Photosynthesis is the major entry point for energy into biological systems. It is the source of all of the food we eat, both through the direct consumption of plant material and its indirect consumption as meat. It is also the source of all the oxygen that we breathe, as well as fuels for heating and transportation. Fossil fuels are the legacy of ancient photosynthesis: Oil has its origin in the bodies of marine algae and the organisms that graze on them, while coal represents the geologic remains of terrestrial (land) plants. Thus, one motivation to understand photosynthesis is the sheer magnitude and importance of this process for life on Earth. Before exploring the details of how photosynthesis actually occurs, let’s look at what types of organism carry out photosynthesis and where they live, as well as what structural components are needed to allow cells to capture energy in this remarkable way.