Dark Reaction – Concepts & Explanation

Key Concepts

• Photosynthesis and its stages

• Light and dark reactions in chloroplast

• Dark reaction – an overview

• Phases of Calvin cycle

• Products of Calvin cycle

Physiological Metabolism – Dark Reaction

Introduction:

Photosynthesis (photo – light ; synthesis – to join) is the process through which green plants make use of senergy from sunlight to make their own food. In the presence of sunlight, chloroplasts of green plants synthesize glucose using water and carbon dioxide. During photosynthesis, water gets oxidized and carbon dioxide gets reduced to form carbohydrates. Photosynthesis forms the basis of all types of  food chains and food webs. Photosynthesis produces starch and oxygen required for the survival of an organism. Therefore, photosynthesis supports all forms of life on earth. 

Photosynthesis in Plants: 

Photosynthesis is an important process that takes place in green plants. In the presence of sunlight, plants combine carbon dioxide and water to produce carbohydrates. Oxygen is the most important by product of photosynthesis. In the presence of light energy, water is split to release oxygen. This process is known as photolysis of water. During the process of photosynthesis water molecule get split up to release oxygen and carbon dioxide forms carbohydrates. Oxygen produced during photosynthesis comes from water molecule and not from carbon dioxide. In plants, photosynthesis takes place in the green part. Mostly green leaves carry out the photosynthesis process. Sometimes other green parts of plants such as green stems and floral buds are also involved in photosynthesis. Chloroplasts are specialised organelles in which photosynthesis process actually takes place.  

Stages of Photosynthesis: 

Photosynthesis involves two important processes, 

1. Light dependent reaction (Light Reaction) 

2. Light-independent reaction (Dark Reaction / Calvin Cycle) 

Though the entire process of photosynthesis takes place in chloroplast, light and dark reactions occur at different sites. Light reaction takes place in grana and dark reaction takes place in stroma regions of chloroplast.  

Light dependent reaction: 

Light dependent reaction is also known as primary photochemical reaction or Hill’s reaction or Arnon’s cycle. Light reaction takes place at a faster rate than dark reaction. During light dependent reaction, light energy gets converted into chemical energy. Light dependent reaction occurs in the thylakoid (tthylakoids are stacked up to form grana) region of chloroplast. Thylakoids contain chlorophyll molecules. Chlorophyll pigments are green in colour. These pigments absorb different wavelengths of light and convert them into chemical energy through photosynthesis. 

Light dependent reaction involves four important stages. 

1. Absorption of light energy 

2. Splitting of water molecules 

3. Release of oxygen 

4. Formation of energy carrying molecules – ATP and NADPH 

During light reaction, solar energy gets converted into chemical energy and is stored in the form of energy carrying molecules – ATP and NADPH. During light reaction, water is split up to release oxygen. High energy carrier molecules, ATP and NADPH are required for the dark reaction, the next stage of photosynthesis.  

Dark reaction / Calvin Cycle – An Overview: 

The second stage of photosynthesis is the light independent reaction. It is also known as dark reaction or Calvin Cycle or C3 cycle. Dark reaction does not require sunlight and it takes place in the absence of sunlight. Plants make use of ATP and NADPH produced by light reaction to reduce carbon dioxide and other compounds and produce glucose. These chemical reactions are collectively known as C3 cycle/ light independent reaction/ dark reaction/ Calvin Cycle. These cyclic chains of reactions were discovered in the year 1950 by a group of scientists at the University of California. Melvin Calvin, Andrews Benson, James Bassham and other scientists were involved in this discovery. Hence, these reactions are known as Calvin Cycle or Calvin Benson Cycle or Calvin Benson Bassham (CBB) Cycle. It is also known as reductive pentose phosphate (RPP) cycle. In 1961, Melvin Calvin was awarded the Nobel Prize for his discovery. 

In simple terms Calvin Cycle refers to the conversion of carbon dioxide to carbohydrate or the fixation of atmospheric carbon dioxide by plants through photosynthesis. Plants which undergo these cycle of reactions are known as C3 plants (Example: rice, wheat, barley, peanuts, spinach etc.). 

Phases of the Calvin Cycle: 

Carbon dioxide from the atmosphere enter into green leaves through stomata (pores on leaves). Carbon dioxide then diffuse into chloroplast stroma. This carbon dioxide is converted into carbohydrate through Calvin Cycle. ATP and NADPH produced during the light reaction are used for this process. Calvin Cycle takes place in the chloroplast stroma. Reactions of Calvin Cycle are divided into three main phases. 

