.\) Pheophytin is the principal electron acceptor in plants.
7. The reduced acceptor, which is a strong reducing agent, now gives its electrons to the electron transport chain’s downstream components.

Photophosphorylation

Photosynthetic Phosphorylation

1. With the discovery that \({\rm{C}}{{\rm{O}}_2}\) can be assimilated in isolated chloroplasts, it became clear that the chloroplast must have the enzymes required for this assimilation as well as the ability to make \({\rm{ATP}}\) (adenosine triphosphate) which is required for the generation of the primary photosynthetic products.
2. In the presence of light, chloroplasts can create \({\rm{ATP}}\). This mechanism was given the name photosynthetic phosphorylation.
3. It was discovered that mitochondria are not the only cytoplasmic particles that generate \({\rm{ATP}}\) for the first time.
4. In chloroplast, \({\rm{ATP}}\) production varies from mitochondrial \({\rm{ATP}}\) production in that it is of respiratory oxidations.
5. The light energy is transformed into \({\rm{ATP}}\) during this process. Hence, chemical energy is converted to light energy.
6. \({\rm{ATP}}\) is solely one of the prerequisites for carbon dioxide to be reduced to carbohydrate levels. In photosynthesis, a reductant must be generated to provide the hydrogens or electrons needed for this reduction. In photosynthesis, \({{\rm{H}}_2}\) is the reduced pyridine nucleotide.
7. \({\rm{NADP}}\) (nicotinamide adenine dinucleotide phosphate) substrate quantities were lowered in the presence of \({{\rm{H}}_2}{\rm{O}},{\rm{ ADP}}\) (adenosine diphosphate), and orthophosphate \(\left( {\rm{P}} \right)\), accompanied by the evolution of oxygen.

The equation is as follow:

\(2{\rm{ADP}} + 2{\rm{Pi}} + 2{\rm{NADP}} + 4{{\rm{H}}_2}{\rm{O}} \to 2{\rm{ATP}} + {{\rm{O}}_2} + 2{\rm{NADPH}} + 2{{\rm{H}}_2}{\rm{O}}\)
A. The development of one molecule of oxygen is accompanied by the reduction of two molecules of \({\rm{NADP}}\).
B. The energy requirements for \({\rm{C}}{{\rm{O}}_2}\) absorption are supplied by \({\rm{ATP}}\) and \({\rm{NADPH}}\). This assimilatory power \(\left( {{\rm{ATP}} + {\rm{NADPH}}} \right)\).

According to Arnon, there are two types of photophosphorylation:

1. Cyclic photophosphorylation
2. Non-cyclic photophosphorylation

Cyclic Photophosphorylation

1. Cyclic photophosphorylation occurs when non-cyclic photophosphorylation is inhibited under particular conditions. Illuminating isolated chloroplasts with light with a wavelength greater than \(680\) nm can inhibit non-cyclic photophosphorylation.
2. The electron transport from water to \({\rm{NADP}}\) is interrupted, and \({\rm{C}}{{\rm{O}}_2}\) fixation is slowed as a result of \({\rm{PS II}}\) inactivation.
3. Electrons are not eliminated from reduced \({\rm{NADPH}}\) when \({\rm{C}}{{\rm{O}}_2}\) fixation stops. As a result, \({\rm{NADPH}}\) will not be oxidised, and \({\rm{NADP}}\) will not be an electron acceptor.
4. Because \({\rm{NADP}}\) is not available in an oxidised state to receive electrons, electrons from photosystem \({\rm{I}}\) \(\left( {{\rm{PS I}}} \right)\) are not transmitted to \({\rm{NADP}}\) from the electron acceptor during cyclic-photophosphorylation.
5. As a result, the electrons are returned to \({\rm{P}}700\).
6. The production of \({\rm{ATP}}\) from \({\rm{ADP}}\) occurs when electrons flow from an electron acceptor to \({\rm{P}}700\). This mechanism is known as cyclic photophosphorylation.
7. Because there is no photolysis of water during cyclic photophosphorylation, no oxygen is liberated, and no \({\rm{NADPH}}\) is produced.
8. After losing an electron, the chlorophyll molecule gains a positive charge, and the electron is transferred to a second acceptor.
9. The cytochrome system is a collection of molecules that serve as the second acceptor. All components of the cytochrome system are cytochrome variations. These cytochromes eventually transfer the electron to the chlorophyll molecule, where it was previously lost.
10. The light’s electromagnetic energy is used in the production of \({\rm{ATP}}\). Light energy is turned into chemical energy in this way. The electron travels in a circular path before returning to the same molecule from which it originated, and this process has been termed cyclic photophosphorylation.
11. The chlorophyll serves as both the ultimate electron acceptor and the beginning electron donor. One electron and two \({\rm{ATP}}\) molecules are generated during cyclic photophosphorylation.

