Differences

The Difference between Light and Dark Reaction

The difference between light and dark reactions lies in the fact that one is carried out by plants in the presence of light and the other is carried out by plants in the absence of light. Light reactions are carried out by green plants and dark reactions are carried out by plants.

In chemistry light and dark reaction is used to produce ATP. The light reaction is the process of converting light energy into chemical energy. The dark reaction is the process of transferring the chemical energy produced to ATP.

It’s all about the light, it’s all about the dark. It’s all about how these two reactions are carried out by plants and how do they differ from each other?

Main differences-light vs. dark reaction

Try to understand:

Light and dark reactions are two types of sequential processes that occur during photosynthesis in plants. The photochemical reaction occurs in the thylakoid membrane of the chloroplast, while the dark reaction occurs in the stroma of the chloroplast. Light energy from sunlight is trapped by chlorophyll during the photoreaction of photosynthesis. The dark reaction is catalyzed by various enzymes. The main difference between light and dark reactions is that the photosynthesis is the first step in photosynthesis that captures light energy to produce ATP and NADPH, while the dark reaction is the ATP and NADPH produced from the photoreaction. It is the second stage of photosynthesis that produces glucose using the energy form of.

In this article

1. What is a light reaction
      -definition, characteristic, function
2. What is a dark reaction
      -definition, characteristic, function
3. What is the difference between a light reaction and a dark reaction?

The Difference between Light and Dark Reaction

What is a photoreaction?

Photoreaction is the first step in photosynthesis, producing ATP and NADPH by trapping the energy of the sun with a dye called chlorophyll. A photoreaction occurs in the thylakoid membrane of the chloroplast. Since the photoreaction depends on sunlight, it only occurs in the presence of sunlight. 

Chlorophylls A and B are the major types of chlorophyll involved in photochemical reactions. Chlorophyll A is the main dye-capturing light energy, and chlorophyll B is an accessory pigment that captures light and reaches chlorophyll A. The energy captured by chlorophyll A is passed to Photosystem II (PS II) and Photosystem I (PSI) in the form of high-energy electrons. 

The emitted PS II receives electrons by decomposing water molecules into molecular oxygen, generating high-energy electrons and transferring them to PS I. The decomposition of water in PSII is called photodegradation. PS Also, the energy of sunlight generates high-energy electrons. These electrons are used to form NADPH by the enzyme NADP. + Reductase ATP synthase utilizes H + Ions produced by photolysis to produce ATP. The light reaction is shown in Figure 1 .

Light reaction

What is Dark Reaction?

The dark reaction is the second step in photosynthesis, producing glucose from the energy of ATP and NADPH produced by the photochemical reaction. It occurs in the interstation of the chloroplast. The dark reaction occurs in two reaction mechanisms: the C3 cycle and the C4 cycle. The C3 cycle is called the Calvin cycle and the C4 cycle is called the hatch stack cycle. 

The Calvin cycle occurs in three steps. During the first step, carbon dioxide is immobilized on ribulose 1,5-bisphosphate to form an unstable 6-carbon compound, which is then hydrolyzed to the 3-carbon compound, 3-phosphoglycerate. The enzyme involved in the process is Rubisco. 

Due to Rubisco’s catabolic imperfections, photorespiration occurs in the presence of low carbon dioxide concentrations. During the second step, some 3-phosphoglycerates are reduced to produce hexose phosphate. The remaining 3-phosphoglycerate is used to recycle ribulose 1,5-phosphate.

During the C4 cycle, double fixation of carbon dioxide is observed, increasing the efficiency of photosynthesis. Prior to entering the carbon cycle, carbon dioxide is immobilized on phosphoenolpyruvate to form the tetracarbon compound, oxaloacetate. Oxaloacetic acid is converted to malic acid and transferred to bundle-sheath cells to enter the Calvin cycle by removing carbon dioxide. The Calvin cycle is shown in Figure 2 .

Calvin cycle

The difference between a bright reaction and a dark reaction

Occurs in

Mild reaction: A photoreaction occurs in the thylakoid membrane of the chloroplast.

Dark reaction: A dark reaction occurs in the interstitium of the chloroplast.

light

Light reaction: The light reaction depends on sunlight.

Dark reaction: The dark reaction is not affected by sunlight.

Pigment

Light reaction: Chlorophyll is a pigment involved in the photochemical reaction.

Dark reaction: The dark reaction does not contain pigments.

Photodecomposition

Mild reaction: Photodegradation occurs in PS II during the photoreaction.

Dark reaction: No photodegradation occurs during the dark reaction.

Oxygen / carbon dioxide

Light reaction: Oxygen is released during the photochemical reaction.

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

result

Light reaction: ATP and NADPH are produced during the photoreaction.

Dark reaction: Glucose is produced using the energy from ATP and NADPH produced by the photochemical reaction.

Conclusion

The light reaction and the dark reaction are two-step reactions related to photosynthesis. A photoreaction occurs in the thylakoid membrane of the chloroplast. The energy of sunlight is captured by chlorophyll, and the captured energy is used for 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 with the involvement of enzymes within the chloroplast substrate. It happens in two ways: C3 cycle and C4 cycle. The C4 cycle is more efficient than the C3 cycle. The main difference between light and dark reactions is their contribution to photosynthesis.

See also:
1. Berg, Jeremy M. “Photoreaction of Photosynthesis” Biochemistry. 5th Edition National Library of Medicine, January 1, 1970. web. April 17, 2017.
2. Berg, Jeremy M. “The Calvin Cycle Synthesizes Hexose from Carbon Dioxide and Water” Biochemistry. 5th Edition National Library of Medicine, January 1, 1970. web. April 16, 2017.
Lodi, Harvey. “CO2 metabolism during photosynthesis” Molecular cell biology

Image
courtesy of 1. David Berard “Cyclic Photophosphorylation”-via his own work (CC0)

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