Physiological Metabolism – Mechanism of Electron Transport

Mechanism of Electron Transport

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. The light reaction takes place in the grana, and the dark reaction takes place in the stroma regions of the chloroplast.

Light-Dependent Reaction

Light dependent reaction is also known as the photochemical reaction or photolysis. The light reaction takes place at a faster rate than the dark reaction. During light-dependent reaction, light energy gets converted into chemical energy. The light-dependent reaction occurs in the thylakoid region of the chloroplast.

(Note: 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 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

Light-dependent reaction makes use of sunlight to produce two molecules, namely, ATP and NADPH. 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.

Electron Transport Chain

Electron transport chain (ETC) of the light reaction occurs in the thylakoid membrane of the chloroplasts. ETC refers to a series of reactions that involves a flow of electrons between electron donors and electron acceptors.

A group of protein molecules is involved in ETC. ETC of light reaction results in the formation of ATP and NADPH. Photosystems play an important role in the ETC of light reactions.

Light energy is the driving force of ETC. 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).

Energy Transfer in Photosystems

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). LHC 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.

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.

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 the 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. This process of energy transfer is repeated multiple times.

When the 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.  Thus, the transferred electron enters the electron transport chain.

Steps Involved in ETC

The basic steps of ETC 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 and free electrons (Photolysis). Free electrons produced during photolysis replace electrons lost by PSII.

Molecules involved in electron transport include plastoquinone (Pq), cytochrome complex, and a copper-containing protein called plastocyanin (Pc).

• 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 splitting 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 the 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 a high-energy electron travels through the electron transport chain, it loses its energy. This energy is utilized for the production of NADPH. Ferredoxin (Fd) and NADP reductase are involved in the synthesis of NADPH.

Process of Electron Flow

Electron flow through ETC of light reaction occurs through two types of photophosphorylation reactions,

1. Non-cyclic photophosphorylation
2. Cyclic photophosphorylation

Non-cyclic photophosphorylation is also known as Z-scheme. In non-cyclic photophosphorylation, 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 photophosphorylation, the electron from chlorophyll flows through a circular path and returns back to the chlorophyll.

Cyclic photophosphorylation results in the formation of ATP molecules alone. Whereas NADPH is not synthesized.