Non-Cyclic Photophosphorylation

1. This is the outcome of photosystem \({\rm{I}}\) \(\left( {{\rm{PSI}}} \right)\) and photosystem \({\rm{II}}\) interacting \(\left( {{\rm{PSII}}} \right)\).
2. The electron is not returned to the chlorophyll molecule in non-cyclic photophosphorylation; instead, it is picked up by \({\rm{NAD}}{{\rm{P}}^ + }\), which then reduces to \({\rm{NADPH}}\).
3. The electron that returns to the chlorophyll molecule comes from an external source, which is water in this case.
4. Oxygen is liberated, and \({\rm{NADPH}}\) and \({\rm{ATP}}\) are generated as a result of this process.
5. The excited chlorophyll loses an electron, which is taken by \({\rm{NADP}}\) coupled with a proton, resulting in \({\rm{NADPH}}\). The \({\rm{NADPH}}\) molecule now stores light energy.
6. Photolysis releases the proton necessary for \({\rm{NADP}}\) reduction from the breakdown of water molecules into hydrogen and hydroxyl ions.
   \(2{{\rm{H}}_2}{\rm{O}} \to 4{{\rm{H}}^ + } + 4{{\rm{e}}^ – } + {{\rm{O}}_2}\)
7. Water and molecular oxygen are produced when the hydroxyl ions react.

The reaction is as follows:

\(4{\rm{O}}{{\rm{H}}^ – }\; \to 2{{\rm{H}}_2}{\rm{O}} + {{\rm{O}}_2} + 4{{\rm{e}}^ – }\)

1. In this case, the hydroxyl ion also releases an electron, which is received by the cytochromes of the chloroplast.
2. The cytochrome then donates this electron to the chlorophyll molecule, which had previously lost one. The energy released during the transport of electrons from the cytochrome is used in the photophosphorylation of \({\rm{ADP}}\) to produce \({\rm{ATP}}\).
3. Hydrogen is firmly bonded to oxygen in water molecules, and it can only be split with the use of energy.
4. Light is the source of this energy. In this way, light energy participates in two processes in non-cyclic photophosphorylation: Chlorophyll molecule activation and photolysis (cleavage) of water.
5. When a photon activates a chlorophyll molecule, one molecule of \({\rm{NADPH}}\) and one molecule of \({\rm{ATP}}\) are generated. In cyclic photophosphorylation, two molecules of \({\rm{ATP}}\) are produced for each photon absorbed by chlorophyll.

Summary

In the electron transport chain,  a group of proteins carry electrons through a membrane within the thylakoid to build a proton gradient that drives adenosine triphosphate synthesis \(\left( {{\rm{ATP}}} \right)\). The cell uses \({\rm{ATP}}\) as an energy source for metabolic processes and cellular functions. The electromagnetic energy of visible light is transformed into chemical energy during photosynthesis. The photosynthetic cells of green plants or photosynthetic bacteria are responsible for this energy conversion. Cyclic photophosphorylation occurs when non-cyclic photophosphorylation is inhibited under particular conditions. Illuminating isolated chloroplasts with light with a wavelength greater than \(680\) nm can inhibit non-cyclic photophosphorylation. Photosystem \({\rm{I}}\) \(\left( {{\rm{P}}700} \right)\), like photosystem \({\rm{II}}\) \(\left( {{\rm{P}}680} \right)\), is stimulated by light absorption and becomes oxidised before transferring its electrons to the primary electron acceptor, which is then reduced

Frequently Asked Questions (FAQs)

Q.1. Does photosynthesis have an electron transport chain?
Ans: Yes, Photosynthesis has an electron transport chain wherein electron carrier molecules by transporting electrons in a chain produces \({\rm{ATP}}\) and \({\rm{NADPH}}\).

Q.2. How does the electron transport chain help photosynthesis?
Ans: \({\rm{ETC}}\) helps in the development of products that are required for the formation of glucose during the dark phase of photosynthesis. The products developed by the transport of electrons through \({\rm{ETC}}\) are \({\rm{ATP}}\) and \({\rm{NADPH}}\).

Q.3. What would occur if the electron transport chain stopped during photosynthesis?
Ans: \({\rm{ATP}}\) and \({\rm{NADPH}}\) would not be produced if the electron transport chain ceased working during photosynthesis, and thus the dark reaction will also be blocked.

Q.4. Where does non-cyclic photophosphorylation occur?
Ans: In the granal thylakoid area of chloroplast, non-cyclic phosphorylation occurs.