4.1.10 Light Independent Phase (dark Reaction)

Introduction

The light-independent phase, also known as the dark reaction or Calvin cycle, is the second phase of photosynthesis. It does not require light directly. It uses the products of the light-dependent phase (ATP and NADPH) to fix carbon dioxide into organic molecules. This phase occurs in the stroma of the chloroplast.

Definition

Light-Independent Phase (Dark Reaction): The light-independent phase (dark reaction) takes its name from the fact that light is not directly required for these reactions to occur. This phase requires the availability of NADPH, ATP (the products of light reaction), and CO₂.

In simple words: The dark reaction is the phase of photosynthesis that uses ATP and NADPH to convert CO₂ into glucose.

Core Concept

• The light-independent phase is also called the dark reaction.
• It occurs in the stroma of the chloroplast.
• It does not require light directly.
• It requires the products of the light reaction:
• NADPH
• ATP
• It also requires CO₂.
• In this phase:
• NADPH is used to reduce carbon dioxide.
• ATP is used to incorporate energy.
• CO₂ is converted into a phosphorylated triose carbohydrate, i.e., glyceraldehyde-3-phosphate (G3P).
• G3P is later used to make glucose.
• Dark reaction generally involves a complicated metabolic pathway, the Calvin cycle or C3 pathway.
• However, in some plants, in addition to the Calvin cycle, another metabolic pathway is also involved, called the C4 pathway.
• The plants in which only the Calvin cycle occurs during dark reaction are called C3 plants.

Detailed Explanation

Calvin Cycle

Definition: Calvin cycle term is applied to the series of metabolic reactions in which CO₂ is reduced to produce G3P.

• These reactions have been explored by Melvin Calvin and co-workers at the University of California.
• Melvin Calvin won the Nobel Prize in 1961 for this work.
• The Calvin cycle can be divided into three phases:

Phase 1: Carbon Fixation

Definition: The first phase of the Calvin cycle in which CO₂ is fixed into an organic molecule.

• One of the key substances in this process is a five-carbon phosphorylating sugar called ribulose bisphosphate (RuBP).
• RuBP is generally referred to as the CO₂ acceptor because it is capable of combining with carbon dioxide.
• The enzyme that catalyzes this reaction is Ribulose bisphosphate carboxylase/oxygenase, also known as RuBisCO.
• Three intermediate molecules of six carbons are formed during this reaction.
• These molecules are unstable and exist for such a short time that they cannot be isolated.
• Each six-carbon molecule breaks down to form two molecules of 3-phosphoglycerate (3-PGA).
• 3-PGA is a phosphorus-containing compound with three carbon atoms.
• Since the carbon of the inorganic compound (CO₂) becomes part of the organic compound (RuBP) during this phase, it is called carbon fixation.
• As the first stable compound in the Calvin cycle is a three-carbon compound (3-PGA), the Calvin cycle is also known as the C3 pathway.

Equation: CO₂ + RuBP RuBisCO→ 2 × 3-PGA

Phase 2: Reduction

Definition: The second phase of the Calvin cycle in which 3-PGA is reduced to G3P using ATP and NADPH.

• In this phase, six molecules of 3-phosphoglycerate (3-PGA) react with six ATP molecules.
• A phosphate from each ATP is transferred to each 3-PGA.
• In this way, 3-PGA molecules are changed into 1,3-bisphosphoglycerate.
• These molecules are then reduced by the hydrogen of NADPH.
• Finally, glyceraldehyde-3-phosphate (G3P) molecules are produced.
• During this reduction process, a phosphate group from each 1,3-bisphosphoglycerate molecule is also given off.
• There are a total of six molecules of G3P produced in this phase.
• But only one molecule is released from the cycle, while the rest of the five molecules are used to regenerate the CO₂ acceptor molecules in the next phase.

Equation: 3-PGA + ATP + NADPH → G3P + ADP + NADP⁺

Phase 3: Regeneration of CO₂ Acceptor (RuBP)

Definition: The third phase of the Calvin cycle in which RuBP is regenerated from G3P.

• Five molecules of G3P from the previous phase are used to regenerate the RuBP (CO₂ acceptor) in this phase.
• These five molecules, each containing three carbon atoms, undergo a series of reactions in which three molecules of ribulose phosphate (RuP) each containing five carbon atoms are produced.
• When three molecules of RuP react with three molecules of ATP, a phosphate group from each ATP is transferred to each RuP.
• Ultimately, RuP are converted into RuBP which again participate in the next cycle.

Equation: 5 × G3P + 3 × ATP → 3 × RuBP + 3 × ADP

Overall Calvin Cycle

• The whole process of the Calvin cycle indicates that there are:
• Three molecules of CO₂
• Six molecules of NADPH (reducing power)
• Nine molecules of ATP (assimilating power)
• These are used to release just one molecule of G3P from the cycle.
• However, in order to produce a glucose molecule, two molecules of G3P are required.

Overall Equation: 3CO₂ + 6NADPH + 9ATP → G3P + 3H₂O + 9ADP + 9Pi + 6NADP⁺

Fate of the Atoms in Photosynthesis

• The oxygen atoms in CO₂ end up in glucose and water.
• The oxygen atoms in water end up in oxygen gas.

C3 Plants

• Plants in which only the Calvin cycle occurs during the dark reaction are called C3 plants.
• Examples: Rice, wheat, soybean, spinach.
• The first stable product is 3-PGA (3-carbon compound).

C4 Plants

• Some plants have an additional metabolic pathway in the light-independent phase of photosynthesis beside the Calvin cycle.
• This metabolic pathway is called the Hatch-Slack cycle or C4 pathway.
• In this pathway, phosphoenol pyruvate (PEP) carboxylase is used instead of RuBisCO to fix CO₂ to phosphoenol pyruvate (a C3 molecule).
• The result is oxaloacetate, a C4 molecule.
• It takes place in the cytoplasm of mesophyll cells.
• Examples: Maize, sugarcane, sorghum.

Intermediate Level

Calvin Cycle in Detail

Overall Calvin Cycle

Overall Equation: 3CO₂ + 6NADPH + 9ATP → G3P + 3H₂O + 9ADP + 9Pi + 6NADP⁺

Comparison of C3 and C4 Plants

Importance of G3P

• G3P is the end product of the Calvin cycle.
• It is used to make:
• Glucose
• Fructose
• Sucrose
• Maltose
• Starch
• Cellulose
• Other organic compounds

