Light Reaction (Hill Reaction)

Light reaction (Hill's reaction)

  • Photochemical Phase

In the photochemical phase, light reactions occur, involving:

- Light absorption

- Water splitting

- Oxygen release

- Formation of high-energy chemical intermediates: ATP and NADPH. 

  • Protein Complexes

- Several protein complexes are involved in light reactions.

- Pigments are organized into two discrete photochemical light harvesting complexes (LHC) within Photosystem I (PS I) and Photosystem II (PS II). 

  • Photosystem Organization

- Photosystems are named based on the sequence of their discovery, not their functional order.

- Each photosystem contains pigments organized into LHCs, consisting of hundreds of pigment molecules bound to proteins.

- Pigments within LHCs absorb different wavelengths of light, making photosynthesis more efficient. 




  • Reaction Centre

- Within each photosystem, a single chlorophyll a molecule forms the reaction centre.

- The reaction centre differs in PS I and PS II:

1. In PS I, the reaction centre chlorophyll a has an absorption peak at 700 nm, known as P700.

2. In PS II, the reaction centre chlorophyll a has an absorption peak at 680 nm, known as P680.


Electron Transport in Photosynthesis: The Z Scheme

  • Photosystem II (PS II)

- Reaction centre chlorophyll a absorbs red light at 680 nm, exciting electrons to a higher energy level.

- Excited electrons are transferred to an electron acceptor and enter an electron transport system consisting of cytochromes. 

  • Electron Movement

- Electron transport occurs downhill in terms of redox potential.

- Electrons pass through the electron transport chain without being used up, eventually reaching Photosystem I (PS I). 

  • Photosystem I (PS I)

- Electrons in the reaction centre of PS I are excited by red light at 700 nm.

- Excited electrons are transferred to another acceptor molecule with a higher redox potential. 

  • NADP+ Reduction

- Excited electrons from PS I are transferred to NADP+ , reducing it to NADPH + H+.

- NADPH + H+ serves as a carrier of high-energy electrons for later stages of photosynthesis. 

  • Z Scheme

- The entire process of electron transfer, starting from PS II, uphill to the acceptor, down the electron transport chain to PS I, and finally to NADP+ , is known as the Z scheme.

- The name "Z scheme" comes from the characteristic shape formed when all carriers are placed in sequence on a redox potential scale. 




Splitting of Water

  • Continuous Electron Supply

- Photosystem II (PS II) continuously supplies electrons to replace those moved to Photosystem I (PS I) during electron transport.

- The electrons needed for this replenishment are obtained through the splitting of water molecules. 

  • Water Splitting Process

- Water splitting occurs within PS II, located on the inner side of the thylakoid membrane.

- Water (H2O) is split into protons (H+), oxygen (O2), and electrons (e-).

The equation representing water splitting is: 2 H2O → 4H+ + O2 + 4e- 

  • Location of Proton and Oxygen Release

- Protons and oxygen formed during water splitting are likely to be released:

Protons (H+) and oxygen (O2) are released into the lumen, the inner compartment of the thylakoid membrane.

This release occurs on the inner side of the thylakoid membrane, where PS II is situated.