Photosynthesis

1. Overview of Photosynthesis

Photosynthesis is the biochemical process by which photoautotrophic organisms (such as plants, algae, and cyanobacteria) convert light energy into chemical energy stored in glucose.

  • Overall Chemical Equation:
      6CO2+6H2O→C6H12O6+6O26CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2

  • Key Organelle: Chloroplast

    • Thylakoids: Sac-like photosynthetic membranes arranged in stacks called grana. Site of light-dependent reactions.

    • Stroma: Dense fluid interior surrounding thylakoids. Site of the Calvin cycle.

    • Chlorophyll: Primary photosynthetic pigment located in the thylakoid membrane that absorbs blue and red light while reflecting green light.

2. Light-Dependent Reactions

Occur within the thylakoid membranes and require direct light energy.

  • Inputs: H2OH_2O, light energy, NADP+NADP^+; ADP+PiADP + P_i

  • Outputs: O2O_2, NADPHNADPH, ATPATP

  • Key Processes:

    1. Photolysis of Water: Light energy splits water molecules to release electrons, protons, and oxygen gas:
           2H2O→4H++4e−+O22H_2O \rightarrow 4H^+ + 4e^- + O_2

    2. Photosystem II (PSII) & Photosystem I (PSI): Absorption of photons excites electrons, moving them down an electron transport chain (ETC).

    3. Chemiosmosis & ATP Synthesis: A proton gradient (H+H^+ concentration) is established across the thylakoid membrane, driving ATP synthesis via ATP synthase.

    4. NADPHNADPH Formation: Final electron acceptor NADP+NADP^+ is reduced to NADPHNADPH.

3. Light-Independent Reactions (Calvin Cycle)

Occur in the stroma and do not require direct light, using energy stored from the light-dependent reactions.

  • Inputs: CO2CO_2, ATPATP, NADPHNADPH

  • Outputs: Glyceraldehyde 3-phosphate (G3PG3P) (used to synthesize C6H12O6C_6H_{12}O_6), ADP+PiADP + P_i, NADP+NADP^+

  • Three Main Stages:

    1. Carbon Fixation: CO2CO_2 combines with ribulose 1,5-bisphosphate (RuBP), catalyzed by the enzyme RuBisCO.

    2. Reduction: ATPATP and NADPHNADPH convert 3-phosphoglycerate (3-PGA) into G3PG3P.

    3. Regeneration of RuBP: Remaining G3PG3P molecules use additional ATPATP to regenerate RuBP, allowing the cycle to continue.

4. Factors Affecting Photosynthesis Rate

  • Light Intensity: Increases the rate up to a saturation point where light is no longer the limiting factor.

  • CO2CO_2 Concentration: Higher concentration increases fixation rate until enzymes become saturated.

  • Temperature: Affects enzymatic activity (e.g., RuBisCO); optimal at moderate temperatures, but denatures enzymes at extreme high temperatures.