Difference Between Light Reaction And Dark Reaction: Definition, Difference

The Basics Of Photosynthesis

Photosynthesis is a biological process by which plants, algae, and some bacteria convert light energy from the sun into chemical energy in the form of glucose. This process is driven by the absorption of light by chlorophyll, the splitting of water molecules, and the fixation of carbon dioxide to form organic molecules.

The net chemical equation for photosynthesis is:

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

This entire process happens in the chloroplasts of plant cells. There, the chlorophyll pigment captures the light energy needed to drive the whole mechanism.

Light Reaction

The details are given below:

Definition And Location

• Happens in Thylakoid Membranes: The light reactions occur inside the thylakoid membranes of chloroplasts.

• Role of Chlorophyll: Chlorophyll absorbs light energy, which will later be transposed into the driving of photochemical reactions.

Key Processes

• Photolysis of Water: The water molecule undergoes splitting into oxygen, protons, and electrons.

• Product formation: Light energy gets converted into chemical energy in the form of ATP and NADPH.

• Photosystems I and II: These are the two complexes involved in capturing the light energy and yielding a chemical energy product.

Steps Involved

• Absorption of Light: Absorbance of photons by chlorophyll excites the electron to higher energy levels.

• Water Splitting: Water is split to replace the excited electrons, releasing oxygen.

• Electron Transport Chain: Electron flow along the transport chain liberates energy to pump protons and create a proton gradient.

• Formation of ATP (Photophosphorylation): The gradient of protons drives the activity of ATP synthase to make ATP.

Dark Reaction

The details are given below:

Definition And Location

• The dark reactions, also known as the Calvin cycle, occur in the stroma of the chloroplast.

• RuBisCO enzyme is responsible for catalysing carbon dioxide fixation.

Important Processes

• Carbon Fixation: CO₂ is fixed into an organic molecule.

• Reduction Phase: 3-phosphoglycerate is reduced to G3P using ATP and NADPH.

• Regeneration of RuBP: RuBP is regenerated to allow the continuation of the cycle.

Steps Involved

• Role of RuBisCO: Catalyses carbon fixation, the first step of the process.

• Formation of G3P: G3P is produced at the reduction phase.

• ATP and NADPH Utilisation: They provide energy and reduce power respectively.

Table: Differences Between Light And Dark Reactions

Significance Of Both Reactions In Photosynthesis

The significance of both reactions is given below:

Interdependence Of Light And Dark Reactions

• Light Reactions Provide ATP and NADPH: Used in the Calvin cycle.

• Dark Reactions are Dependent on Light Reactions: They consume the products to fix carbon and produce glucose.

Contribution To Glucose Formation

• Light Reactions Produce the Energy Carriers: ATP and NADPH.

• Dark Reactions Synthesise Glucose: Using the energy carriers produced in light reactions.

Importance In Plant Growth And Survival

• Energy Production: ATP and glucose are vital in plant metabolism.

• Growth and Development: Glucose provides the carbon skeletons for other organic molecules necessary for growth.

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