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Light reaction and dark reactions are crucial in maintaining life on earth. Photosynthesis is a process by which green plants absorb light energy from the sun with the assistance of water and carbon dioxide, transforming the light energy into chemical energy. For this conversion, plants make glucose and oxygen. Light reaction and dark reaction both are subsequent stages that occur during the process of photosynthesis of the plants.
Light Reactions in green plants are captured by the light absorbing pigment called Chlorophyll in the Chloroplasts (the thylakoid membrane of Chloroplast) of the leaves. The end product of light reaction are ATP and NADPH. In the Dark Reaction, sugar molecules are formed from the carbon dioxide and water molecules. The process occurs in the stroma of the chloroplast (aqueous fluid surrounding stacks of thylakoids) in the absence of light.
Read More: Photosynthesis in Higher Plants
| Table of Content |
Key Terms: Photosynthesis, Dark Reaction, Light Reaction, Plants, Chlorophyll, Chloroplast, Oxygen, Hydrogen, Carbon Dioxide
Light Reaction
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Some of the important characteristics of light reaction are as follows:
- It is the first and foremost stage in the process of photosynthesis.
- This stage requires sunlight for a series of biochemical reactions.
- Chlorophyll, the light-absorbing pigment, captures the light to convert chemical energy into ATP (Adenosine triphosphate) and NADPH (Nicotinamide adenine dinucleotide phosphate).
- Water (H2O) is oxidized and Oxygen (O2) is released.
- The electrons are freed from water and are transferred to ATP and NADPH.
- Light Reaction occurs in the membrane of the chloroplast thylakoid.
- The Carbohydrate molecules are obtained from the Carbon dioxide during the formation of chemical energy from the reactions.
- Photosystems I and II absorb the photons from the sunlight.
- The light energy tends to split into the water and later extracts the electrons from photosystems II.
- The electrons move from the PSII to b65 (cytochrome) for photosystems I (PSI) and reduce in the form of energy.
- These electrons are used in the formation of NADPH by the enzyme.
Thus, in a light-dependent reaction, a water molecule is broken down producing two H+ ions and half an oxygen molecule. Simultaneously, another water molecule is broken down. ATP is an important source of all cellular energy.
Read More: Factors affecting Photosynthesis
Dark Reaction
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Some interesting characteristics of a dark reaction are as follows:
- Dark Reaction is the subsequent stage of photosynthesis, that produces glucose from the energy of ATP and NADPH produced in the light reaction.
- The stage where food is synthesized by the plants and uses the energy that is generated by the light reaction.
- It is also known as the carbon fixing reaction or as the Calvin cycle, the Calvin Benson cycle.
- The plants that have the Calvin cycle are called C3 plants and Hatch-Stack cycle are called C4 plants.
- This stage does not require sunlight.
- The reaction occurs in the stroma of the chloroplasts which are the cell organelles, also known as the kitchen of the cells.
- It is outside of the thylakoids. The energy from the ATP and NADPH is used to fix carbon dioxide (CO2).
- The outcome of this reaction are sugar molecules and other organic molecules required for cell function and metabolism.
- Carbon fixation, Carbon reduction and RuB regeneration are the phases of the Dark reaction.
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| Concepts Related Light Vs Dark Reaction | ||
|---|---|---|
| Light Sources | Photochemical Reactions | Photosynthesis Vs Respiration |
| Chromoplasts | Mitochondria Vs Plastids | Cytoplasm Vs Photoplasm |
| Mitochondria Diagram | Xylem Vs Phloem | Why are Plants Green? |
Difference between Light Reaction and Dark Reaction
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Tabulated below are some of the important differences between light reaction vs dark reaction:
| Basis | Light Reaction | Dark Reaction |
|---|---|---|
| Occurrence | Takes place in the thylakoid membrane of the Chloroplast. | Takes place outside the thylakoid i.e. the stroma of the Chloroplast. |
| Stage | First stage of the photosynthesis process. | Subsequent stage of the photosynthesis process. |
| Alternate Name | Also known as the photo-chemical phase. | Also known as the bio-synthetic phase. |
| Light | The light reaction only takes place in the presence of the light. | The dark reaction takes place in the presence or absence of sunlight. |
| Pigments | Chlorophylls are the pigments involved in the light reaction. | No pigments are involved in the dark reaction. |
| Photolysis | The process of photolysis occurs in PS II during the light reaction. | There is no process of photolysis during the dark reaction. |
| Effect | Water molecule is split and oxygen is released. | Carbon dioxide is utilized and glucose is produced. |
| Reaction | H+ ions are being utilized by NADP(Nicotinamide Adenine Dinucleotide Phosphate) to form NADPH | The hydrogen of NADPH is used to combine with CO2. |
| End | The result of the light reaction is ATP and NADPH which are also known as Assimilatory powers. | The result of the dark reaction is glucose i.e. Produced by using energy from the ATP and NADPH, produced in the light energy |
Read More: Plant Tissues
Things to Remember
- Light reaction includes light absorption, hydrolysis, the release of oxygen followed by formation of ATP and NADPH.
