Lecture 20 Photosynthesis 3 Dark reactions, intro to glycolysis and cellular respiration_2_0

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Today's Topic

  • The Dark Reactions (In detail)

  • Relevant Reading: Chapter 8

Light Reactions Overview

  • Components:

    • Stroma

    • Thylakoid Membrane

    • Light Energy

    • Chlorophyll Reaction Center

    • H2O splitting produces 1/2 O2, including protons (H+).

    • Process:

      • Linear Electron Flow (LEF):

        • Produces about 1.2 ATP per NADPH.

        • Dark reactions utilize approximately 1.5 ATPs per NADPH.

        • Requires additional ATP production.

Cyclic Electron Flow (CEF)

  • CEF Process:

    • Instead of reducing NADP+, electrons from PS1 return to the proton pump.

    • Increases ATP production for use in dark reactions.

Dark Reactions (Calvin Cycle)

  • Location: Occur in the stroma of the chloroplast.

  • Requirements: NADPH & ATP (from light reactions), CO2.

  • Three Steps:

    1. Carbon Fixation: Converts CO2 into organic form (PGA).

    2. Reduction of PGA: Electrons from NADPH reduce PGA into G3P.

    3. Regeneration of RuBP: Necessary for the carbon fixation step.

Carbon Fixation Details

  • Initial Components: RuBP (5-C sugar, carbon acceptor) and CO2.

  • Process:

    • RuBP & CO2 bond to form a 6-C intermediate.

    • This is catalyzed by Rubisco enzyme.

    • The 6-C intermediate splits into two 3-C PGA molecules.

More on Rubisco

  • Rubisco:

    • Most abundant enzyme globally (~20 lbs per human).

    • Fixes about 100 billion tons of CO2 annually (200 trillion lbs).

Reduction of PGA

  • Using:

    • NADPH (electron source).

    • ATP (energy source).

  • Conversion:

    • PGA is reduced to G3P.

  • G3P Functions:

    • Used for glucose synthesis.

    • Used for RuBP regeneration.

Summary of Dark Reactions

  • Key Components:

    • Enzyme: Rubisco.

    • Produces a 6-C intermediate that breaks down into two 3-C PGA molecules.

    • G3P derived from NADPH electrons and ATP energy.

    • Some G3P used for glucose, others for regenerating RuBP.

Beyond Glucose Production

  • Dark Reactions Yield:

    • More than just glucose, including other sugars, amino acids, lipids, and nucleic acids.

Glucose as a Fuel Source

  • Energy Content:

    • Glucose: 686 kcal/mol.

    • ATP Hydrolysis: ∆G = -7.3 kcal/mol.

  • Energy Extraction:

    • Through glucose oxidation, which proceeds through two phases: Glycolysis and Cellular Respiration.

Glucose Oxidation Phases

  • Phase 1: Glycolysis:

    • Location: Cytoplasm.

    • Converts glucose to pyruvate.

  • Phase 2: Cellular Respiration (CR):

    • Location: Mitochondria.

    • Divided into three stages: Oxidation of pyruvate, Citric Acid Cycle (Krebs Cycle), Electron Transport Chain.