Dark Reaction (Blackman Reaction)

Dark Reaction (Blackman Reaction)

  • ATP and NADPH, produced during the light-dependent reactions, are utilized in the biosynthetic phase of photosynthesis. 
  • The biosynthetic phase involves the synthesis of sugars from CO2 and H2O, driven by ATP and NADPH. 
  • Dependence on Light Reaction Products:

- The biosynthetic phase does not directly rely on the presence of light but depends on the products of the light reactions, namely ATP and NADPH.

- Verification of this dependency can be observed by the cessation of biosynthetic processes when light becomes unavailable, resuming upon reintroduction of light. 

  • Misnomer of "Dark Reaction":

- Calling the biosynthetic phase the "dark reaction" may be a misnomer because it doesn't necessarily occur only in darkness but rather depends on the availability of light reaction products.

- The term "dark reaction" could be misleading as it implies that the process occurs solely in the absence of light. 

  • Utilization of ATP and NADPH:

- CO2 is combined with H2O to produce sugars or (CH2O)n.

- The first product formed when CO2 is fixed is 3-phosphoglyceric acid (PGA), containing three carbon atoms.

- The Calvin cycle, named after Melvin Calvin, describes the complete biosynthetic pathway of photosynthesis, with PGA identified as the initial product.

  •  Different Pathways of CO2 Assimilation:

- Research revealed that in certain plants, the first stable product of CO2 fixation is oxaloacetic acid (OAA), containing four carbon atoms.

- Two main pathways of CO2 assimilation were identified: the C3 pathway, where PGA is the first product, and the C4 pathway, where OAA is the initial product.

- These pathways are associated with distinct characteristics observed in different groups of plants.

 

The Primary Acceptor of CO2

Scientists aimed to identify the molecule that accepts CO2 during the biosynthetic phase of photosynthesis, leading to the formation of PGA. 

  • Identification of Acceptor Molecule:

- Initially, scientists expected the primary acceptor to be a 2-carbon compound, assuming that the first product (PGA) contained three carbon atoms.

- However, studies revealed that the actual acceptor molecule is a 5-carbon ketose sugar called ribulose bisphosphate (RuBP).

- The discovery of RuBP as the primary acceptor molecule was unexpected and required extensive experimentation. 

  • Significance of RuBP:

- RuBP serves as the starting point for carbon fixation during the Calvin cycle, facilitating the attachment of CO2 molecules.

- Upon accepting CO2, RuBP undergoes a series of enzymatic reactions, leading to the formation of PGA, a crucial step in carbon assimilation. 

  • Experimental Process:

- Scientists conducted numerous experiments over an extended period to determine the identity of the primary acceptor molecule.

- The discovery of RuBP as the acceptor molecule provided valuable insights into the biochemical pathways of photosynthesis.

 

The Calvin Cycle:

 

The Calvin cycle, named after Melvin Calvin, is a series of biochemical reactions that occur during the biosynthetic phase of photosynthesis.

It operates in all photosynthetic plants, irrespective of whether they follow C3 or C4 pathways. 

  • Stages of the Calvin Cycle:

Carboxylation:

- Carboxylation is the process of fixing CO2 into a stable organic intermediate, facilitated by the enzyme RuBP carboxylase (RuBisCO).

- RuBisCO catalyzes the carboxylation of ribulose bisphosphate (RuBP) to form two molecules of 3-phosphoglyceric acid (3-PGA).

- This step is essential for initiating carbon fixation in the Calvin cycle. 

Reduction:

- Reduction involves a series of reactions leading to the formation of glucose.

- Each CO2 molecule fixed requires the utilization of two molecules of ATP for phosphorylation and two molecules of NADPH for reduction.

- Six turns of the cycle and fixation of six CO2 molecules are necessary to produce one molecule of glucose. 

Regeneration:

- Regeneration of the CO2 acceptor molecule, RuBP, is crucial for the uninterrupted continuation of the Calvin cycle.

- One molecule of ATP is utilized for phosphorylation to regenerate RuBP, ensuring the cyclic nature of the pathway. 

 

 

 

  • Energy Requirements:

- For every CO2 molecule entering the Calvin cycle, three molecules of ATP and two molecules of NADPH are required.

- Cyclic phosphorylation may occur to balance the difference in the number of ATP and NADPH used in the dark reaction. 

  • Calculating Requirements for Glucose Synthesis:

- To produce one molecule of glucose, six turns of the Calvin cycle are required.

- Six CO2 molecules are fixed, leading to the utilization of 18 molecules of ATP and 12 molecules of NADPH. 

  • Summary of Input and Output:

- Input: Six CO2 molecules, 18 ATP, and 12 NADPH

- Output: One glucose molecule, 18 ADP, and 12 NADP

 

The C4 Pathway:

  • Plants adapted to dry tropical regions utilize the C4 pathway for carbon fixation. 
  • Despite initially fixing CO2 into oxaloacetic acid (OAA), they primarily employ the C3 pathway (Calvin cycle) for biosynthesis. 
  • Distinctive Features of C4 Plants:

1. Leaf Anatomy:

- C4 plants exhibit unique leaf anatomy known as "Kranz" anatomy.

- Bundle sheath cells, characterized by numerous chloroplasts and thick walls, surround the vascular bundles.

- Bundle sheath cells lack intercellular spaces and play a crucial role in CO2 concentration. 

2. Physiological Traits:

- C4 plants are adapted to higher temperatures and respond well to intense light.

- They demonstrate improved productivity and resilience to environmental stressors compared to C3 plants.

- Notably, C4 plants avoid photorespiration, a process that reduces photosynthetic efficiency in C3 plants under certain conditions. 

  • Hatch and Slack Pathway:

- Also known as the C4 pathway, it operates in a cyclic manner to efficiently concentrate CO2 for Calvin cycle. 

- Steps of the Pathway:

1. CO2 Fixation: Phosphoenolpyruvate (PEP), a 3-carbon molecule, serves as the primary CO2 acceptor in mesophyll cells. 

2. PEP Carboxylation: PEP carboxylase catalyzes the fixation of CO2 to form oxaloacetic acid (OAA) in mesophyll cells. 

3. Formation of C4 Acids: OAA is converted into 4-carbon compounds like malic acid or aspartic acid in mesophyll cells. 

4. Transport to Bundle Sheath Cells: C4 acids are transported to bundle sheath cells for further processing. 

5. Release of CO2: C4 acids are broken down in bundle sheath cells to release CO2 and a 3-carbon molecule. 

6. Regeneration of PEP: The 3-carbon molecule is transported back to mesophyll cells and converted back to PEP, completing the cycle. 

7. Calvin Pathway: CO2 released in bundle sheath cells enters the Calvin pathway, where sugars are synthesized. 

 

 

 

  • Distribution of Calvin Pathway:

- In C3 plants, the Calvin pathway occurs in all mesophyll cells.

- In C4 plants, the Calvin pathway is restricted to bundle sheath cells, while mesophyll cells primarily facilitate CO2 concentration through the C4 pathway.