4.1.7 Arrangements of Pigments (photosystems)

Introduction

For efficient absorption and utilization of light energy, the photosynthetic pigments are embedded on the surface of thylakoid membranes where they are arranged in the form of clusters. These clusters of pigment molecules are called photosystems. The arrangement of pigments into photosystems allows the plant to capture light energy efficiently and convert it into chemical energy.

Definition

Photosystems: These clusters of pigment molecules are called photosystems.

In simple words: Photosystems are groups of pigment molecules arranged in a specific way in the thylakoid membrane to capture light energy.

Core Concept

• Photosynthetic pigments are embedded on the surface of thylakoid membranes.
• They are arranged in the form of clusters.
• These clusters are called photosystems.
• The peripheral part of photosystem is called antenna complex.
• The central part of photosystem is called reaction centre.
• There are two distinct photosystem types:

Detailed Explanation

Structure of a Photosystem

A photosystem consists of two main parts:

• Antenna Complex (Peripheral Part)
• Reaction Centre (Central Part)

Antenna Complex

• The peripheral part of photosystem.
• Consists of accessory pigments such as:
• Chlorophyll-b
• Carotenoids
• These pigments absorb light of various wavelengths.
• They pass on their gained energy to chlorophyll-a of the reaction centre.

Reaction Centre

• The central part of photosystem.
• Contains only chlorophyll-a and associated proteins.
• Since chlorophyll-a generally has an optimal absorption wavelength of 660 nm, it associates with different proteins in each type of photosystem to slightly shift its optimal wavelength.
• This produces two distinct photosystem types:
• Photosystem-I (PS-I)
• Photosystem-II (PS-II)

Types of Photosystems

Photosystem-I (PS-I)

• The chlorophyll-a in the reaction centre of PS-I can absorb maximum 700 nm wavelength of light.
• Hence called P700.
• P700 is the reaction centre of Photosystem-I.

Photosystem-II (PS-II)

• The chlorophyll-a in the reaction centre of PS-II can absorb maximum 680 nm wavelength of light.
• Hence called P680.
• P680 is the reaction centre of Photosystem-II.

• The photosystems are named for the order in which they were discovered.
• They are not named for the order in which they occur in the thylakoid membrane.

Arrangement of Photosystems in Thylakoid Membrane

• Photosystems are embedded in the thylakoid membrane.
• They are arranged in a specific order:
• PS-II comes first in the electron transport chain.
• PS-I comes second in the electron transport chain.
• This is despite the fact that PS-I was discovered first.

Function of Photosystems

• As chlorophyll-a can only absorb light of a narrow wavelength, it works with the pigments of the antenna complex to gain energy from a larger part of the spectrum.
• The pigments absorb light of various wavelengths and pass on their gained energy to chlorophyll-a of the reaction centre.
• When the energy reaches the chlorophyll-a, its electrons become so excited due to high energy that they escape.
• These electrons are accepted by the first molecule of a nearby electron transport chain which is even higher energy and larger than chlorophyll molecule.
• In this way, chlorophyll molecule becomes oxidized.

Equation: Chlorophyll (Reduced form) Light energy→ Chlorophyll (Oxidized form) + e^-

Electron Transport System

• The electron transport system plays an important role in the generation of ATP by the conversion of light energy into chemical energy.
• The excited electrons pass through a series of electron carriers.
• This leads to the production of ATP and NADPH.

Summary Table of Photosystems

Intermediate Level

Structure of Photosystem in Detail

Types of Photosystems in Detail

Function of Antenna Complex

• Absorbs light of various wavelengths.
• Passes energy to the reaction centre.
• Contains accessory pigments:
• Chlorophyll-b
• Carotenoids
• Extends the range of light absorption.

Function of Reaction Centre

• Contains chlorophyll-a.
• Absorbs light at specific wavelengths (700 nm or 680 nm).
• Excites electrons.
• Passes electrons to the electron transport chain.
• Chlorophyll becomes oxidized.

Electron Transport Chain

Naming of Photosystems

• Photosystem-I was discovered first.
• Photosystem-II was discovered second.
• They are named by discovery order, not by order in ETC.
• In the ETC, PS-II comes first, then PS-I.

Important Points

• 1. For efficient absorption and utilization of light energy, the photosynthetic pigments are embedded on the surface of thylakoid membranes where they are arranged in the form of clusters.
• 2. These clusters of pigment molecules are called photosystems.
• 3. The peripheral part of photosystem is called antenna complex which consists of accessory pigments such as chlorophyll-b and carotenoids.
• 4. The central part of photosystem is called reaction centre which contains only chlorophyll-a and associated proteins.
• 5. Since chlorophyll-a generally has an optimal absorption wavelength of 660 nm, it associates with different proteins in each type of photosystem to slightly shift its optimal wavelength, producing two distinct photosystem types:
• - Photosystem-I (PS-I)
• - Photosystem-II (PS-II)
• 6. The chlorophyll-a in the reaction centre of PS-I can absorb maximum 700 nm wavelength of light, hence called P700.
• 7. Similarly, the chlorophyll-a in the reaction centre of PS-II can absorb maximum 680 nm wavelength of light, hence called P680.
• 8. The photosystems are named for the order in which they were discovered and not for the order in which they occur in the thylakoid membrane.
• 9. As chlorophyll-a can only absorb light of a narrow wavelength, it works with the pigments of antenna complex to gain energy from a larger part of the spectrum.
• 10. The pigments absorb light of various wavelengths and pass on their gained energy to chlorophyll-a of the reaction centre.
• 11. When the energy reaches the chlorophyll-a, its electrons become so excited due to high energy that they escape and are accepted by first molecule of a nearby electron transport chain which is even high energy and larger than chlorophyll molecule.
• 12. In this way chlorophyll molecule becomes oxidized.
• 13. The electron transport system plays an important role in generation of ATP by the conversion of light energy into chemical energy.

