The Light Reactions of Photosynthesis



In the thylakoid membranes, chlorophyll is organized along with other molecules into two photosystems (I and II). Photosystems are the light-harvesting units of the thylakoid membrane. Each photosystem has an antenna of a few hundred pigment molecules.

How a Photosystem Harvests Light

When a photon of light strikes a pigment molecule, the energy is passed from molecule to molecule until it reaches the reaction center which contains a particular form of chlorophyll a. The reaction-center chlorophyll of photosystem I is known as P700 because this pigment is best at absorbing light having a wavelength of 700 nm (the far-red part of the light spectrum). The chlorophyll at the reaction-center of photosystem II is called P680 because its absorption spectrum has a peak of 680 nm (in the red part of the light spectrum). These two pigments, P700 and P680, are actually identical chlorophyll a molecules. However, their association with different protein molecules in the thylakoid membrane accounts for the slight differences in light-absorbing properties. At the reaction center, the absorbed light energy drives an oxidation-reduction reaction (loss and gain of electrons). An excited electron from the reaction-center chlorophyll is captured by a specialized molecule called the primary acceptor. Light drives the synthesis of NADPH2 and ATP, the two main products of the light- dependent reactions, by energizing the two photosystems embedded in the thylakoid membranes of the chloroplast. The removal of hydrogen and electrons from water by photosystem II in the light produces O2, the other major product of the light reactions of photosynthesis.

In summary, the light-dependent reactions uses solar power to generate ATP and NADPH2, which provide chemical and reducing power, respectively, to the sugar-making reactions of the Calvin cycle. An incidental by-product of the light-dependent reactions is oxygen.