We generally group organisms on the basis of shared evolutionary histories, creating taxa such as vertebrate animals, insects, coniferous trees, and orchids. However, we can also classify organisms by how they obtain energy–that is, by their trophic (feeding) biology. Organisms that use inorganic sources of both carbon and energy are called autotroph’s (“self-feeders”) and are of two types, photosynthetic and chemosynthetic. Photosynthetic autotroph’s use carbon dioxide (PCO2) as a source of carbon and light as a source of energy. This group includes the plants, photosynthetic protists, and photosynthetic bacteria.
Whether on coral reef, rain forest, or abandoned urban lot, organisms engage in an active search for energy and nutrients. For must organisms, life boils down to converting energy and nutrients into descendants. The energy used by different organisms comes in the form of light, organic molecules, or inorganic molecules. Nutrients are the raw materials an organism must acquire from the environment to live. Because organisms acquire energy and nutrients in diverse ways, we need to organize our discussion under the umbrella of major concepts.