Photosynthesis is
- a process used by plants and other organisms to convert light energy into chemical energy 
- a word derived from the Greek phōs (φῶς), "light", and synthesis (σύνθεσις), "putting together"
- a process that always begins when energy from light is absorbed by proteins called reaction centers
- performed differently by different species
- largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere