HL Bio: Unit 3.7 - Light INdependent Reaction of Photosynthesis/ Calvin Cycle

Calvin Cycle

• Occurs in the stroma of the chloroplast

• Even if it is light independent it still requires the ATP and Reduced NADP produced in the light dependent reactions 

  • The two processes are interdependent as they require products from the other reactions 

Limiting reactions in different light intensities

• In low light intensity: Production of ATP and reduced NADP is restricted making it the limiting reaction

• In high light intensity: Everything is working at its optimum, however, carbon fixation is very slow making it the limiting factor

Three main Steps within the Calvin Cycle

1. Carbon Fixation by Rubisco

2. Reduction of Glycerate-3-phosphate

3. Regeneration of Ribulose Bisphosphate

Carbon Fixation by Rubisco

• Takes the carbon compound Ribulose biphosphate (RuBP) and carbonates it using a carbon dioxide molecules 

  • It takes the carbon and oxygen which is added to the RuBP forming 2 Glycerate-3-phosphate (3 carbon compounds)

    • (turns a 5 carbon molecules to a “6 carbon molecule”)

      • It breaks apart the 6 carbon compound and forms 2 three carbon compounds 

Carbon Dioxide

Readily dissolves in water and passes through the cell wall after diffusing from the atmosphere

Ribusco Fact

• Really slow enzyme so the stroma overcompensates by having very high concentration → only takes 3 carbon dioxides at once

  • The most abundant enzyme 

Reduction of Glycerate-3-phosphate

• Addition of the excess hydrogens by reduction to the glycerate-3-phosphate to replace the carboxyl group with an aldehyde group

  • Requires ATP to perform the reduction 

  • Per 3 ribulose biphosphates 2 atp and 2 reduced NADP is needed

• Requires reduced NADP to provide the electrons within the hydrogen atoms 

  • Oxygen removed from the carboxyl groups combines with the hydrogen from the reduced NADP → forms water

• Forming triose bipshosphate

Regeneration of Ribulose Bisphosphate

• Producing 2 triose biphosphate as the end product of the Calvin Cycle 

• RuBP is produced by utilising triose phosphate which also starts the process

  • Without RuBP the Calvin cycle wouldn’t occur 

  • Takes 5 triose phosphates (3 carbons) to produce 3 ribulose phosphates (5 carbons)

    • It’s like it spreads out the carbons 

    • 5 triose phosphates need to be reused to reproduce the RuBP (1 of every 6 triose phosphates made can be taken out of the cycle) so it takes 6 spins of the cycle to be able to take 2 triose phosphates out to produce 6-carbon molecules 

  • Then the 3 ribulose phosphates (5) takes 3 ATP energy and dephosphoralize them and takes the phosphate to form 3 ribulose bisphosphate

How many turns of the calvin cylce to produce a glucose molecule

The calvin cycle needs to turn 6 times to produce two trios phosphates that can be taken out to make one 6 carbon compound

Different things triose phosphate can produce

• Glucose: two triose phosphates together

• Sucrose: Glucose tends to be converted to sucrose for transportation from leaves to other parts 

• Starch: When glucose is produced more quickly than it is stored the glucose is stored as extra starch in the chloroplast which is then used up during times without light 

Different things triose phosphate can produce

• Fatty acids: produce fatty acids by Taking acetyl coenzyme A from the glycolysis and link reaction and putting it with two carbon acetyl groups 

• Triglycerides: Triosphaste can also produce glycerol which is then linked to the fatty acids and produces triglycerides