Light Dependent Reaction – Easy Process & Explanation

Introduction:

Photosynthesis (photo–light; synthesis – to join) is the process through which green plants make use of energy from sunlight to make their own food. Green plants make use of light energy, and inorganic materials namely carbon dioxide and water to produce sugar and oxygen. In short, photosynthesis is the process through which chloroplasts of green plants synthesize glucose by making use of water and carbon dioxide in the presence of sunlight. During photosynthesis, water gets oxidized and carbon dioxide gets reduced to form carbohydrates. Photosynthesis forms the basis of all types of food chains and food webs. Photosynthesis produces starch and oxygen required for the survival of an organism. Therefore, photosynthesis supports all forms of life on earth.  

Photosynthesis involves two important processes: 

1. Light-dependent reaction (Light Reaction) 

2. Light-independent reaction (Dark Reaction / Calvin Cycle) 

Though the entire process of photosynthesis takes place in chloroplast, light and dark reactions occur at different sites. Light reaction takes place in grana and the dark reaction takes place in stroma regions of the chloroplast.  

An Overview of Light-Dependent Reaction

Light-dependent reaction is also known as the photochemical reaction or photolysis. Light reaction takes place at a faster rate than dark reaction. During light-dependent reactions, light energy gets converted into chemical energy. A light-dependent reaction occurs in the thylakoid region of the chloroplast. Thylakoids are disc-shaped membrane-bound compartments present inside chloroplasts. Thylakoids are stacked up to form grana. Thylakoids contain chlorophyll molecules. Chlorophyll pigments are green in color. These pigments absorb different wavelengths of light and convert them into chemical energy through photosynthesis. 

Light-dependent reaction is also known as a primary photochemical reaction or Hill’s reaction or Arnon’s cycle. As the light reaction is induced by sunlight, it is known as the primary photochemical reaction. In the year 1939, Robert Hill (1899 – 1991), a British plant biochemist ascertained the ‘Hill Reaction’. He proved that in the presence of sunlight, chloroplasts produce oxygen from water. In the year 1954, Daniel Arnon (1910 – 1994), a plant physiologist demonstrated the ‘Arnon’s cycle’.  He proved that chloroplasts carry out the complete photosynthesis process. He also demonstrated the photophosphorylation process that takes place in chloroplasts. (Formation of ATP from ADP and inorganic phosphate in the presence of light energy is known as photophosphorylation).  

Stages of Light-dependent Reaction: 

The light-dependent reaction involves four important stages. 

1. Absorption of light energy 

2. Splitting of water molecules 

3. Release of oxygen 

4. Formation of energy-carrying molecules – ATP and NADPH 

The light-dependent reaction makes use of sunlight to produce two molecules namely, ATP and NADPH. ATP (adenosine triphosphate) is the energy storage molecule. NADPH (nicotinamide adenine dinucleotide phosphate) is the reduced electron carrier. These two molecules are required for the dark reaction, the next stage of photosynthesis. Complexes of light-absorbing pigments and protein molecules are known as photosystems. Photosystems play an important role in light reactions. 

Photosystems: 

Among all photosynthetic pigments, chlorophyll-a is the most essential pigment that traps solar energy. All autotrophic plants except photosynthetic bacteria are found to have chlorophyll-a pigment. Chlorophyll – a is also known as reaction center. Other types of chlorophyll pigments and carotenoids are collectively referred to as accessory pigments or harvesting centers.  The primary role of accessory pigments is to absorb light energy and transfer it to chlorophyll – a pigment for photosynthesis. Chlorophyll-a (reaction center) and accessory pigments are packed together into functional units known as photosystems. Within photosystems, pigments are organized into distinct light-harvesting complexes (LHC). LHCs are composed of hundreds of pigment molecules that are bound to proteins. 

Photosystems are classified into two different types:  

1. Photosystem I (PSI) 

2. Photosystem II (PSII) 

Each photosystem is composed of around 250 – 400 chlorophyll-a molecules. 

In PS I maximum absorption of light energy occurs at a wavelength of 700 nm. This reaction center with chlorophyll-a is referred to as P700. In PS II maximum absorption of light energy occurs at a wavelength of 680 nm. The reaction center of PS II with chlorophyll-a is referred to as P680.  

Role of Photosystems in Light-dependent Reaction: 

Photosystems play an important role in light reactions. Small packages or particles of energy present in light rays are known as ‘photons. The energy carried by a single photon is known as ‘quantum’.  When this energy is absorbed by chlorophyll molecules, they get energized and move to an excited state thereby losing an electron to the outer orbit. This excited state is unstable and therefore excited electrons lose excess energy to return back to the ground state. This excitation energy is utilized for the process of photophosphorylation. (Photo – light energy; phosphorylation – production of ATP from ADP). 

