What mechanism transfers excitation energy between chlorophyll molecules in the light-harvesting complex?

Förster resonance energy transfer transfers excitation energy between chlorophyll molecules in the light-harvesting complex. In this nonradiative process, an excited chlorophyll donor passes energy to a nearby acceptor through dipole–dipole coupling rather than emitting a photon that another molecule absorbs.

Light-harvesting complexes contain proteins arranged with chlorophylls and carotenoids in the thylakoid membrane. Their pigments absorb different portions of sunlight, then funnel that energy through successive transfers toward a reaction-center chlorophyll, where charge separation begins the light reactions. This arrangement lets photosynthetic organisms collect more light than the reaction center could capture alone.

The process is named for German physicist Theodor Förster, whose 1948 work described intermolecular energy migration. FRET is extremely distance-sensitive: efficiency falls approximately with the sixth power of donor–acceptor distance and commonly operates across roughly 1–10 nanometres. A common mix-up is calling it electron transfer; FRET moves excitation energy while the electron remains associated with its original molecule.