Difference Between Light Reaction And Dark Reaction: Definition, Difference

Overview 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 Reactions (Light-Dependent Phase)

The details are given below:

Definition And Location

• The light reactions occur inside the thylakoid membranes of chloroplasts.

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

Key Steps

• 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.

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

Dark Reactions (Light-Independent Phase / Calvin Cycle)

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.

Key Steps

• 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.

Interdependence Of Light And Dark Reactions

Both light and dark reactions are interdependent on each other:

Difference Between Light And Dark Reactions

The difference between light and dark reactions is included in the table below:

Significance Of Both Reactions In Photosynthesis

The significance of both reactions is given below:

• Energy formation: Light reactions produce the energy carriers, ATP and NADPH.

• Glucose synthesis: Dark reactions synthesise glucose using the energy carriers produced in light reactions.

• 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.

Light vs Dark Reaction NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Key steps of Light and Dark Reactions

• Light vs Dark Reactions

Practice Questions for NEET

Q1. Dark reaction takes place in the

1. Thylakoids

2. Lumen of thylakoids

3. Stroma

4. Intermembrane space

Correct answer: 3) Stroma

Explanation:

The dark reactions are also termed the Calvin cycle, taking place within the stroma of the chloroplasts. In the dark reactions, carbon dioxide is fixed into organic molecules utilizing the ATP and NADPH, which are formed in the light reaction. The entire cycle has a series of enzymatic reactions that assimilate carbon dioxide to glucose to provide the plant energy and building blocks for the plant's growth and metabolism.

Hence, the correct answer is option 3) Stroma.

Q2. How many molecules of ATP and NADPH are required for every molecule of CO2 fixed in the Calvin cycle?

1. 2 molecules of ATP and 3 molecules of NADPH

2. 2 molecules of ATP and 2 molecules of NADPH

3. 3 molecules of ATP and 3 molecules of NADPH

4. 3 molecules of ATP and 2 molecules of NADPH

Correct answer: 4) 3 molecules of ATP and 2 molecules of NADPH

Explanation:

Three ATP molecules and two NADPH molecules must be added to the Calvin cycle to fix one CO₂ molecule. The different stages of the cycle depend on these chemicals. In the reduction phase, 3-phosphoglycerate is changed into 1,3-bisphosphoglycerate, and in the regeneration phase, ribulose-1,5-bisphosphate (RuBP) is produced using ATP.

Hence, the correct option is 4) 3 molecules of ATP and 2 molecules of NADPH.

Q3. The correct sequence of steps of dark reaction are

1. Regeneration - carboxylation - reduction

2. Reduction - oxidation - hydrogenation

3. Carboxylation - reduction - regeneration

4. Reduction - carboxylation - regeneration

Correct answer: 3) Carboxylation - reduction - regeneration

Explanation:

The Calvin cycle is an important series of reactions in photosynthesis, which is divided into three major stages:
Carboxylation: In the first step of this process, carbon dioxide, CO₂ is fixed into an organic molecule. Here, the action of the RuBisCO enzyme catalyses the reaction of CO₂ with the ribulose bisphosphate, RuBP, to give 3-PGA.
Reduction: This is the step where ATP and NADPH, produced during the light-dependent reactions, reduce 3-PGA to form glyceraldehyde-3-phosphate (G3P). It is the reduction of 3-PGA in which energy-rich molecules are produced.
Regeneration: In the final step, some G3P molecules are used to regenerate RuBP, enabling the cycle to continue. This regeneration requires ATP and ensures that the cycle can repeatedly fix CO₂.
These together enable plants to convert atmospheric CO₂ into organic compounds that can be used in growth and energy storage.

Hence, the correct answer is option 3) Carboxylation - reduction - regeneration.

Also Read:

• MCQs on Dark Reaction

• CAM Plants

• Cyclin and Noncyclic Photophosphorylation