In the thylakoid membrane, PSI is located in the unstacked region of grana facing chloroplast stroma. Whereas, PSII is located in the stacked region of grana facing the thylakoid lumen (Fig. No. 10). Pigment molecules in photosystems are arranged in such a way that they act as an energy funnel that passes energy to the reaction center. When a pigment in a photosystem absorbs light energy, it gets excited. The energy is then transferred to neighboring pigments through direct electromagnetic interactions. This process is known as resonance energy transfer (Fig. No. 8). This process of energy transfer is repeated multiple times. When energy reaches a special pair of chlorophyll-a molecules present in the reaction center, energy is no longer passed to other pigments. Instead, the chlorophyll-a special pair gets excited and loses an electron to the primary electron acceptor (Fig. No. 9).  Thus, the transferred electron enters into the electron transport chain. 

Process of Light-dependent Reaction: 

The light-dependent reaction is the first stage of photosynthesis. During light reactions, solar energy gets converted into chemical energy and is stored in the form of energy-carrying molecules – ATP and NADPH. During light reactions, water is split up to release oxygen. The light reaction involves two types of phosphorylation reactions,  

1. Non-cyclic phosphorylation 

2. Cyclic phosphorylation 

Non-cyclic phosphorylation is also known as Z- scheme. In non-cyclic phosphorylation, electrons from chlorophyll travel through a non–circular path and do not return back to chlorophyll.  Electrons are utilized for the formation of ATP and NADPH molecules. 

In cyclic phosphorylation, an electron from chlorophyll flows through a circular path and returns back to the chlorophyll. Cyclic phosphorylation results in the formation of ATP molecules alone. Whereas, NADPH is not synthesized. 

The basic steps of light-dependent reaction are as follows. 

• Light absorption in PSII: 

When a pigment in PSII absorbs light energy, it gets excited. The energy is then transferred to neighboring pigments multiple times through the process of resonance energy transfer. After the energy reaches the reaction center, it is transferred to P680. The electron is then transferred to an electron acceptor. The energy produced is used to split water and release oxygen. 

• Synthesis of ATP: 

High–energy electron loses its energy as it travels through the electron transport chain. The released energy aids the pumping of hydrogen ions from chloroplast stroma to thylakoid lumen thereby building a concentration gradient. Hydrogen ions produced as a result of the splitting of water also get added up to the gradient. Hydrogen ions then flow down their gradient from the lumen to the stroma. During this process, they pass through ATP synthase and aid the production of ATP. This process is called chemiosmosis. 

• Light absorption in PSI: 

In PSI, pigments absorb light energy and transfer it to the reaction center. When the energy reaches a special pair of chlorophylls in P700, it is transferred to an acceptor molecule. This missing electron is then replaced with a new electron transferred through the electron transport chain from PSII.  

• Synthesis of NADPH: 

As high-energy electron travels through the electron transport chain, it loses its energy. This energy is utilized for the production of NADPH. 

Thus, light reaction results in the conversion of light energy into chemical energy through the production of ATP and NADPH. The light reaction also results in the splitting of the water molecule and the release of oxygen as a by-product. ATP and NADPH produced through light reactions are utilised for the dark reaction / Calvin cycle, the second stage of photosynthesis.  

Summary

• Photosynthesis is the process through which chloroplasts of green plants synthesize glucose by making use of water and carbon dioxide in the presence of sunlight

■ During photosynthesis, water gets oxidized and carbon dioxide gets reduced to form carbohydrates.

• Photosynthesis involves two important processes – light reaction and dark reaction

• Light reaction takes place in grana and dark reaction takes place in stroma regions of chloroplast

 • Light-dependent reaction is also known as primary photochemical reaction Hill’s reaction or Arnon’s cycle.

• Light-dependent reaction occurs in the thylakoid region of chloroplast

• Light-dependent reaction involves four important stages – absorption of light energy, splitting of water molecules, release of oxygen, and formation of energy-carrying molecules – ATP and NADPH.

■ Chlorophyll-a (reaction center) and accessory pigments are packed together into functional units known as photosystems.

■ Within photosystems pigments are organised into distinct light-harvesting complexes (LHC).

■ LHCs are composed of hundreds of pigment molecules that are bound to proteins.

■ Photosystems are classified into two different types – Photosystem I (PSI), and Photosystem II (PSII).

■ Maximum absorption of light energy occurs at a wavelength of 700 nm and 680 nm in PSI and PSI! respectively. The reaction centers of PSI and PSII are P700 and Pox) respectively.

• Photosystems play an important role in light reactions.

■ Photosynthetic pigments absorb light energy and aid the process of photophosphorylation. (Photo – light energy; phosphorylation – production of ATP from ADP).

• Energy transfer in photosystems occurs through the process of resonance energy transfer.

■ Light reaction involves two types of phosphorylation reactions – cyclic and non-cyclic phosphorylation.

■ Cyclic phosphorylation results in the formation of ATP molecules alone.

■ Non-cyclic phosphorylation results in the formation of both NADPH and ATP molecules.

■ Light reaction results in the conversion of light energy into chemical energy through the production of ATP and NADPH.

■ Light reaction also results in the splitting of water molecules and release of oxygen as a by-product

■ ATP and NADPH produced through light reactions are utilised for the dark reaction / Calvin cycle, the second stage of photosynthesis.