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Difference Between Light Reaction and Dark Reaction

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Last updated date: 09th Apr 2024
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Understanding Light Reaction and Dark Reaction in Photosynthesis

Photosynthesis is the fundamental process through which plants capture light energy and convert it into glucose, a form of chemical energy. The process occurs in specialised organelles called chloroplasts, which contain pigments like chlorophyll. Photosynthesis is divided into two distinct stages: the light reaction and the dark reaction, also known as the Calvin cycle. These stages work together to harness light energy, produce ATP (adenosine triphosphate), and synthesize glucose.

Light Reaction: Harnessing Light Energy

Definition and Function: 

The light reaction is the initial stage of photosynthesis, occurring in the thylakoid membranes of the chloroplasts. It involves capturing light energy and converting it into chemical energy in the form of ATP and NADPH. During this process, water molecules are split, releasing oxygen as a byproduct.


Characteristics of Light Reaction:

  1. Occurs in the thylakoid membranes of chloroplasts.

  2. Requires sunlight as a source of energy.

  3. Involves the absorption of light by chlorophyll pigments.

  4. Generates ATP and NADPH, which are used in the dark reaction.


Dark Reaction: Fixing Carbon and Producing Glucose

Definition and Function:

The dark reaction, or Calvin cycle, is the second stage of photosynthesis that takes place in the stroma of the chloroplasts. It utilizes the ATP and NADPH produced during the light reaction to convert carbon dioxide into glucose. This process is also crucial for the regeneration of the starting molecule, RuBP (ribulose-1,5-bisphosphate).


Characteristics of Dark Reaction:

  1. Occurs in the stroma of chloroplasts.

  2. Does not require direct sunlight.

  3. Utilizes ATP and NADPH produced during the light reaction.

  4. Fixes carbon dioxide into organic compounds, particularly glucose.


Phases of Photosynthesis:

  1. Light Reaction:

    • Light Absorption: Chlorophyll pigments in the thylakoid membranes absorb light energy.

    • Electron Transport Chain: Excited electrons from chlorophyll are passed along a series of proteins, creating an electron transport chain.

    • Chemiosmosis and ATP Synthesis: The flow of electrons generates a proton gradient across the thylakoid membrane, driving the synthesis of ATP through chemiosmosis.

    • Production of NADPH and Oxygen: Water molecules are split, releasing electrons, protons, and oxygen. The electrons are used to generate NADPH, a high-energy electron carrier.


  1. Dark Reaction (Calvin Cycle):


  • Carbon Fixation: Carbon dioxide molecules from the atmosphere combine with an organic molecule called RuBP to form an unstable compound.

  • Reduction and Carbohydrate Production: ATP and NADPH from the light reaction provide the energy and electrons needed to convert the unstable compound into more stable organic molecules, including glucose.

  • Regeneration of RuBP: Some organic molecules produced in the cycle are used to regenerate the initial molecule, RuBP, ensuring the continuity of the cycle.


These phases work together to convert light energy into chemical energy stored in glucose, which is essential for the growth and development of plants.


Light Reaction Vs. Dark Reaction: Exploring The Key Differences

The below table describes the difference between light reaction and dark reaction in detail:

S.No.

Light Reaction

Dark Reaction


Occurs in the thylakoid membrane

Occurs in the stroma of chloroplasts

  1.  

Requires light energy

Does not directly require light


Converts light energy into chemical energy (ATP and NADPH)

Utilizes ATP and NADPH produced in the light reaction to fix carbon dioxide and produce glucose


Produces oxygen as a byproduct

Does not produce oxygen


Involves electron transport chain and chemiosmosis

Involves the Calvin cycle (carbon fixation, reduction, and regeneration)

6.

Takes place during the day when light is available

Can occur both during the day and night, as long as ATP and NADPH are available

7.

Provides energy for the dark reaction

Utilizes the products of the light reaction for carbohydrate synthesis


Closing the Loop: A Conclusion on Light Reaction and Dark Reaction

Photosynthesis is a vital process for the sustenance of life on Earth. The light reaction and dark reaction work in harmony to convert light energy into chemical energy and produce glucose. The light reaction captures light energy, generates ATP and NADPH, and releases oxygen. In contrast, the dark reaction utilizes ATP and NADPH to fix carbon dioxide and synthesize glucose. Together, these two processes demonstrate the remarkable ability of plants to convert sunlight into usable energy, supporting the growth.

FAQs on Difference Between Light Reaction and Dark Reaction

1. What does it mean when you say "mild reaction"?

The term "mild reaction" is likely a misunderstanding or misinterpretation in the context of photosynthesis. The correct term is "light reaction," which occurs in the thylakoid membranes of chloroplasts. During the light reaction, light energy is absorbed and used to produce ATP and NADPH, which are important energy carriers. These molecules are then utilized in the dark reaction (Calvin cycle) to convert carbon dioxide into glucose. Thus, it is crucial to use the accurate term "light reaction" to describe this essential process in photosynthesis.

2. What's a "dark reaction?"

The "dark reaction," also known as the Calvin cycle, is the process in photosynthesis where plants utilize ATP and NADPH produced during the light reaction to convert carbon dioxide into glucose. It takes place in the chloroplast's stroma.

3. A plant's energy production can't take place without light. why?

Light is essential for a plant's energy production because it is used in the process of photosynthesis. During photosynthesis, plants convert light energy into chemical energy in the form of ATP (adenosine triphosphate). Without light, plants are unable to generate ATP and carry out the necessary reactions for energy synthesis. Additionally, plants release oxygen as a byproduct of photosynthesis, which is crucial for sustaining life on Earth. While some anaerobic photosynthetic bacteria can use light energy, they do not possess the same light-dependent reactions as plants and do not produce oxygen as a result.