9.2 Light-independent reactions: Calvin cycle

The Calvin cycle is the light-independent phase of photosynthesis, fixing carbon dioxide into organic compounds. It occurs in the chloroplast stroma and involves three main stages: carbon fixation, reduction, and regeneration. RuBisCO, the key enzyme, catalyzes the initial step.

The cycle's efficiency depends on environmental factors like CO2 levels, temperature, and light intensity. It produces glyceraldehyde 3-phosphate (G3P), which forms glucose and other carbohydrates. Understanding the Calvin cycle is crucial for grasping plant metabolism and energy production.

Carbon Fixation in the Calvin Cycle

Overview and Initial Carbon Fixation

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• Calvin cycle occurs in the stroma of chloroplasts to fix atmospheric carbon dioxide into organic compounds
• Carbon fixation represents the first step of the Calvin cycle
• RuBisCO enzyme catalyzes the addition of CO2 to ribulose-1,5-bisphosphate (RuBP)
• Addition of CO2 to RuBP forms an unstable 6-carbon compound
• 6-carbon compound immediately splits into two 3-carbon molecules of 3-phosphoglycerate (3-PGA)

Reduction and Regeneration Phases

• Reduction phase follows carbon fixation
  • ATP and NADPH from light-dependent reactions convert 3-PGA into glyceraldehyde 3-phosphate (G3P)
  • G3P serves as a three-carbon sugar molecule
• Regeneration phase completes the cycle
  • Some G3P molecules regenerate RuBP
  • Regeneration ensures continuity of the carbon fixation process
• Stoichiometry of the Calvin cycle
  • Three CO2 molecules fixed produce one G3P molecule for glucose synthesis
  • Five G3P molecules regenerate three RuBP molecules

Carbon Fixation Efficiency and Environmental Factors

• Carbon fixation efficiency varies among plant species (C3, C4, and CAM plants)
• Environmental factors affecting carbon fixation
  • CO2 concentration in the atmosphere
  • Temperature (affects enzyme activity)
  • Light intensity (indirectly through the availability of ATP and NADPH)
• Photorespiration reduces carbon fixation efficiency in some conditions
  • Occurs when RuBisCO fixes oxygen instead of carbon dioxide
  • More prevalent in high temperature and low CO2 environments

Key Enzymes of the Calvin Cycle

RuBisCO and Carbon Fixation

• Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) serves as the primary enzyme of the Calvin cycle
• RuBisCO catalyzes carbon fixation by adding CO2 to RuBP
• Structure of RuBisCO
  • Large subunit contains the catalytic site
  • Small subunit enhances catalytic efficiency
• RuBisCO activation requires carbamylation and binding of magnesium ions
• RuBisCO activase enzyme regulates RuBisCO activity

Enzymes in the Reduction Phase

• Phosphoglycerate kinase catalyzes 3-phosphoglycerate phosphorylation
  • Uses ATP to convert 3-PGA to 1,3-bisphosphoglycerate
• Glyceraldehyde 3-phosphate dehydrogenase reduces 1,3-bisphosphoglycerate
  • Uses NADPH to produce glyceraldehyde 3-phosphate (G3P)
• Triose phosphate isomerase interconverts dihydroxyacetone phosphate and G3P
  • Maintains equilibrium between these triose phosphates

Enzymes in the Regeneration Phase

• Transketolase plays a crucial role in regeneration
  • Catalyzes transfer of two-carbon units between sugar phosphates
  • Involved in the formation of xylulose 5-phosphate and sedoheptulose 7-phosphate
• Phosphoribulokinase catalyzes the final step of RuBP regeneration
  • Phosphorylates ribulose 5-phosphate to ribulose-1,5-bisphosphate
  • Uses ATP as the phosphate donor
• Other enzymes involved in regeneration
  • Aldolase (fructose-bisphosphate aldolase)
  • Phosphatase (fructose-1,6-bisphosphatase)
  • Epimerase (phosphopentose epimerase)

RuBP Regeneration in the Calvin Cycle

Importance and Process of RuBP Regeneration

• Regeneration of ribulose-1,5-bisphosphate (RuBP) ensures continuous operation of the Calvin cycle
• RuBP serves as the primary CO2 acceptor molecule
• Complex series of reactions convert five out of six G3P molecules back into three RuBP molecules
• Key intermediates in the regeneration process
  • Xylulose 5-phosphate
  • Ribose 5-phosphate
  • Ribulose 5-phosphate
• Enzymatic reactions interconvert these sugar phosphates

Steps and Enzymes in RuBP Regeneration

• Transketolase transfers a two-carbon unit from fructose 6-phosphate to G3P
  • Forms xylulose 5-phosphate and erythrose 4-phosphate
• Aldolase combines erythrose 4-phosphate with dihydroxyacetone phosphate
  • Produces sedoheptulose 1,7-bisphosphate
• Phosphatase removes a phosphate group from sedoheptulose 1,7-bisphosphate
  • Forms sedoheptulose 7-phosphate
• Transketolase transfers a two-carbon unit from sedoheptulose 7-phosphate to G3P
  • Produces xylulose 5-phosphate and ribose 5-phosphate
• Phosphopentose epimerase converts xylulose 5-phosphate to ribulose 5-phosphate
• Phosphoribulokinase catalyzes the final step
  • Uses ATP to phosphorylate ribulose 5-phosphate to RuBP

Significance of RuBP Regeneration

• Ensures Calvin cycle can continue fixing carbon dioxide without external CO2 acceptor sources
• Makes the Calvin cycle a self-sustaining process
• Allows for efficient carbon fixation in varying environmental conditions
• Regeneration rate affects overall photosynthetic efficiency
• Regulated by various factors (light intensity, CO2 concentration, enzyme activity)

Glucose Production from the Calvin Cycle

Primary End Products and Glucose Synthesis

• Glyceraldehyde 3-phosphate (G3P) serves as the primary end product of the Calvin cycle
• G3P functions as a three-carbon sugar phosphate building block for various carbohydrates
• Glucose 6-phosphate formation occurs by combining two G3P molecules
• Glucose-6-phosphatase converts glucose 6-phosphate to glucose
• Excess G3P molecules not used for RuBP regeneration synthesize other carbohydrates
  • Sucrose (transport sugar in plants)
  • Starch (storage carbohydrate)
  • Cellulose (structural component of cell walls)

Carbohydrate Production and Plant Metabolism

• Calvin cycle provides the primary source of energy and carbon skeletons for plants
• Supports plant growth, development, and storage functions
• Carbohydrate production rate regulated by various factors
  • Light intensity
  • CO2 concentration
  • Water availability
  • Nutrient status
• Carbohydrate allocation varies based on plant developmental stage and environmental conditions

Glucose Utilization and Storage in Plants

• Glucose serves as an immediate energy source through glycolysis and cellular respiration
• Glucose conversion to other sugars
  • Fructose (through glucose isomerase)
  • Sucrose (combined with fructose via sucrose synthase)
• Starch synthesis for long-term energy storage
  • Amylose (linear glucose polymer)
  • Amylopectin (branched glucose polymer)
• Cellulose production for structural support
  • Beta-1,4-glycosidic bonds between glucose units
• Pentose phosphate pathway utilizes glucose 6-phosphate
  • Produces NADPH and ribose 5-phosphate for nucleotide synthesis