Do you think light is necessary for living beings? What if we do not provide light to a green plant? Do you know what happens to the light energy absorbed by the leaves of the plants? Sun is the ultimate source of light energy for all the living organisms on the earth.

The leaves of plants convert this light energy absorbed by the pigments (such as chlorophyll) into chemical energy (stored in the food), which includes a series of chemical reactions called light reactions of photosynthesis. The green plants prepare the food by the natural process called photosynthesis which consists of two phases depending on the requirement of energy, i.e., photochemical phase or light reaction phase and biosynthetic phase or dark reaction phase. Scroll down to explore more about the light reaction of photosynthesis, its definition, process, importance, etc.

Light Reaction of Photosynthesis Definition

Light Reaction of Photosynthesis is defined as a series of chemical reactions that require light energy captured by the pigment present in the leaves (such as chlorophyll) to get converted into chemical energy in the form of ATP and NADPH.

This reaction is also known as the Photochemical phase or light-dependent reaction. The main motto of photochemical reactions is to produce ATP (photophosphorylation) and \({\rm{NADPH}}{\rm{.}}{{\rm{H}}^ + }.\)

Characteristics of Light Reaction of Photosynthesis

1. Light Reaction involves the pigment molecules, both chlorophylls and accessory pigments.
2. Light Reaction provides the raw material, like ATP and \({\rm{NADPH}}.{{\rm{H}}^{\rm{ + }}}{\rm{,}}\) for the biochemical reactions that take place in the stroma.
3. This photochemical reaction is dependent upon the light, whose main function is to provide an energy source to the antenna pigment molecules for excitation.
4. The photochemical phase involves the photolysis of water in which the water molecule breaks up into hydrogen ions and oxygen in the illuminated chloroplast.
5. For this reaction, light energy, an oxygen evolving complex (OEC) and an electron carrier are required.
6. The light reactions take place in the grana, or more specifically, in the thylakoid of the grana inside the chloroplasts.

Emerson Red Drop and Enhancement Effect

Emerson \(\left( {1957} \right)\) has conducted several experiments to understand light reactions. Two major findings of his experiments are:

1. There is a fall in the rate of photosynthesis above \(680\,{\rm{nm}}\) is called the red drop.
2. That if two monochromatic beams of wavelengths \(650 – 680\,{\rm{nm}}\) and \(700 – 720\,{\rm{nm}}\) are used, the rate of photosynthesis increases. This is known as Emerson’s Enhancement Effect.

Photosystems or Pigment Systems

1. In green plants, the photosynthetic units occur in the form of two distinct photosystems (or pigment systems), PS-I and PS-II (named in the chronology of discovery).

2. The PS-I, along with some electron carriers, is located on both the non-appressed part of grana and stroma thylakoid. Conversely, PS-II, along with the electron carriers, are located on the appressed part of the grana thylakoid.

3. PS-I has more chlorophyll-a, a reducing agent, which is a special chlorophyll-a molecule \(\left( {{{\rm{P}}_{{\rm{700}}}}} \right),\) plastoquinone, plastocyanin, ferredoxin, etc.

4. PS-II has nearly equal chlorophyll-a and chlorophyll-b, a reducing agent which is a special chlorophyll-a \(\left( {{{\rm{P}}_{{\rm{680}}}}} \right)\) molecule, plastoquinone, plastocyanin, cytochrome complex, \({\rm{M}}{{\rm{n}}^{ + 2}},\,{\rm{C}}{{\rm{l}}^ – },\) etc.

Fig: Comparison of PS-I and PS-II

5. Each photosystem has a photo centre or reaction centre, generally occupied by a chlorophyll-a molecule.

6. The reaction centre is different in both the photosystems as given below:
i. In PS-I, the reaction centre of chlorophyll-a has a peak of absorption at \(700\,{\rm{nm,}}\) known as \({{\rm{P}}_{{\rm{700}}{\rm{.}}}}\)
ii. The PS-I is involved in both cyclic photophosphorylation and non-cyclic photophosphorylation and is basically concerned with \({\rm{NAD}}{{\rm{P}}^{\rm{ + }}}\) reduction.
iii. In PS-II, the reaction centre has an absorption peak at \(680\,{\rm{nm,}}\) hence known as \({{\rm{P}}_{{\rm{680}}}}.\)
iv. The PS-II is found in non-cyclic photophosphorylation only and is involved in the photolysis of water.

