Chlorophyll molecules in photosystern I (PSI) and photosystern II (PSII) absorb light
energy. The energy excites electrons, raising their energy level so that they leave the
chlorophyll. The chlorophyll is said to be photo-activated.
PSII contains an enzyme that splits water when activated by light. This reaction is
called photolysis (‘splitting by light’). The water molecules are split into oxygen and
hydrogen atoms. Each hydrogen atom then loses its electron, to become a positively
charged hydrogen ion (proton), H+.
The electrons are picked up by the chlorophyll in PSII, to replace the electrons they lost.
The oxygen atoms join together to form oxygen molecules, which diffuse out of the
chloroplast and into the air around the leaf.
The light- dependent reactions. Credit: Pears education.
The electrons emitted from PSII are picked up by electron carriers in the membranes of
the thylakoids. They are passed along a chain of these carriers, losing energy as they
go. The energy they lose is used to make ADP combine with a phosphate group,
producing ATP. This is called photophosphorylation. At the end of the electron carrier
chain, the electron is picked up by PSI, to replace the electron the chlorophyll in PSI had
lost.
The electrons from PSI are passed along a different chain of carriers to NADP. The
NADP also picks up the hydrogen ions from the split water molecules. The NADP
becomes reduced NADP.
We can show all of this in a diagram called the Z-scheme. The higher up the diagram,
the higher the energy level. If you follow one electron from a water molecule, you can
see how it
• is taken up by PSII
• has its energy raised as the chlorophyll in PSII absorbs light energy
• loses some of this energy as it passes along the electron carrier chain
• is taken up by PSI
• has its energy raised agaln as the chlorophyll in PSI absorbs light energy
• becomes part of a reduced NADP molecule
Video: Photosynthesis
https://www.youtube.com/watch?v=YeD9idmcX0w