The thylakoids convert light energy into the chemical energy of ATP and NADPH. Light is a form of electromagnetic radiation. Like other forms of energy, light moves in rhythmic waves. The distance between crests of electromagnetic waves is the wavelength. Wavelengths of electromagnetic radiation range from less than a nanometer to more than a kilometer. The entire range of electromagnetic radiation is called the electromagnetic spectrum. The most important segment for life is a narrow band between 380 to 750nm, which is the band of visible life. While the light travels as a wave, some of the properties are those of a separate particle, which is the photon. Photons are not tangible objects but they have fixed quantities of energy. The amount of energy packed in a photon is inversely related to its wavelength. The shorter wavelengths of photons pack more energy. When a molecule absorbs a photon, one of those molecules electrons is elevated to an orbital with more potential energy. The electron moves from its ground state to an excited state. The only photons that a molecule can absorb are those energy matches exactly the energy difference between the ground state and excited state of this electron.

There is an energy difference among atoms and molecules, and due to this a particular compound absorbs only photons corresponding to specific wavelengths. Thus, each pigment has a unique absorption spectrum. Excited electrons are unstable. When they drop back down, they release heat energy. Each light-harvesting complex consists of pigment molecules (which may include chlorophyll a, chlorophyll b, and carotenoid molecules) bound to particular proteins. Together, these light-harvesting complexes act like light-gathering Ĺ“antenna complexes for the reaction center. When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule, the reaction center. At the reaction center is a primary electron acceptor,which accepts an excited electron from the reaction center chlorophyll a. The solar-powered transfer of an electron from a special chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions. Photosystem I (PS I) has a reaction center chlorophyll a that has an absorption peak at 700 nm. Photosystem II (PS II) has a reaction center chlorophyll a that has an absorption peak at 680 nm. The differences between these reaction centers (and their absorption spectra) lie not in the chlorophyll molecules, but in the proteins associated with each reaction center. Under certain conditions, photoexcited electrons from photosystem I, but not photosystem II, can take an alternative pathway, cyclic electron flow. As electrons flow along the electron transport chain, they generate ATP by cyclic photophosphorylation.

-Biology 8th edition textbook, Solomon Berg Martin
-Mt. Sinai library databse