Light Reaction of Photosynthesis Process

Following are the steps of light reaction of photosynthesis:

Electron Transport

1. The photosynthetic electron transport chain initiates with the absorption of light energy by the photosystem-II.

2. Due to photoexcitation of the reaction centre, the electron from \({\rm{P680}}\) goes out and is accepted by an electron acceptor.

3. Electron(s) pass to other electron transporters (like cytochrome).

4. The movement of electrons is downhill according to the redox potential scale.

5. The electrons of the electron transport chain are not used up in the chain. Instead, they are further passed on to the pigments of PS-I.

6. Simultaneously, excited electrons from PS-I also leave the molecule and are accepted by electron acceptors with higher redox potential.

7. Now, the electrons in the reaction centre of PS-I also get excited on receiving the red light of wavelength \({\rm{700}}\,{\rm{nm}}\) and get transferred to another electron acceptor with higher redox potential.

8. Now, the electrons move to a molecule rich in energy and \({\rm{NAD}}{{\rm{P}}^{\rm{ + }}},\) which gets reduced to \({\rm{NADPH}}{\rm{.}}{{\rm{H}}^{\rm{ + }}}.\)

9. In \(1960,\) Bendall and Hill discovered the Z-scheme of electron transport.

Splitting of Water

1. PS-II supplies electrons to the PS-I. The PS-II gets electrons from water molecules.

2. This light-dependent splitting of water is called the photolysis of water.

3. In this process, the water splits into protons, electrons and oxygen.

4. This process is associated with the photosystem-II that is located on the inner side of the thylakoid membrane.

5. \({\rm{M}}{{\rm{n}}^{{\rm{ + 2}}}}\) and \({\rm{C}}{{\rm{l}}^ – }\) ions also play an important role in the photolysis of water molecules.

6. The obtained electrons replace those electrons which are removed from the photosystem-II.

7. The protons get accumulated in the lumen of the thylakoid, and oxygen is evolved as a by-product.
\(2{{\rm{H}}_{\rm{2}}}{\rm{O}} \to 4{{\rm{H}}^ + } + {{\rm{O}}_2} \uparrow + 4{{\rm{e}}^ – }\)

Photophosphorylation

1. Phosphorylation is the process through which ATP is synthesised from ADP and inorganic phosphate \(\left( {{{\rm{P}}_{\rm{i}}}} \right)\) by the cell organelles like mitochondria and chloroplast.

2. Since this process takes place in the presence of sunlight in the chloroplast, it is called photophosphorylation.

3. Phosphorylation in mitochondria is not light-dependent, but it uses the energy by oxidation of nutrients to produce ATP. Hence it is called oxidative phosphorylation.

4. The process of photophosphorylation is of two types:
a. Non-cyclic photophosphorylation
b. Cyclic photophosphorylation

Fig: Non-Cyclic Photophosphorylation

CYCLIC PHOTOPHOSPHORYLATION	NON-CYCLIC PHOTOPHOSPHORYLATION
It is performed by photosystem-I independently	It is performed by a collaboration of both photosystems II and I.
An external source of electrons is not required because the same electrons get recycled.	This process requires an external electron donor.
It is not connected with the photolysis of water. Therefore, no oxygen is evolved.	It is connected with the photolysis of water and the liberation of oxygen.
It synthesises only ATP.	It is not only concerned with ATP synthesis but also the production of \({\rm{NADPH}}{\rm{.}}{{\rm{H}}^ + }.\)
It operates under low light intensity or when carbon dioxide availability is poor.	It takes place under optimum light, aerobic conditions and in the presence of carbon dioxide.
It occurs mostly in the stromal or intergranal thylakoids.	It occurs in the granal thylakoids.

Fig: Cyclic Photophosphorylation

Chemiosmotic Hypothesis

1. This hypothesis was given by Peter Mitchell \(\left( {1961} \right)\) in order to explain the ATP synthesis in photosynthesis (also in respiration).

2. The synthesis of ATP is directly linked to the development of a proton gradient across the thylakoid membranes of a chloroplast.

