Photosynthesis is the process that provides the energy necessary for growth and development of plants and algae. The primary role of photosynthesis is to fix atmospheric CO~2~ into organic molecules, such as sugar or starch. Carbon is stored in plant tissues during the day and it is released at night to fuel respiration. Photosynthesis uses solar energy (in the form of photons) to create chemical energy from water and carbon dioxide.
Photosynthetic organisms absorb light energy from the environment in order to convert it into chemical energy. In plants, this is realized by the light-harvesting complex (LHC) II and the reaction center (RC). The RC is the part of LHCII that contains the primary quinone electron acceptor Q~A~.
The action spectrum is the distribution of photons that excite a biological reaction or process. The spectrum of the light that reaches the Earth's surface is called the solar spectrum, and its shape is modified by absorption and scattering in the atmosphere and the Earth's surface .
What action spectrum indicates is the rate of photosynthesis.