Living cells of organisms contain structures within them that perform specific functions for the cell. These structures are called ORGANELLES. Organelles are organ-like structures that are specific about their function in a cell.
Both the cells of prokaryotes and eukaryotes possess organelles, however, the organelles in eukaryotes are bounded by a membrane e.g mitochondrion, chloroplast etc. while those of prokaryotic cells are not bounded by a membrane.
The organelles inside a cell can be seen with the aid of an electron microscope. Examples of organelles are chloroplast (functions in photosynthesis), mitochondria (functions in production and storage of cellular energy), ribosome (functions in protein synthesis) etc.
Without an oxygen atmosphere very few things could live on the early Earth. Anaerobic bacteria were probably the first living things on Earth. The early Earth had no oceans and was frequently hit with meteorites and asteroids.