Important Points

• 1. The light-independent phase (dark reaction) takes its name from the fact that light is not directly required for these reactions to occur.
• 2. This phase requires the availability of NADPH, ATP (the products of light reaction), and CO₂.
• 3. In this phase of photosynthesis, NADPH is used to reduce carbon dioxide while ATP is used to incorporate energy.
• 4. Finally, CO₂ is converted into a phosphorylated triose carbohydrate, i.e., glyceraldehyde-3-phosphate (G3P) which are later on used to make glucose.
• 5. Dark reaction generally involves a complicated metabolic pathway, the Calvin cycle or C3 pathway.
• 6. However, in some plants, in addition to Calvin cycle another metabolic pathway is also involved, called C4 pathway.
• 7. The plants in which only Calvin cycle occurs during dark reaction are called C3 plants.
• 8. Calvin cycle term is applied to the series of metabolic reactions in which CO₂ is reduced to produce G3P.
• 9. These reactions have been explored by Melvin Calvin and co-workers at the University of California.
• 10. Melvin Calvin won the Nobel Prize in 1961 for this work.
• 11. The Calvin cycle can be divided into three phases:
• - Carbon fixation
• - Reduction
• - Regeneration of carbon dioxide acceptor i.e., RuBP
• 12. Carbon fixation:
• - One of the key substance in this process is a five carbon phosphorylating sugar called ribulose bisphosphate (RuBP).
• - It is generally referred as CO₂ acceptor because it is capable of combining with carbon dioxide with the help of Ribulose bisphosphate (RuBP) carboxylase/oxygenase also known as RuBisCO.
• - Three intermediate molecules of six carbons are formed during this reaction.
• - These molecules are unstable and exist for such a short time that, they cannot be isolated.
• - Each six carbon breaks down to form two molecules of 3-phosphoglycerate (3-PGA), a phosphorous containing compound with three carbon atoms.
• - Since, the carbon of inorganic compound (CO₂) becomes the part of organic compound (RuBP) during this phase, hence, it is called carbon fixation.
• - As the first stable compound in the Calvin cycle is a three carbon compound (3-PGA) that is why Calvin cycle is also known as C3 pathway.
• 13. Reduction:
• - In this phase six molecules of 3-phosphoglycerate (3-PGA) react with six ATP molecules, a phosphate from each ATP is transferred to each 3-PGA.
• - In this way, 3-PGA molecules are changed into 1,3-Bisphosphoglycerate.
• - These molecules are then reduced by the hydrogen of NADPH and finally glyceraldehyde 3 phosphate (G3P) molecules are produced.
• - During this reduction process a phosphate group from each 1,3-Bisphosphoglycerate molecule is also given off.
• - There are total six molecules of G3P are produced in this phase but only one molecule is released from the cycle while rest of the five molecules are used to regenerate the CO₂ acceptor molecules in the next phase.
• 14. Regeneration of CO₂ acceptor:
• - Five molecules of G3P from the previous phase are used to regenerate the RuBP (CO₂ acceptor) in this phase.
• - These five molecules each containing three carbon atoms undergo a series of reactions in which three molecules of ribulose phosphate (RuP) each containing five carbon atoms are produced.
• - When three molecules of RuP react with three molecules of ATP, a phosphate group from each ATP is transferred to each RuP.
• - Ultimately RuP are converted into RuBP which again participate in the next cycle.
• 15. The whole process of Calvin cycle indicates that there are three molecules of CO₂, six molecules of NADPH (reducing power) and nine molecules of ATP (assimilating power) are used to release just one molecule of G3P form the cycle.
• 16. However, in order to produce a glucose molecule, two molecules of G3P are required.
• 17. The overall process of Calvin cycle can be represented as:
• 3CO₂ + 6NADPH + 9ATP → G3P + 3H₂O + 9ADP + 9Pi + 6NADP⁺

Remember

• - Dark reaction = stroma; does not need light directly
• - Calvin cycle = carbon fixation, reduction, regeneration
• - RuBisCO = enzyme that fixes CO₂
• - 3-PGA = first stable product (3C)
• - **G3P** = end product
• - ATP and NADPH = used in the dark reaction
• - **CO₂** = carbon source
• - C3 plants = only Calvin cycle
• - C4 plants = Calvin cycle + C4 pathway
• - Melvin Calvin = discovered the Calvin cycle; Nobel Prize 1961

Common Mistakes

• 1. Thinking the dark reaction occurs only in the dark – The dark reaction occurs during the day as well; it does not require light directly.
• 2. Confusing the Calvin cycle with the Krebs cycle – The Calvin cycle is in photosynthesis; the Krebs cycle is in respiration.
• 3. Thinking RuBisCO is only a carboxylase – RuBisCO is both a carboxylase and an oxygenase.
• 4. Confusing 3-PGA and G3P – 3-PGA is the first stable product; G3P is the end product.
• 5. Thinking the Calvin cycle produces glucose directly – The Calvin cycle produces G3P; glucose is made from two G3P molecules.
• 6. Forgetting the role of ATP and NADPH – They are required for the reduction phase.
• 7. Thinking C3 and C4 plants are the same – C3 plants use only the Calvin cycle; C4 plants use the C4 pathway in addition.
• 8. Confusing C3 and C4 pathways – C3 = 3-PGA (3C); C4 = oxaloacetate (4C).
• 9. Forgetting the Nobel Prize – Melvin Calvin won the Nobel Prize in 1961.
• 10. Thinking the Calvin cycle uses light – The Calvin cycle does not use light directly.
• 11. Confusing the number of ATP and NADPH – 9 ATP and 6 NADPH are used per G3P.
• 12. Thinking G3P is only used for glucose – G3P is also used for other organic compounds.

