What Happens When a Chlorophyll Molecule Absorbs Light?

By Rachel Asher
What Happens When a Chlorophyll Molecule Absorbs Light?

A Photon of Light Excites the Chlorophyll Molecule

When a chlorophyll molecule absorbs light, the process of photosynthesis, or the transfer of light into sugar, begins. Chlorophyll is a green liquid inside one part of a plant cell: the chloroplast.

When light hits the chlorophyll molecule, it becomes excited. This energy passes through other chlorophyll molecules, and into the reaction center of Photosystem II: this is the location of the first stage of photosynthesis, and the electron transport chain.

For each photon of light that enters and excites a chlorophyll molecule, one electron is released from the reaction center of Photosystem II. When two electrons are released, they are transferred to Plastoquinone Qb, a mobile carrier, which picks up two protons and starts moving towards the Cytochrome b6f complex. Cytochrome b6f, like Photosystem II, is a complex where photosynthesis processes occur.

Plastoquinone Qb on the Move with Two Electrons

While Plastoquinone Qb is moving, the two electrons that were lost in Photosystem II have to be replaced. This is done by splitting water molecules. Hydrogen ions and oxygen are released as a by-product of replacing the two electrons.

Passing through the Cytochrome b6f Complex

Finally, Plastoquinone Qb reaches its destination: Cytochrome b6f complex, which is another complex in the electron transport chain. Here, it releases the two protons into lumen space (open space between a plant cell's organelles and molecules) and releases the two electrons into the Cytochrome b6f complex. The electrons travel through the complex, two hydrogen ions are released, and the electrons arrive at Plastocyanin, a mobile carrier like Plastoquinone Qb, which takes the electrons over to Photosystem I.

Electron Transport in Photosystem I and the Production of ATP

In Photosystem I, a complex in the electron transport chain that works similarly to Photosystem II, the chlorophyll molecules are also stimulated by light, in turn resulting in the release of electrons. Two electrons are transferred to Ferrodoxin, then to an enzyme called FNR (Ferrodoxin NADP Reductase). The two electrons, and one hydrogen ion, are attached to NADP to produce NADPH. This entire process stimulates the production of ATP from ADP and Pi in ATP synthase.