Difference Between Light Reaction and Dark Reaction

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Difference Between light reaction and dark reaction are mainly due to the following properties:
The demand for light: A light reaction demands the source of light energy for the breakdown of water into oxygen, while dark reaction does not need a light source.
Site of occurrence: A light reaction resides within the thylakoid membrane, whereas dark reaction resides inside the aqueous stroma of the chloroplast.

Light and dark reaction are the two phases that complete the process of photosynthesis in photosynthetic organisms.

Content: Light Reaction Vs Dark Reaction

  1. Comparison Chart
  2. Definition
  3. Key Differences
  4. Conclusion

Comparison Chart

PropertiesLight reactionDark reaction
MeaningIt involves the conversion of light energy from sunlight into cellular energy (ATP and NADPH), by the oxidation of water moleculeIt involves the conversion of cellular energy (ATP and NADPH) into chemical energy or sugars, by the reduction of carbon dioxide
Alternative namesLight dependent or Hill’s reactionLight independent or Blackman’s reaction
Phase of photosynthesisIt is the first phase of photosynthesis and also refers as “Photochemical phase”It is the second phase of photosynthesis and also refers as “Biosynthetic phase”
Light requirementIt strongly requires the presence of light sourceIt occurs in absence of light source
Site of occurrenceOccurs in the thylakoid membrane of chloroplastOccurs in the stroma of chloroplast
Functional roleIt liberates oxygen by splitting waterIt fixes carbon dioxide to produce sugar
PhotolysisOccurs in photosystem-IIDoes not occur
End productIts end product is oxygen with a release of ATP and NADPHIts end product is sugar with a release of ADP and NADP
Involvement of chlorophyllIt involves the presence of photosynthetic pigment or chlorophyll to absorb the light energyIt lacks such pigment

Definition of Light Reaction

It is the first phase of photosynthesis, where it derives cellular energy, i.e. ATP and NADPH from the light energy, i.e. sunlight and thus also refers to as “Photochemical phase”. The light reaction is a stage which involves oxidation of single water molecule into half oxygen molecule. Therefore, splitting of two water molecules will result in a release of one oxygen molecule.

During oxidation, ATP and NADPH produce that provides cellular energy that is further used by a plant cell to prepare food for itself. It occurs during the day time or in the presence of sunlight and also refers to as “Light-dependent” reaction.

light reaction

For the reconstruction of light energy, a light reaction includes two photosystems, namely PS-I and PS-II. PS-I can harness a light wavelength of 700nm, while PS-II harnesses a light wavelength of 680nm. The site of light reaction is within the thylakoid membrane of the photosynthetic apparatus (Chloroplast). It generally involves the following steps:

  • Absorption of light energy coming from sunlight.
  • Hydrolysis of a water molecule
  • Release of oxygen into the atmosphere
  • Formation of cellular energy, i.e. ATP and NADPH

Definition of Dark Reaction

It is the second phase of photosynthesis, where it exploits the cellular energy (ATP and NADPH) released from the light reaction to produce chemical energy by synthesizing sugars and thus also refers to as “Biosynthetic phase”. The biosynthetic phase involves a reduction of six CO2 molecules and twelve H2 molecules from the NADPH into single glucose (C6H12O6) molecule.

dark reaction

During reduction, ADP and NADP release by the hydrolysis of ATP and NADPH. It occurs during the in the absence of sunlight and hence also refers to as “Light independent” reaction. A location of the light-independent reaction is inside the aqueous stroma of the photosynthetic apparatus (Chloroplast). It generally involves the following steps:

  • Usage of chemical energy coming from the light reaction.
  • Fixation of carbon dioxide
  • Formation of glucose
  • Release of ADP and NADP

Key Differences Between Light Reaction and Dark Reaction

  1. A light reaction involves the conversion of light energy from sunlight into cellular energy (ATP and NADPH), by the oxidation of water molecule. A dark reaction is associated with the transformation of cellular energy (ATP and NADPH) into chemical energy or sugars, by the reduction of carbon dioxide.
  2. The alternative names for the light reaction are light-dependent or Hill’s reaction, while alternative names for the dark reaction are light-independent or Blackman’s reaction.
  3. A light reaction strongly requires the presence of light source to produce cellular energy for the plant, whereas dark reaction occurs in the absence of light.
  4. Light reaction plays a functional role in the liberation of oxygen by splitting water, whereas dark reaction plays a fundamental role in fixation of carbon dioxide to form glucose.
  5. The release of oxygen with ATP and NADPH and a release of sugar with ADP and NADP are the end products of light and dark reaction, respectively.
  6. The light reaction involves the presence of photosynthetic pigment, i.e. chlorophyll to trap the light energy in the thylakoid membrane. The dark reaction occurs in the stroma of chloroplast, which lacks the chlorophyll pigments.

Conclusion

Therefore, we can conclude that both light and dark reaction shares a lot of differences, but besides that, these collectively accomplish the process of photosynthesis. These follow a cyclic pathway, where both the light and chemical energy are continuously used and released.

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