Difference Between Light and Dark Reaction

Major Difference – Light vs Dark Reaction

Light reaction and dark reaction are the two types of sequential processes that occur during the photosynthesis of plants. Light reaction occurs in the thylakoid membrane of chloroplast whereas the dark reaction occurs in the stroma of the chloroplast. Light energy from sunlight is trapped by chlorophylls during the light reaction of the photosynthesis. Dark reaction is catalyzed by various enzymes. The main difference between light and dark reaction is that light reaction is the first stage of photosynthesis, which traps light energy in order to produce ATP and NADPH whereas dark reaction is the second stage of photosynthesis, which produces glucose by using the energy form ATP and NADPH produced from the light reaction. 

 This article examines,

1. What is Light Reaction
      – Definition, Characteristics, Function
2. What is Dark Reaction
      – Definition, Characteristics, Function
3. What is the difference between Light and Dark Reaction

What is Light Reaction

Light reaction is the first stage of photosynthesis, which produces ATP and NADPH by trapping the energy of sunlight by pigments called chlorophyll. Light reaction occurs in the thylakoid membrane of chloroplasts. Since light reaction depends on sunlight, it only occurs in the presence of sunlight. Chlorophyll A and B are the major types of chlorophylls involved in the light reaction. Chlorophyll A is the principal pigment-trapping light energy, and chlorophyll B is the accessory pigment, which traps light and passes to chlorophyll A. The energy trapped by chlorophyll A is passed to the photosystem II (PS II) and photosystem I (PSI) in the form of high energy electrons. Exited PS II takes electrons by splitting water molecules into molecular oxygen, generating high energy electrons, which are transferred through a series of electron carriers into PS I. Splitting of water at PS II is called photolysis. PS I also generates high energy electrons by the energy of sunlight. These electrons are used in the formation of NADPH by the enzyme, NADP⁺ reductase. ATP synthase utilizes H⁺ ions, which are generated by photolysis in order to produce ATP. Light reaction is shown in figure 1.

Figure 1: Light Reaction

What is Dark Reaction

Dark reaction is the second stage of photosynthesis, which produces glucose from the energy of ATP and NADPH produced in the light reaction. It occurs in the stroma of the chloroplast. The dark reaction occurs in two reaction mechanisms: C3 cycle and C4 cycle. C3 cycle is called Calvin cycle whereas C4 cycle is called Hatch-Stack cycle. Calvin cycle occurs in three steps. During the first step, carbon dioxide is fixed into ribulose 1,5-bisphosphate, forming an unstable six carbon compound, which is then hydrolyzed into three carbon compound, 3-phosphoglycerate. The enzyme involved in the process is rubisco. Due to the catabolic imperfection of rubisco, photorespiration occurs in the presence of low carbon dioxide concentrations. During the second step, some of the 3-phosphoglycerates are reduced in order to produce hexose phosphates. The remaining 3-phosphoglycerates are used in the recycling of ribulose 1,5-phosphate.

During C4 cycle, double fixation of carbon dioxide is observed, increasing the efficiency of photosynthesis. Before entering into Calvin cycle, carbon dioxide is fixed into phosphoenol pyruvate, forming a four carbon compound, oxaloacetate. Oxaloacetate is converted into malate and transferred into bundle sheath cells in order to enter into Calvin cycle by removing carbon dioxide. Calvin cycle is shown in figure 2.

Figure 2: Calvin Cycle

Difference Between Light and Dark Reaction

Occurs in

Light Reaction: Light reaction occurs in the thylakoid membrane of the chloroplast.

Dark Reaction: Dark reaction occurs in the stroma of the chloroplast.

Light

Light Reaction: Light reaction depends on the sunlight.

Dark Reaction: Dark reaction is independent of sunlight.

Pigments

Light Reaction: Chlorophylls are the pigments involved in the light reaction.

Dark Reaction: No pigments are involved in the dark reaction.

Photolysis

Light Reaction: Photolysis occurs in PS II during the light reaction.

Dark Reaction: No photolysis occurs during the dark reaction.

Oxygen/Carbon dioxide

Light Reaction: Oxygen is liberated during the light reaction.

Dark Reaction: Carbon dioxide is fixed during the dark reaction.

Results

Light Reaction: ATP and NADPH are produced during the light reaction.

Dark Reaction: Glucose is produced by using the energy from ATP and NADPH, produced in the light reaction.

Conclusion

Light reaction and dark reaction are the two steps of reactions involved in photosynthesis. Light reaction occurs in the thylakoid membrane of chloroplasts. The energy of sunlight is trapped by chlorophylls, and the trapped energy is used in the production of ATP and NADPH. These ATP and NADPH are used in the production of glucose in the dark reaction. The dark reaction occurs in the stroma of chloroplast with the involvement of enzymes. It occurs in two ways, C3 cycle and C4 cycle. C4 cycle is more efficient than C3 cycle. The main difference between light and dark reaction is their contribution to photosynthesis.

Reference:
1. Berg, Jeremy M. “The Light Reactions of Photosynthesis.”Biochemistry. 5th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 17 Apr. 2017.
2. Berg, Jeremy M. “The Calvin Cycle Synthesizes Hexoses from Carbon Dioxide and Water.” Biochemistry. 5th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 16 Apr. 2017.
3. Lodish, Harvey. “CO2 Metabolism during Photosynthesis.” Molecular Cell Biology.

Image Courtesy:
1. “Cyclic Photophosphorylation” By David Berard – Own work (CC0) via Commons Wikimedia
2. “Calvin cycle background” By Adenosine – File:Calvin-cycle4.svg (CC BY-SA 3.0) via Commons Wikimedia