The cell is the fundamental unit of life and requires certain basic functions, including:
1. a boundary that distinguishes inside from outside the cell yet allows materials to enter or leave the cell.
2. an "engine" to capture energy from the environment and convert it to a form useful to the organism.
3. a genome that can be transmitted to the next generation and is stable, but able to change.
4. a way to change in response to environmental changes, over both a short (individual organism) and long (generational) time frame.