Summary for the Interaction of Light with Biological Molecules

Cells and organisms that make up biological systems stay alive by a constant input of energy. For almost all organisms, this energy comes directly or indirectly from sunlight. Sunlight is in the form of individual packets of energy called photons, which have characteristic wavelengths. The photons in visible light range from 400 nm (blue) to 700 nm (red); the shorter wavelength photons have more energy than those with longer wavelengths.  Molecules with conjugated double bonds can absorb the energy in visible light photons, moving an electron from the ground state to an excited state. Once in an excited state the energy can be lost through the four de-excitation processes

  1. Thermal Decay
  2. Fluorescence
  3. Energy transfer
  4. Conversion to triplet state (photochemistry).

Plants use photochemistry to carry out the redox reactions which produce photosynthetic energy. However the high energy in the excited molecules of the photosynthetic apparatus is great enough to produce biological damage if it is not properly utilized.