1. Carbon Fixation 

2. Reduction 

3. Regeneration of Ribulose1, 5-bisphosphate (RuBP) 

Carbon Fixation: 

Carbon fixation is the first phase of Calvin Cycle. During this phase carbon dioxide absorbed by plants reacts with ribulose1, 5-bisphosphate (RuBP), a phosphorylated 5 – carbon compound. This reaction is catalyzed by the enzyme ribulose biphosphate carboxylase / oxygenase or RUBISCO(RuBp). RUBISCO has been identified to be the most abundant enzyme on earth. This reaction results in the formation of a six-carbon intermediate. This molecule is highly unstable and therefore, instantly splits into two molecules of three-carbon molecule known as 3 – phosphoglycerate (3– PGA). Thus, carbon from carbon dioxide gets fixed into 3-carbon compound. Hence, this reaction cycle is also known as C3 pathway. 

Reduction: 

In the reduction phase of Calvin cycle, 3-PGA molecules (from carbon fixation phase) are converted into glyceraldehyde-3-phosphate (G3P). ATP and NADPH are used for this reaction. As NADPH gets reduced (donates electrons), this reaction is termed as reduction phase. This process takes place in two steps.  

In the first step, ATP gets converted into ADP and transfers a phosphate group to 1,3-bisphosphoglycerate.  

In the second step, 1,3-bisphosphoglycerate molecules get reduced. NADPH donates two electrons to 1,3-bisphosphoglycerate and gets converted into NADP⁺. 

1,3-bisphosphoglycerate loses one of its phosphate groups and gets converted into glyceraldehyde 3-phosphate (G3P), a three – carbon sugar. These G3P molecules are then used to produce glucose or fructose (6-carbon sugar molecules). In some plants glucose is used to produce cellulose or starch. In some other plants, fructose and glucose are combined to form sucrose. 

In the reduction phase, the chemical energy of ATP and the reduction power of NADPH are utilised. ADP and NADP⁺ formed during this reduction phase of Calvin cycle return back to the light reaction as substrates. These molecules are re-energized and used again. 

Regeneration of Ribulose1, 5-bisphosphate (RuBP): 

The third phase of Calvin cycle involves a series of complex reactions. During this phase, some of the G3P molecules (formed during the reduction phase) leave the Calvin cycle and are exported to the cytoplasm from chloroplast. In cytoplasm, these G3P molecules are used for the production of glucose / fructose/ sucrose/starch. Other G3P molecules that remain in the Calvin cycle are used for the regeneration of Ribulose1, 5-bisphosphate (RuBP). These RuBP molecules are again combined with the absorbed carbon dioxide and enter into the carbon fixation phase. Thus, the reaction cycle gets repeated again. 

Products of Calvin Cycle: 

• After each turn of the Calvin Cycle, one molecule of carbon gets fixed. 
• After three turns of Calvin Cycle, one molecule of G3P is exported into the cytoplasm. 
• G3P molecules that leave Calvin Cycle are used for the production of glucose / fructose/ sucrose/starch.  
• Two G3P molecules combine together to form one glucose molecule. Therefore, six turns of Calvin Cycle are required to produce one molecule of glucose.

Summary

• Photosynthesis is the process through which chloroplasts of green plants synthesize glucose by making use of water and carbon dioxide in the presence of sunlight.

• During photosynthesis, water gets oxidized and carbon dioxide gets reduced to form carbohydrates.

• Photosynthesis involves two important processes – light reaction and dark reaction

• Light reaction takes place in grana and dark reaction takes place in stroma regions of chloroplast

• Light dependent reaction is also known as primary photochemical reaction or Hill’s reaction or Arnon’s cycle.

• Light dependent reaction occurs in the thylakoid region of chloroplast

• Light dependent reaction involves four important stages – absorption of light energy, splitting of water molecules, release of oxygen, formation of energy carrying molecules – ATP and NADPH.

• Light reaction results in the conversion of light energy into chemical energy through the production of ATP and NADPH.

• Light reaction also results in splitting of water molecule and release of oxygen as a by-product.

• ATP and NADPH produced through light reaction are utilised for the dark reaction / Calvin cycle, the second stage of photosynthesis.

• Calvin Cycle takes place in the stroma of chloroplasts in the absence of light.

• The overall purpose of Calvin Cycle is to convert atmospheric carbon dioxide into carbohydrate (sugars).

• Calvin Cycle makes use of ATP and NADPH produced by light reaction to convert carbon dioxide into carbohydrate.

• Calvin Cycle involves three important phases – carbon fixation, reduction, and regeneration of Ribulosel, 5-bisphosphate (RuBP).

• After each turn of the Calvin Cycle, one molecule of carbon gets fixed.

• After three turns of Calvin Cycle, one molecule of G3P is exported into the cytoplasm.

• G3P molecules that leave Calvin Cycle is used for the production of glucose / fructose/ sucrose/starch. • Two G3P molecules combine together to form one glucose molecule. Therefore, six turns of Calvin Cycle are required to produce one molecule of glucose.