Chlorophyll ‘a’ is the primary pigment for light reaction. What are accessory pigments? What is their role in photosynthesis?

Pigments which assist chlorophyll in trapping solar radiation are called accessory pigments. Chlorophyll b, xanthophyll and carotenoids are the accessory pigments. They help in trapping solar energy and pass it onto chlorophyll a. Thus, accessory pigment play a supportive role in light harvesting.

Look at leaves of the plant on the shady side and compare it with the leaves on the sunny side. Or compare the potted plants kept in the sunlight with those in the shade. Which of them has leaves that are darker green? Why?

The leaves of the shaded side are darker green than those kept in sunlight due to two reasons: (i) The chloroplasts occur mostly in the mesophyll cells along their walls for receiving optimum quantity of incident light. (ii) The chloroplasts align themselves in vertical position along the lateral walls in high light intensity and along tangential walls in moderate light

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Mechanism Of Photosynthesis

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Mechanism of Photosynthesis

1.0There are two types of reactions in photosynthesis

1. Light reaction

2. Dark reaction

Photosynthesis is a redox reaction during which oxidation of H₂O occurs (as it provides H⁺ and e^–) during light reaction and reduction of CO₂ occurs (as it accepts H⁺ & e^–) during dark reaction (biosynthetic phase).

Photosynthesis is an anabolic (synthesising) & endergonic (Energy absorbing) process.

2.0What is Light Reaction?

Light reactions or the 'Photochemical' phase include light absorption, water splitting, oxygen release and formation of high energy chemical intermediates like ATP and NADPH.

Several protein complexes are involved in the process. The groups of photosynthetic pigments in thylakoid membranes are known as photosystems. The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within the Photosystem I (PS I) and Photosystem II(PS II) . These are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction. The LHC are made up of hundreds of pigment molecules bound to proteins. Each photosystem has all the pigments (except one molecule of chlorophyll a) forming a light harvesting system also called antennae or LHC. These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light. The single chlorophyll a molecule forms the reaction centre. The reaction centre is different in both the photosystems. In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700 while in PS II it has absorption maxima at 680 nm and is called P680.

Photosystem = Reaction centre + LHC

In every photosystem, there is a reaction centre (molecule of chlorophyll –a) surrounded by accessory pigments. The PS II is located in the appressed region of granal thylakoids and PS I in non-appressed region of grana and in stroma thylakoids. (In the other way, we can say that granal thylakoids have both PS I and PS II whereas stroma thylakoids have only PS I).

Photosystem 1 and 2 explained in detailed way using diagram

3.0Difference Between PS-I & PS-II

PS-I		PS-II
(1)	Located at non-appressed parts of grana thylakoid and stroma lamellae.	(1)	Usually located at appressed parts of grana thylakoids.
(2)	Its reaction center is P₇₀₀.	(2)	Its reaction center is P₆₈₀.
(3)	It has reducing nature (reduce NADP).	(3)	It has oxidizing nature (oxidise H₂O).
(4)	It participates in both cyclic and non-cyclic photophosphorylation.	(4)	It participates only in non-cyclic hotophosphorylation.

4.0Also Read

Respiration & Its Type	Locomotion and Movement	Skeletal System
Anaerobic & Aerobic Respiration	Muscle	Appendicular Skeleton
Electron Transport System (ETS) & Oxidative Phosphorylation	Structure of Contractile Protein	Factors Affecting Photosynthesis

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Mechanism of Photosynthesis

1.0There are two types of reactions in photosynthesis

1. Light reaction

2. Dark reaction

Photosynthesis is a redox reaction during which oxidation of H₂O occurs (as it provides H⁺ and e^–) during light reaction and reduction of CO₂ occurs (as it accepts H⁺ & e^–) during dark reaction (biosynthetic phase).

Photosynthesis is an anabolic (synthesising) & endergonic (Energy absorbing) process.

2.0What is Light Reaction?

Light reactions or the 'Photochemical' phase include light absorption, water splitting, oxygen release and formation of high energy chemical intermediates like ATP and NADPH.

Several protein complexes are involved in the process. The groups of photosynthetic pigments in thylakoid membranes are known as photosystems. The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within the Photosystem I (PS I) and Photosystem II(PS II) . These are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction. The LHC are made up of hundreds of pigment molecules bound to proteins. Each photosystem has all the pigments (except one molecule of chlorophyll a) forming a light harvesting system also called antennae or LHC. These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light. The single chlorophyll a molecule forms the reaction centre. The reaction centre is different in both the photosystems. In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700 while in PS II it has absorption maxima at 680 nm and is called P680.

Photosystem = Reaction centre + LHC

In every photosystem, there is a reaction centre (molecule of chlorophyll –a) surrounded by accessory pigments. The PS II is located in the appressed region of granal thylakoids and PS I in non-appressed region of grana and in stroma thylakoids. (In the other way, we can say that granal thylakoids have both PS I and PS II whereas stroma thylakoids have only PS I).

3.0Difference Between PS-I & PS-II

PS-I		PS-II
(1)	Located at non-appressed parts of grana thylakoid and stroma lamellae.	(1)	Usually located at appressed parts of grana thylakoids.
(2)	Its reaction center is P₇₀₀.	(2)	Its reaction center is P₆₈₀.
(3)	It has reducing nature (reduce NADP).	(3)	It has oxidizing nature (oxidise H₂O).
(4)	It participates in both cyclic and non-cyclic photophosphorylation.	(4)	It participates only in non-cyclic hotophosphorylation.

4.0Also Read

Respiration & Its Type	Locomotion and Movement	Skeletal System
Anaerobic & Aerobic Respiration	Muscle	Appendicular Skeleton
Electron Transport System (ETS) & Oxidative Phosphorylation	Structure of Contractile Protein	Factors Affecting Photosynthesis

Frequently Asked Questions

Chlorophyll ‘a’ is the primary pigment for light reaction. What are accessory pigments? What is their role in photosynthesis?

Look at leaves of the plant on the shady side and compare it with the leaves on the sunny side. Or compare the potted plants kept in the sunlight with those in the shade. Which of them has leaves that are darker green? Why?

Frequently Asked Questions

Chlorophyll ‘a’ is the primary pigment for light reaction. What are accessory pigments? What is their role in photosynthesis?

Look at leaves of the plant on the shady side and compare it with the leaves on the sunny side. Or compare the potted plants kept in the sunlight with those in the shade. Which of them has leaves that are darker green? Why?

Join ALLEN!

Mechanism of Photosynthesis

1.0There are two types of reactions in photosynthesis

2.0What is Light Reaction?

3.0Difference Between PS-I & PS-II

4.0Also Read

Table of Contents

Join ALLEN!

Mechanism of Photosynthesis

1.0There are two types of reactions in photosynthesis

2.0What is Light Reaction?

3.0Difference Between PS-I & PS-II

4.0Also Read

Table of Contents