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The Process of Photosynthesis

The Process of Photosynthesis Overview

  1. Light-dependent reactions (Photochemical phase), which involve Photosystem II (PSII) and Photosystem I (PSI).
  2. Light-independent reactions (Calvin cycle), which use the ATP and NADPH produced by the light-dependent reactions to fix carbon into glucose.
Credit of Picture: https://mrkubuske.com/tag/dark-reactions/

Light-dependent Reactions

These reactions occur in the thylakoid membranes of the chloroplast, where light energy is captured by chlorophyll pigments and used to produce ATP, NADPH, and oxygen. Photosystem II (PSII) and Photosystem I (PSI) are key components in this process.

Photosystem II (PSII):

  • Role: PSII is the first protein complex involved in the light-dependent reactions.

  • Location: Thylakoid membrane.

  • Process:

    • Photon absorption: PSII absorbs light energy (photons) through chlorophyll and other pigments.
    • Excitation of electrons: This energy excites electrons in chlorophyll molecules, raising them to a higher energy state.
    • Water splitting (Photolysis): To replace the excited electrons, water molecules are split into oxygen (O₂), protons (H⁺), and electrons. This occurs in the oxygen-evolving complex (OEC) associated with PSII. The oxygen is released as a byproduct, which is critical for life on Earth.
    • Electron transport: The high-energy electrons are passed from PSII to the electron transport chain (ETC), which consists of various proteins embedded in the thylakoid membrane.

  • Raw Materials: Light energy (photons) and water (H₂O).

  • Products: Oxygen (O₂) and high-energy electrons.

Photosystem I (PSI):

  • Role: PSI is the second protein complex in the light-dependent reactions and is crucial for the production of NADPH.

  • Location: Thylakoid membrane.

  • Process:

    • Photon absorption: PSI absorbs light energy and excites electrons in chlorophyll pigments.
    • Electron transfer: The excited electrons are passed through another set of proteins in the electron transport chain, ultimately reaching NADP⁺ (nicotinamide adenine dinucleotide phosphate).
    • Reduction of NADP⁺: The high-energy electrons from PSI are used to reduce NADP⁺ to NADPH, a molecule that stores energy and will be used in the Calvin cycle.

  • Raw Materials: Light energy (photons) and electrons from PSII.

  • Products: NADPH.

Electron Transport Chain (ETC) and ATP Synthase

  • After the electrons leave PSII, they travel through the electron transport chain (ETC), where they move through various proteins like plastoquinone (PQ), cytochrome b6f complex, and plastocyanin (PC).
  • As electrons move along the ETC, protons (H⁺) are pumped across the thylakoid membrane into the thylakoid space, creating an electrochemical gradient (proton gradient).
  • The protons then flow back through ATP synthase, a protein complex, which uses the flow of protons to synthesize ATP from ADP and inorganic phosphate (Pi). This process is called photophosphorylation.

Summary of Light-dependent Reactions:

  1. Photosystem II (PSII) absorbs light, splits water, and releases oxygen while passing excited electrons to the ETC.
  2. The electron transport chain (ETC) pumps protons (H⁺) into the thylakoid space, creating a proton gradient.
  3. Photosystem I (PSI) absorbs light and produces NADPH by transferring excited electrons to NADP⁺.
  4. ATP synthase uses the proton gradient to generate ATP.

Raw Materials of Light-dependent Reactions:

  • Water (H₂O)
  • Light energy (photons)

Products of Light-dependent Reactions:

  • ATP
  • NADPH
  • Oxygen (O₂)

The Calvin Cycle (Light-independent Reactions)

After the light-dependent reactions, the ATP and NADPH produced are used in the Calvin cycle to fix carbon dioxide (CO₂) into organic molecules, eventually producing glucose.

Key Steps in the Calvin Cycle:

  1. Carbon Fixation: CO₂ is attached to ribulose bisphosphate (RuBP) by the enzyme rubisco.
  2. Reduction: ATP and NADPH are used to convert 3-PGA into G3P (glyceraldehyde-3-phosphate).
  3. Regeneration of RuBP: Some G3P molecules are used to regenerate RuBP, allowing the cycle to continue.

Summary of Photosynthesis with PSI and PSII:

  • Raw Materials: CO₂ (from the atmosphere), H₂O (from the roots), light energy.
  • Products: Glucose (C₆H₁₂O₆), O₂ (from water splitting), ATP, NADPH.

Photosystem II (PSII) is responsible for the initial absorption of light and water splitting, generating O₂ and electrons. Photosystem I (PSI) absorbs light energy and reduces NADP⁺ to NADPH, which is used in the Calvin cycle.

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