Fundamental unit of life: every organism either consists of cells or is itself a single cell. All organisms consists of one or more cells. The cell is the basic unit of structure fir all organisms. All cells arise only from pre-existing cells ( basic unit of reproduction) Prokaryote: cells lacking a nuclear envelope, cytoplasmic organelles, and a cytoskeleton (primarily bacteria), smallest organism in the world. Eukaryote: cells that have a nuclear envelope, cytoplasmic organelles, and a cytoskeleton. Bacteria: one of the two domains of prokaryotes; collectively, bacteria are the most metabolically diverse organism. Most familiar to us, including those that cause diseases for humans and animals. Achaea: one of the two domains of prokaryotes; achaeans have some unique molecular and biochemical traits, but they also share some unique traits with bacteria and others with eukaryotes. Not well known, only discovered about 40 years ago. Live in extreme conditions, that no other organisms can survive, not even bacteria.