- Dark reaction uses carbon-fixing reaction and forms sugar molecules from the carbon dioxide and water molecules
- The process of photosynthesis can be summarized with the formula given below: 6CO2 + 6H2O + sunlight (light energy) → C6H12O6 + 6O2
- The electrons in the light reaction are used in the formation of NADPH by the enzyme, NADP+.
- ATP is synthesized by utilizing H+ ions, that are generated by photolysis to produce ATP.
- The key diferences between light reaction vs dark reaction are based on the location, light requirement, pigment involved, reaction mechanism and reaction products
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Sample Questions
Ques. What is the stable intermediate product of photosynthesis? (2 marks)
(1)Glucose
(2)Phosphoglyceric acid
(3)Formaldehyde
(4)PGAL
Ans: (2)
In the majority of plants, the first stable intermediate of carbon dioxide reduction is a three-carbon compound. This type of carbon reduction is known as the C3 cycle. In the C3 cycle, initial carboxylation is brought by RuBisCo, which carboxylates Ribulose- 1,5 bisphosphate (RUBP) into Phosphoglyceric acid. The Phosphoglyceric acid is later reduced to Phosphoglyceraldehyde, which is used to synthesize sugars and recycle RUBP.
Ques. Who discovered the dark reaction? (1 mark)
Ans: NADPH and ATP drive the second stage, the ‘dark’ reactions (Calvin cycle) were discovered by Melvin Calvin that do not require light.
Ques. How does light affect the growth of plants? (1 mark)
Ans: Light provides energy to plants through a process called photosynthesis. This light energy helps the plant to grow.
Ques. Tropical plants like the sugarcane show high efficiency of CO2 fixation because of(2 marks)
(1)Calvin cycle
(2)Hatch – Slack cycle
(3)Cyclic photophosphorylation
(4)TCA cycle
Ans: (2)
Hatch slack pathway also known as C4 Carbon fixation is a photosynthetic process in some plants. It is the first step in extracting carbon dioxide to be able to use it in sugar and other biomolecules. It is one of the three known processes for carbon fixation. C4 refers to the four-carbon molecule that is the first product of this type of carbon fixation.
Ques. How much ATP does a dark reaction produce? (2 marks)
Ans: 3 ATP Molecules.
Photosynthesis occurs in two stages. Light and Dark reaction. In the light reaction ATP and reduced coenzymes, NADPH2 are synthesized, which are used to prepare sugar. 3 ATP molecules and 2NADPH2 are required to fix one molecule of Carbon dioxide in the Calvin cycle.
Ques.Kranz Anatomy is found in: (2 marks)
(1)Seed
(2)Flower
(3)Stem
(4)Leaves
Ans:(4)Leaves
In the leaves of C4 plants, the bundle sheath cells are present in the form of rings around in the vascular bundle. The arrangement is called Kranz and the Anatomy is known as the Kranz type of Anatomy.
Ques. How does the plant make energy without light? (2 marks)
Ans: Plants make ATP from other sources such as metal ions and turn carbon dioxide into glucose. The anaerobic photosynthetic bacteria use light energy but do not have the light reactions as in the plants and produce no oxygen.
Ques. Isotopes that are employed to study the process of photosynthesis reaction.(2 marks)
(1)S35& P32
(2)N14& O18
(3)C14 & O18
(4)N14 & CO60
Ans: (3)
Photosynthesis is a redox reaction in which carbon dioxide is reduced to sugars in a dark reaction and water is oxidized to release oxygen in the light reaction of photosynthesis. C14 label is used to trace the path of carbon in the dark reaction in C3 plants by Calvin, Benson and Basham. The O18 label was used by Rumen and Kamen to experimentally prove that the source of oxygen released during the light reaction of photosynthesis is water.
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