Difference Between Light Reaction and Dark Reaction

The difference between light reaction and the dark reaction is mainly due to the following properties:
The demand for light: A light reaction demands a source of light energy for breaking down water into oxygen, while dark reaction does not need a light source.

Site of occurrence: A light reaction occurs within the thylakoid membrane, whereas dark reaction occurs inside the aqueous stroma of the chloroplast.

Light and dark reaction are the two phases that complete the process of photosynthesis in the photosynthetic organisms. In this context, you would get to know the key differences between the light and dark reaction along with the comparison chart and definition of the two.

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 a plant derives cellular energy, i.e. ATP and NADPH from the light energy, i.e. sunlight. The photochemical phase or light reaction is a stage that involves oxidation of single water molecule into half oxygen molecule.

Therefore, splitting of two water molecules will result in releasing one oxygen molecule. Oxidation of water releases ATP and NADPH that in turn provides cellular energy to the plant cells to prepare food for themselves.

It occurs during the day time or in the presence of sunlight. For the reconstruction of light energy, a light reaction includes two photosystems, namely PS-I and PS-II.

light reaction

PS-I harnesses a light wavelength of 700 nm, while PS-II harnesses a light wavelength of 680 nm. 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 a plant exploits the cellular energy (ATP and NADPH) released from the light reaction to produce chemical energy by synthesizing sugars.

The biosynthetic phase involves a reduction of six CO2 molecules and twelve H2 molecules from the NADPH into single glucose (C6H12O6) molecule.

dark reaction

ADP and NADP are released by the hydrolysis of ATP and NADPH. It occurs during the absence of sunlight. 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 light source to produce cellular energy for the plants, whereas dark reaction occurs in the absence of light.
  4. Light reaction plays a functional role in liberating oxygen by splitting water, whereas dark reaction plays a fundamental role in fixing 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 photochemical and biosynthetic phase shares a lot of differences, but they collectively accomplish the process of photosynthesis. They follow a cyclic pathway, where both the light and chemical energy are continuously used and released.

Leave a Comment

Your email address will not be published. Required fields are marked *