Photosynthesis - Light Reactions

Step 1 - Sunlight Absorption

Sunlight is absorbed by pigments (mostly chlorophyll)in Photosystem II (PSII). This sunlight excites electrons in chlorophyll molecules, raising their energy state.

Photolysis (water splitting)

Water molecules are split into oxygen, protons (H+), and electrons. This replaces the excited electrons. Produces oxygen gas which is then released into the atmosphere.

Step 3 - Electron Transfer Chain (ETC) and ATP Synthesis

High-energy electrons move down the ETC, losing energy as they progress. This energy pumps the protons (H+) into the thylakoid space, creating a proton gradient.

Build-up of protons drives ATP production through ATP synthase (progress called chemiosmosis).

Step 4 - Photosystem I (PSI) and NADPH formation

Electrons from ETC reach PSI. They are re-energised by more light absorption. High-energy electrons are transferred into NADP+, reducing it to NADPH (a molecule that will be used in the next stage).

Step 5 - End products

End products include ATP, NADPH (electron carriers), and oxygen (byproduct released into atmosphere). The ATP and NADPH molecules will eventually be used to power the Calvin Cycle.