Photosynthesis

Photosynthesis: Goals and Overview

  • Goals for Today:

    • Understand how photosynthetic pathways evolved.

    • Understand how the history of Earth was affected by the evolution of these pathways.

Overview of Photosynthesis

A. Photosynthetic Pathways and Energy Transfer

  1. Step One: Transferring radiant energy to chemical energy.

    • Energy of Photon:

      • An electron (e-) is excited by the energy of a photon, initiating the energy transfer process.

B. Storing Chemical Energy

  1. Step Two: Storing that chemical energy in the bonds of molecules.

    • Chemical Reaction:

      • 6 CO2 + ATP ightarrow C6 ext{(glucose)} + ADP + P

    • This represents the transformation and storage of energy during photosynthesis.

Light Dependent Reactions

A. Step 1: The Light Dependent Reaction

1. Primitive Systems

  • a. Cyclic Phosphorylation:

    • Used by photoheterotrophs:

      • Examples include purple non-sulfur bacteria, green non-sulfur bacteria, and heliobacteria.

      • These organisms ‘eat’ organic molecules for carbon and energy but also synthesize ATP using sunlight energy.

2. Electron Transport Mechanism

  • Electron Transport Chain (ETC):

    • The electron transport chain is located in the inner membrane, akin to mitochondria.

    • Chemiosmosis occurs:

      • An excited electron is passed down the electron transport chain, with $H^+$ ions being pumped out, which then flood back in to synthesize ATP.

      • ADP + P
        ightarrow ATP

3. Sulfur Bacteria

  • b. Cyclic Phosphorylation (Cyclic Process):

    • Electron (e-) from Photosystem I (PS I) can be recycled or passed to an electron acceptor.

    • Upon excitation by sunlight, the electron is used to manufacture ATP, returning to PS I in a cyclic manner.

  • c. Reduction of NADP:

    • If the electron is passed to NADP, it reduces NADP to NADPH, enabling the continuation of energy transfer.

    • NADP + 2H
      ightarrow NADPH

    • If NADP is reduced, the electron from PS I is not recycled.

4. Electron Source

  • To sustain the process, electrons must be drawn from another molecule:

    • Example with H2S:

      • H_2S
        ightarrow 2e + 2H^+ + S

    • The photosystem can strip electrons from H2S, resulting in sulfur gas being released as a waste product.

    • Environmental Limitation:

      • These organisms can only inhabit environments where H2S is available (e.g., sulfur springs).

5. Evolution to More Efficient Systems

  • Advancements in photosynthetic systems enabled organisms to strip electrons from water, allowing them to thrive in diverse habitats.

  • Advanced Systems:

    • Photosystem II (PS II):

      • Utilizes water as the electron donor instead of H2S, leading to the release of oxygen gas as a waste product.

      • 2H2O ightarrow 4e + 4H^+ + O2

B. Overall Summary of Light Dependent Reactions

  1. The reactions transform radiant energy into chemical bond energy in ATP and NADPH, preceding the light-independent reactions for glucose synthesis.

Light Independent Reactions (Calvin Cycle)

A. Purpose and Process

  1. The aim is to create a larger, more stable organic molecule and use the energy from ATP to form stable carbon-carbon (C-C) bonds in glucose.

B. Key Steps

  1. 6 CO2 + 6 RuBP Reaction:

    • A CO2 molecule binds to Ribulose bisphosphate (RuBP), forming an unstable 6-carbon molecule that splits into two 3-carbon molecules (3-PGA).

  2. Formation of Glucose:

    • Using the energy from ATP and the reduction potential of NADPH, 2 of the 12 PGA molecules transform into glucose.

    • Rearrangement of the remaining 10 C3 molecules regenerates 6 RuBP.

C. Molecule Transformation

  1. Full cycle visualization:

    • 6C5 ext{ (RuBP)} + 6CO2
      ightarrow 6C6 ightarrow 12C3
      ightarrow 2C3 ext{ (glucose)} + 10C3 + ATP + ADP + NADPH + NADP

    • This represents the creation and cycling process within the Calvin Cycle.

History of Photosynthesis

A. Evolution Timeline

  1. Photosynthesis began evolving early, at least 3.5 billion years ago. Evidence includes stromatolites that present bacterial mats, and earlier microfossils resembling cyanobacteria.

  2. Characteristics of the Earth's atmosphere changed concomitantly with photosynthesis development:

    • Notable drop in CO2 levels correlated with the Calvin Cycle and atmospheric changes.

B. Notable Events

  1. 2.3 billion years ago:

    • Earliest banded iron formations recorded show exposure of iron crystals to atmospheric oxygen, indicating oxygenic photosynthesis development.

  2. Carboniferous Period (354-290 million years ago):

    • The formation of significant fossil fuel deposits due to extensive biomass production from photosynthesis, limiting carbon returned to the atmosphere.

C. Modern Observations

  1. Current oscillations in O2 and CO2 levels reflect the balance between photosynthesis and organismal respiration:

    • Carbon dioxide levels: Climbed from 320 to 400 ppm, a 25% increase in 50 years.

    • Oxygen levels: Notably decreased by 70 ppm but remains abundant at 21%.

    • Projected levels: Expected to reach 426 ppm by February 2, 2025.

Conclusion

  • The evolution of photosynthesis has dramatically influenced Earth's atmosphere and ecosystems, illustrating the interconnectedness of biological processes and environmental evolution.