Photosynthesis Study Notes

Overview of Photosynthesis

  • Photosynthesis involves two main processes:
    • Light Reactions
    • Calvin Cycle

Chloroplast Structures

  • Key structures in chloroplasts:
    • Thylakoid Membrane
    • Plays a significant role in photosynthesis by hosting the light reactions.
  • Electron carriers are utilized in the thylakoid membrane during photosynthesis.

Light Reactions

  • Light reactions occur in the thylakoid membranes.

Key Concepts of Light Reactions

  • Purpose:

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

  • Involves two major components:

    • Pigments (e.g., Chlorophyll a and Chlorophyll b):
    • Absorb light energy.
    • Photocystins:
    • Major components of the reaction center.

Photosystems in Light Reactions

  • Photosystem II (PSII):

    • Upon absorbing light, photosystem II causes excitation of electrons.
    • Electrons are ejected from the reaction center:
    • Specifically, a special pair of chlorophyll molecules donates electrons.
    • Process:
    • Light strikes pigments in PSII, exciting electrons.
    • The ejected electron moves to an electron acceptor.
    • Water splitting occurs, providing electrons to replenish those lost during excitation:
    • During this process, oxygen is released into the atmosphere.
    • The electron is transferred through the electron transport chain (ETC) to produce ATP.

  • Photosystem I (PSI):

    • Similar excitation of P700 chlorophyll occurs.
    • The lost electron is replaced by an electron from PSII.
    • The electron moves to an electron acceptor and leads to the formation of NADPH.

Electron Transport Chain (ETC)

  • The electron transport chain connects PSII to PSI.
  • Generates an electrochemical gradient used to synthesize ATP via ATP Synthase.

Summary of Processes in Light Reactions

  • Light strikes PSII:
    • Causes the removal of electrons from P680.
    • Splitting of water replenishes electrons in PSII and produces O2.
  • ATP is generated from electron transport and chemiosmosis.
  • In PSI:
    • Excited electrons from P700 lead to NADPH production.

Calvin Cycle Overview

  • The Calvin Cycle follows the light reactions and requires ATP and NADPH produced during those processes.

  • Three main phases:

    1. Carbon Fixation:
    • Involves the enzymatic incorporation of CO2 into an organic molecule (Rubisco enzyme is critical).
    1. Reduction Phase:
    • The energy from ATP and NADPH is used to reduce 3-phosphoglycerate (PGA) to glyceraldehyde-3-phosphate (G3P).
    1. Regeneration of RuBP:
    • Ensures the cycle can continue optimizing carbon fixation.

  • Net Gain:

    • Each cycle processes 3 molecules of CO2 yielding a net gain of 1 G3P after several cycles.

Important Notes

  • The initial carbon fixation in the Calvin cycle is crucial for producing sugars and energy-storing molecules.
  • There might be instances where not all light reactions immediately feed into the Calvin Cycle due to the behavior of Rubisco.