Chloroplasts Part 2

Overview of Photosynthesis

  • Photosynthesis is composed of light-dependent and light-independent reactions (Calvin Cycle).

Light-Dependent Reactions

  • Location: Thylakoids in the chloroplasts.

  • Function: Convert light energy into chemical energy (ATP and NADPH).

Light-Independent Reactions (Calvin Cycle)

  • Location: Stroma of the chloroplasts (fluid-filled area surrounding thylakoids).

  • Description: Does not require direct sunlight, but depends on ATP and NADPH produced during light-dependent reactions.

  • Goal: Carbon fixation - converting inorganic carbon (CO₂) into organic molecules (e.g., carbohydrates).

Carbon Fixation

  • Process of CO₂ entering the Calvin Cycle.

  • CO₂ is attached to ribulose bisphosphate (RuBP), a five-carbon molecule.

  • Enzyme: Ribulose bisphosphate carboxylase/oxygenase (commonly known as Rubisco) catalyzes this reaction.

  • Rubisco: The most abundant enzyme on Earth, critical for carbon fixation.

  • Result: Formation of a short-lived six-carbon intermediate that splits into two three-carbon molecules:

    • Three-phosphoglycerate (3-PGA).

Reduction Phase

  • ATP and NADPH from light reactions are utilized to convert each 3-PGA molecule into glyceraldehyde-3-phosphate (G3P).

  • G3P: A high-energy sugar molecule fundamental for carbohydrate synthesis.

  • For every three molecules of CO₂ entering the cycle, one molecule of G3P exits to form glucose and other carbohydrates.

Cycle Continuation

  • Remaining G3P molecules are recycled to regenerate RuBP, allowing the cycle to continue.

  • This step also requires ATP to complete the energy utilization loop.

Overall Inputs and Outputs of the Calvin Cycle

  • Inputs:

    • CO₂ (as carbon source)

    • ATP (as energy source)

    • NADPH (as electron donor)

  • Outputs:

    • G3P (which can be converted into glucose and other organic compounds)

    • ADP

    • NADP⁺ (recycled back to thylakoids for re-energizing in light reactions)

Summary of Process

  • Light-dependent reactions charge the "batteries" (ATP and NADPH).

  • Calvin Cycle discharges these batteries to build sugars.

  • The entire process enables plants to synthesize organic compounds that support all life forms.

  • Elements of the process include:

    • Carbon fixation

    • Reduction phase

    • Utilization of components from the light phase to power light-independent reaction.