Quick Revision

• The light independent phase (dark reaction) takes its name from the fact that light is not directly required for these reactions to occur.
• This phase requires the availability of NADPH, ATP (the products of light reaction) and CO₂.
• In this phase of photosynthesis, NADPH is used to reduce carbon dioxide while ATP is used to incorporate energy.
• Finally, CO₂ is converted into a phosphorylated triose carbohydrate i.e., glyceraldehyde-3-phosphate (G3P) which are later on used to make glucose.
• Dark reaction generally involves a complicated metabolic pathway, the Calvin cycle or C3 pathway.
• However, in some plants, in addition to Calvin cycle another metabolic pathway is also involved, called C4 pathway.
• The plants in which only Calvin cycle occurs during dark reaction are called C3 plants.
• Calvin cycle:
• Calvin cycle term is applied to the series of metabolic reactions in which CO₂ is reduced to produce G3P.
• These reactions have been explored by Melvin Calvin and co-workers at the University of California.
• Melvin Calvin won the Nobel Prize in 1961 for this work.
• The Calvin cycle can be divided into three phases, carbon fixation, reduction and regeneration of carbon dioxide acceptor i.e., RuBP.
• Carbon fixation:
• One of the key substance in this process is a five carbon phosphorylating sugar called ribulose bisphosphate (RuBP).
• It is generally referred as CO₂ acceptor because it is capable of combining with carbon dioxide with the help of Ribulose bisphosphate (RuBP) carboxylase/oxygenase also known as RuBisCO.
• Three intermediate molecules of six carbons are formed during this reaction.
• These molecules are unstable and exist for such a short time that, they cannot be isolated.
• Each six carbon breaks down to form two molecules of 3-phosphoglycerate (3-PGA), a phosphorous containing compound with three carbon atoms.
• Since, the carbon of inorganic compound (CO₂) becomes the part of organic compound (RuBP) during this phase, hence, it is called carbon fixation.
• As the first stable compound in the Calvin cycle is a three carbon compound (3-PGA) that is why Calvin cycle is also known as C3 pathway.
• Reduction:
• In this phase six molecules of 3-phosphoglycerate (3-PGA) react with six ATP molecules, a phosphate from each ATP is transferred to each 3-PGA.
• In this way, 3-PGA molecules are changed into 1,3-Bisphosphoglycerate.
• These molecules are then reduced by the hydrogen of NADPH and finally glyceraldehyde 3 phosphate (G3P) molecules are produced.
• During this reduction process a phosphate group from each 1,3-Bisphosphoglycerate molecule is also given off.
• There are total six molecules of G3P are produced in this phase but only one molecule is released from the cycle while rest of the five molecules are used to regenerate the CO₂ acceptor molecules in the next phase.
• Regeneration of CO₂ acceptor:
• Five molecules of G3P from the previous phase are used to regenerate the RuBP (CO₂ acceptor) in this phase.
• These five molecules each containing three carbon atoms undergo a series of reactions in which three molecules of ribulose phosphate (RuP) each containing five carbon atoms are produced.
• When three molecules of RuP react with three molecules of ATP, a phosphate group from each ATP is transferred to each RuP.
• Ultimately RuP are converted into RuBP which again participate in the next cycle.
• The whole process of Calvin cycle indicates that there are three molecules of CO₂, six molecules of NADPH (reducing power) and nine molecules of ATP (assimilating power) are used to release just one molecule of G3P form the cycle.
• However, in order to produce a glucose molecule, two molecules of G3P are required.
• The overall process of Calvin cycle can be represented as:
• 3CO₂ + 6NADPH + 9ATP → G3P + 3H₂O + 9ADP + 9Pi + 6NADP⁺

60-Second Revision

• Dark reaction = stroma; does not need light directly
• Calvin cycle = carbon fixation, reduction, regeneration
• RuBisCO = enzyme that fixes CO₂
• 3-PGA = first stable product (3C)
• **G3P** = end product
• ATP and NADPH = used in the dark reaction
• **CO₂** = carbon source
• C3 plants = only Calvin cycle
• C4 plants = Calvin cycle + C4 pathway
• Melvin Calvin = discovered the Calvin cycle; Nobel Prize 1961
• Warning: The dark reaction occurs during the day as well. The Calvin cycle produces G3P, not glucose directly. RuBisCO fixes CO₂. 9 ATP and 6 NADPH are used per G3P. C3 and C4 plants differ in their first stable product. Melvin Calvin won the Nobel Prize in 1961.