Remember

• - Photosystems = clusters of pigment molecules
• - Antenna complex = accessory pigments (chlorophyll-b, carotenoids)
• - Reaction centre = chlorophyll-a + proteins
• - PS-I = P700; absorbs at 700 nm; discovered first
• - PS-II = P680; absorbs at 680 nm; discovered second
• - PS-II comes first in the electron transport chain
• - PS-I comes second in the electron transport chain
• - Photosystems are named by discovery order, not by order in ETC

Common Mistakes

• 1. **Confusing PS-I and PS-II** – PS-I = P700; PS-II = P680.
• 2. Thinking PS-I comes first in the ETC – PS-II comes first; PS-I comes second.
• 3. Confusing the discovery order with the ETC order – PS-I was discovered first, but PS-II comes first in the ETC.
• 4. Thinking the reaction centre contains all pigments – The reaction centre contains only chlorophyll-a.
• 5. Forgetting the role of antenna complex – Antenna complex contains accessory pigments and extends the range of light absorption.
• 6. Confusing the absorption maxima – P700 = 700 nm; P680 = 680 nm.
• 7. Thinking both photosystems produce oxygen – Only PS-II produces oxygen (from water).
• 8. Thinking both photosystems produce NADPH – Only PS-I produces NADPH (in non-cyclic photophosphorylation).
• 9. Forgetting the role of electron transport chain – ETC produces ATP.
• 10. Thinking the photosystems are named by their order in ETC – They are named by discovery order.
• 11. Confusing the primary electron acceptors – PS-I = ferredoxin; PS-II = plastoquinone.
• 12. Thinking the photosystems are independent – They work together in the electron transport chain.

Quick Revision

• For efficient absorption and utilization of light energy, the photosynthetic pigments are embedded on the surface of thylakoid membranes where they are arranged in the form of clusters.
• These clusters of pigment molecules are called photosystems.
• The peripheral part of photosystem is called antenna complex which consists of accessory pigments such as chlorophyll-b and carotenoids.
• The central part of photosystem is called reaction centre which contains only chlorophyll-a and associated proteins.
• Since chlorophyll-a generally has an optimal absorption wavelength of 660nm it associates with different proteins in each type of photosystem to slightly shift its optimal wavelength, producing two distinct photosystem types i.e., photosystem-I (PS-I) and photosystem-II (PS-II).
• The chlorophyll-a in the reaction centre of PS-I can absorb maximum 700nm wavelength of light, hence called P700.
• Similarly, the chlorophyll-a in the reaction centre of PS-II can absorb maximum 680nm wavelength of light, hence called P680.
• The photosystems are named for the order in which they were discovered and not for the order in which they occur in the thylakoid membrane.
• As chlorophyll-a can only absorb light of a narrow wavelength, it works with the pigments of antenna complex to gain energy from a larger part of the spectrum.
• The pigments absorb light of various wavelengths and pass on their gained energy to chlorophyll-a of the reaction centre.
• When the energy reaches the chlorophyll-a, its electrons become so excited due to high energy that they escape and is accepted by first molecule of a nearby electron transport chain which is even high energy and larger than chlorophyll molecule.
• In this way chlorophyll molecule becomes oxidized.
• Chlorophyll (Reduced form) → (Light energy) → Chlorophyll (Oxidized form) + e⁻
• The electron transport system plays an important role in generation of ATP by the conversion of light energy into chemical energy.

60-Second Revision

• Photosystems = clusters of pigment molecules
• Antenna complex = accessory pigments (chlorophyll-b, carotenoids)
• Reaction centre = chlorophyll-a + proteins
• PS-I = P700; absorbs at 700 nm; discovered first
• PS-II = P680; absorbs at 680 nm; discovered second
• PS-II comes first in the electron transport chain
• PS-I comes second in the electron transport chain
• Photosystems are named by discovery order, not by order in ETC
• Warning: PS-I and PS-II are not named by their order in the ETC. PS-II comes first in the ETC, but PS-I was discovered first. The reaction centre contains only chlorophyll-a. The antenna complex contains accessory pigments. P700 = PS-I; P680 = PS-II. Only PS-II produces oxygen. Only PS-I produces NADPH in non-cyclic photophosphorylation.