Calvin Cycle

Major Highlights of the Calvin Cycle:

  • occurs in the stroma

  • CO2 is “fixed” from the atmosphere and converted into sugars

  • utilizes the energy from ATP & electrons from NADPH (both from light reaction)

  • it’s a cyclical metabolic pathway bc it regenerates the starting products (RuBP)

3 Phases of the Calvin Cycle:

  1. Carbon Fixation

  2. Reduction (Synthesis of Triose Phosphate)

  3. Regeneration

  • PAY ATTENTION TO CARBONS

Carbon Fixation:

  • the process of attaching a CO2 to a 5-carbon sugar called Ribulose biphosphate (RuBp)

  • 3 CO2 + 3RuBp total per cycle/”turn”

  • carbon fixation process is catalyzed by RuBisCO (enzyme)

Rubisco

Identify the reasons it’s necessary for rubisco to be in high concentrations in the stroma:

  • it’s slow and doesn’t work very efficiently

  • high energy requirement for Calvin Cycle

Describe the process of photorespiration:

  • Rubisco can mistakenly add an O2 instead of a CO2

    • makes Rubisco even LESS efficient

  • if O2 is added, the molecule CANT proceed through the rest of the Calvin Cycle —> photorespiration

Describe why Rubisco works best when there’s a high concentration of CO2 in the stroma:

  • to reduce the chance of adding an O2 to RuBp accidentally

Describe what happens after the initial carbon fixation process:

  • each of the resulting 6-carbon compounds breaks into 2 × 3-carbon compounds (glycerate 3-phosphate)

Identify the inputs and outputs of the Carbon Fixation phase:

  • Inputs: 3 x RuBp & 3 x CO2

  • Output: 6 x GP (glycerate 3-phosphate)

Reduction

Describe what happens during the reduction phase:

  • each GP molecule is converted into a TP (triose phosphate)

  • TP = Glyceraldehyde 3-phosphate (G3P)

  • each molecule requires energy from 1 ATP molecule and the electrons from 1 NADPH molecule

Inputs and Outputs of the reduction phase:

  • Input: 6 x GP, ATP, NADPH

  • Output: 6 x TP, ADP, NADP+

Describe why this phase is called “reduction”

  • bc GP gains electrons as it’s converted into TP

State what exits the cycle at the end of this phase & what continues through:

  • at the end: 6 x those phosphates —> and 1 of them will exit the cycle

  • 5 of those phosphates will stay in the cycle & go to regeneration

Regeneration

Describe what happens during the regeneration phase:

  • recreates the CO2 receptor (RuBp) —> allows for the cycle to continue in a cyclical manner

State the inputs and outputs:

  • Inputs:

  • Outputs:

Describe why this phase is called “regeneration”:

  • bc the triose phosphate gets turned into glucose after it exits the Calvin Cycle

Making Glucose and other organic molecules

  • to successfully create 1 molecule of glucose —> need: any other organic molecule that plants need to synthesize

  • other carbohydrates, amino acids, and nucleotides can be made from the outputs of the Calvin Cycle

Light-independent v. Light-dependent reactions

  • the light reaction is called the light dependent reaction

    • makes sense bc they need light

    • doesn’t make sense bc they also need other things —> water, ADP

  • the calvin cycle is also called the light-independent reaction

    • makes sense bc it doesn’t directly use light

    • doesn’t make sense bc it uses the products from the light reaction —> ATP

Light and CO2 Availability

If there’s no light available: impact on photosynthesis—>

  • immediate: light reaction shuts down

  • delayed: Calvin Cycle shuts down bc there’s no ATP or NADPH being produced to fuel it

If there’s no CO2 available —> impact on photosynthesis:

  • immediate: no Calvin Cycle bc it directly goes into the CC

  • delayed: no light reaction bc there’s no empty molecules (NADP+ or ADP) to go back into the light reaction