3. The development of proton gradient results due to the reasons as given below:
a) As the water molecule splits into the inner side of the membrane, the protons (or the Hydrogen ions \(\left( {{{\rm{H}}^ + }} \right)\)) are accumulated in the thylakoid lumen.
b) The lumen becomes enriched with \({{\rm{H}}^ + }\) ions.
c) The primary electron acceptors, which are towards the outer side of the membrane, transfer their electrons to the \({\rm{H}}\)-carrier.
d) The carrier removes a proton from the matrix while transporting electrons to the inner side of the membrane.
e) Thus, protons are released to the lumen, while electrons are passed to the next carrier.
f) NADP-reductase enzyme is present on the outside of the thylakoid membrane. This receives electrons from PS-I and protons from the matrix. They are needed to reduce \({\rm{NAD}}{{\rm{P}}^{\rm{ + }}}\) to \({\rm{NAD}}{{\rm{P}}^{\rm{ + }}} + {{\rm{H}}^ + }.\)
g) Hence, protons in the stroma within the chloroplasts decrease in number, while the accumulation of protons takes place in the lumen. As a result, a proton gradient is created across the thylakoid membrane, which leads to a decrease in the pH at the site of the lumen.
h) The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the \({{\rm{F}}_0}\) portion of the ATPase enzyme that results in the release of energy in the form of ATP molecules.
i) This allows ATP synthase to synthesise several molecules of ADP from ADP and inorganic phosphate.
j) Thus, for chemiosmosis functioning, a membrane, a proton pump, a proton gradient and ATPase enzyme are required.
k) The ATP, thus produced, will be used simultaneously in the biosynthetic reaction or dark reaction in the stroma, responsible for the fixing of carbon dioxide and synthesis of sugar.

Fig: Chemiosmotic Hypothesis

Importance of Light Reaction of Photosynthesis

The importance of light reaction is as follows:

1. It is one of the most important phases of the photosynthetic reactions, as photosynthesis demands a huge requirement of light energy.

2. It is the preparatory phase for the formation of sugar that occurs inside the thylakoids, especially those of the grana region.

3. Its main function is to provide an energy source to the antenna pigment molecules for excitation.

4. It also helps in the production of assimilatory power consisting of reduced coenzyme \({\rm{NADPH}}.{{\rm{H}}^ + }\) and energy-rich ATP molecules.

5. It also helps in the photolysis of water in which the water molecule breaks up into hydrogen ions and oxygen in the illuminated chloroplast, releasing a pair of electrons.

Summary

Thus, light reaction of photosynthesis is defined as a series of chemical reactions that require light energy captured by the pigment present in the leaves (such as chlorophyll) to get converted into chemical energy in the form of ATP and NADPH. It is one of the most important phases of the photosynthetic reactions as photosynthesis demands a huge requirement of light energy and the energy source is the sun itself, which provides energy in terms of photons.

The primary function of light reaction is to supply an energy source to the antenna pigment molecules so that they get excited, and due to the excitation, a series of electron transport chains result in the formation of ATP and NADPH. Thus, light reaction of photosynthesis involves the absorption of light by the photosystems and then the formation of ATP and NADPH by the electron transport chain.

FAQs

Q.1. What is the major function of light reaction?
Ans: The major function of light reaction is to provide an energy source to the antenna pigment molecules so that they get excited, and due to the excitation, a series of electron transport chains will result in the formation of ATP.

Q.2. What is the light and dark reaction of photosynthesis?
Ans: The light reaction is the preparatory stage of the photosynthesis which provides light energy to the antenna pigment molecules for the production of ATP and \({\rm{NADPH}}.{{\rm{H}}^ + },\) and the dark reaction is the second step of photosynthesis which utilises the energy stored in ATP and \({\rm{NADPH}}. {{\rm{H}}^ + }\) to produce glucose.

Q.3. What are the four steps of light reaction?
Ans: The four steps of light reactions are as follows:
1. Absorption of light by PS-II
2. Formation of ATP
3. Absorption of light by PS-I
4. Formation of \({\rm{NADPH}}.{{\rm{H}}^ + }.\)

Q.4. What is light reaction also called?
Ans: Light reaction is also known as Photochemical Reaction.

Q.5. What is the Hill reaction also called?
Ans: The photolysis of water is also termed as Hill reaction.

Q.6. Where does the light reaction of photosynthesis take place?
Ans: Light reaction of photosynthesis takes place in the thylakoids, especially in the grana region.

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