2 Light Dependent Reactions

Light Dependent Reactions

The light dependent reactions, a crucial stage in photosynthesis, take place within the thylakoid membranes of chloroplasts and involve several complexes, carrier molecules, and enzymes.

Key Components

  • Photosystem II (PS II)

    • Functions with Plastocyanin and Plastoquinone.

  • Cytochrome Complex

  • NADP+ Reductase

  • Ferredoxin

When used together, these components facilitate the process referred to as linear/non-cyclic electron transport. The reactions are initiated by photons of light exciting electrons, leading to the splitting of water molecules into electrons, protons, and oxygen gas.

Linear/Non-Cyclic Electron Transport

Mechanism Overview

  1. Light Absorption

    • Light hits the antenna complex, energizing P680 in PS II, leading to its excited state, P680*.

  2. Electron Transfer

    • The excited electron is captured by a primary acceptor, resulting in P680+, a strong oxidizer.

    • P680+ pulls an electron from a water molecule through a water-splitting complex, generating protons and oxygen.

Photosystem II Operations

  • As electrons are transferred, the negatively charged acceptor transfers an electron to Plastoquinone (PQ), and for each oxidized water molecule, the process can happen twice.

  • PQ shuttles electrons to the cytochrome complex while also transporting protons, which leads to a proton concentration gradient in the lumen.

Electron Transport Continuation

Cytochrome Complex Role

  • Cytochrome Complex passes electrons to Plastocyanin, which carries them to Photosystem I (PS I).

  • PS I absorbs more light, energizing P700*, the excited state of the pigment.

  • The electron is transferred to the primary acceptor of PS I, resulting in a charged P700+ molecule, which then becomes neutral again by receiving an electron from Plastocyanin.

Ferredoxin Functionality

  • The excited electrons move to Ferredoxin, an iron-sulfur protein, transferring electrons to NADP+ Reductase.

  • The enzyme reduces NADP+ to NADPH, adding protons from the stroma, resulting in NADPH formation and decreased proton concentration in the stroma.

Proton Gradient and ATP Formation

Chemiosmotic Synthesis

  • The proton motive force generated by the gradient across the thylakoid membrane drives protons back into the stroma through ATP synthase, synthesizing ATP from ADP and inorganic phosphate (Pi).

  • Mechanisms for Establishing Proton Gradient:

    • Protons are taken into the lumen during PQ’s redox cycling.

    • Water splitting contributes additional protons.

    • Removal of protons during NADPH formation decreases stroma proton levels.

Outcomes of the Light Dependent Reactions

  • High-energy electrons are transported from low-energy water molecules to become NADPH, storing energy for the Calvin cycle.

  • The overall reaction can be summed as:

    • 2 H2O + 2 NADP+ + 3 ADP + 3 Pi → O2 + 2 NADPH + 3 ATP.

Cyclic Electron Transport

Alternative Pathway

  • PS I can function independently of PS II in cyclic electron transport.

  • Electrons cycle back from Ferredoxin to PQ, including ATP production without NADPH formation, which helps prevent NADPH excess in the stroma.

Summary of Linear vs. Cyclic Transport

  • Linear transport produces NADPH and ATP in a 1:1 ratio; however, more ATP is needed during the Calvin cycle, promoting cyclic electron transport to maintain balance.