Light Dependent Reactions: Definition, Examples, Diagram, Process, Explanation

What Are Light-Dependent Reactions?

Light-dependent reactions are the first step of photosynthesis, capturing light energy and then transforming it into chemical energy in the shape of ATP and NADPH. Light-dependent reactions take place in the thylakoid membranes of the chloroplast.

Chloroplasts are plant cell and algae organelles specialised to perform the process of photosynthesis. An embedded thylakoid is stacked into grana, and around it—the stroma—which is the fluid surrounding the thylakoids.

The main constituents involved in light-dependent reactions are found in thylakoid membranes: photosystems, electron transport chains, and the enzyme ATP synthase. These membranes create a structure where chemical processes can be run efficiently.

Components In Light-Dependent Reactions

The components of light-dependent reactions are:

Light And Pigments

• Light: Provides the energy to fuel the reactions.

• Pigment: A molecule which absorbs light at a specific wavelength.

Photosystems I And II

• Photosystems I and II are protein-pigment complexes involved in the light-dependent reactions of photosynthesis.

• PSII captures the light energy and passes it on to initiate an electron transport chain, whereas PSI receives electrons and boosts their energy to be transferred for the generation of NADPH.

Role Of Chlorophyll

• Chlorophyll is the main pigment in photosystems, and it absorbs light energy, especially in the blue and red wavelengths.

• It can excite electrons and hence transfer the light energy into chemical energy.

• Other auxiliary pigments extend the range of absorption of light and hence protect chlorophyll from damage while enhancing the efficiency of photosynthesis.

Steps Of Light-Dependent Reactions

Following are the steps of the light-dependent reactions:

Light Absorption

• A photon is absorbed by chlorophyll.

• Excitement of electrons in the photosystems.

Splitting Of Water (Photolysis)

• The light energy of PSII causes water molecules to split, producing oxygen and protons with electrons.

Electron Transport Chain (ETC)

• The energy released from high-energy electron transport across the ETC pumps protons into the thylakoid lumen, developing a proton gradient.

Photophosphorylation Generation of ATP

• Protons driven by the gradient drive the ATP synthase to convert ADP and inorganic phosphate into ATP.

Reduction Of NADP+ To NADPH

• Electrons are passed on to reduce NADP+ into NADPH. Another essential energy carrier.

Detailed Mechanisms

The detailed mechanism is explained below:

Absorption Of Photon And Excitation Of Electron

• Photons excite electrons in chlorophyll.

• This electron is then transferred to primary electron acceptors.

Role Of Water And Oxygen Evolution

• Water molecules are split in PSII, donating electrons to replace those lost by chlorophyll and releasing oxygen in the process.

The Proton Gradient And ATP Synthase

• Protons diffuse inside the thylakoid lumen, generating a gradient.

• Utilising this gradient, ATP synthase synthesises ATP from ADP and inorganic phosphate.

Formation Of NADPH

• Electrons arrive at PSI, get re-energised, and finally reduce NADP+ into NADPH needed for the Calvin cycle.

Products And Their Importance

The products in light-dependent reactions are:

ATP And Its Role In Cellular Processes

• The energy source for cellular processes.

• They perform endergonic reactions in the cell.

NADPH And Its Role In The Calvin Cycle

• NADPH serves as reducing power for the Calvin cycle; that is, it facilitates the conversion of CO2 to glucose.

Oxygen Release And Ecological Impact

• The by-product of light-dependent reactions is oxygen, which is then released into the atmosphere.

• It is necessary for the respiration of most living organisms; this balances life on Earth.

Factors Affecting Light-Dependent Reactions

The factors affecting light-dependent reactions are:

Light Intensity

• Higher intensity elevates the rate of light-dependent reactions.

• Too much light may result in photoinhibition.

Wavelength Of Light

• Wavelengths of red and blue light are the most effective.

• Greenlight has the lowest effect due to its reflection by chlorophyll.

Water Availability

• It is essential for photolysis.

• Limited water can slow down or stop the reactions.

Temperature

• Optimal temperatures enhance enzyme activities.

• Extreme temperatures can denature enzymes.

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