PHOTOSYNTHESIS 3

Learning Objective 10 (LO10)

• Students will be able to outline the three phases of the Calvin Cycle and its product.

Phases of the Calvin Cycle

I. Fixation of CO₂ Phase

• Definition: This phase involves the incorporation of carbon dioxide (CO₂) into an organic molecule.

• Process: CO₂ is fixed by reacting with ribulose bisphosphate (RuBP) to form 3-phosphoglycerate (3PGA) with the help of the enzyme rubisco.

II. Reduction Phase

• Description: This phase utilizes energy, in the form of ATP and NADPH, to convert 3PGA into glyceraldehyde-3-phosphate (G3P), which is a three-carbon (3C) sugar.

• Inputs for this phase:

  • ATP

  • NADPH

• Product of this phase:

  • 2 G3P molecules are generated for every 6 cycles of the Calvin cycle, usable for glucose synthesis or energy storage.

III. Regeneration Phase

• Function: In this phase, G3P is used to regenerate RuBP, allowing the cycle to continue.

• ATP is used again in this phase to restore the substrates to phase I.

• This regeneration process is crucial to ensure that CO₂ fixation can occur continuously in the cycle.

Summary of the Calvin Cycle Outcomes

• After completing 6 cycles of the Calvin Cycle:

  • 3 CO₂ molecules are incorporated into the cycle

  • Production of 6 molecules of 3PGA, which are then converted into G3P and ultimately used for glucose production.

  • 6 ATP and 2 NADPH are consumed directly in the cycle to produce energy and reducing power for the process.

• Final Product:

  • 2 G3P (3C) molecules can exit the cycle and are utilized to combine to form a glucose molecule (C₆H₁₂O₆), which is a hexose (6C).

  • The overall equation summarizing the Calvin Cycle is:
    $ext{6 CO}<em>2 + ext{12 NADPH} + ext{18 ATP} ightarrow ext{C}</em>6 ext{H}<em>{12} ext{O}</em>6 + ext{12 NADP}^+ + ext{18 ADP} + ext{18 Pi}$

• Special Note: G3P molecules not utilized in glucose synthesis are used to regenerate RuBP and continue the cycle.