What is the basic unit of life?

The cell is the basic structural and functional unit of all living organisms. Cell theory (one of biology's foundational principles): (1) All living things are composed of one or more cells; (2) The cell is the basic unit of life; (3) All cells come from pre-existing cells. Levels of biological organization (smallest to largest): atom → molecule → organelle → cell → tissue → organ → organ system → organism → population → community → ecosystem → biome → biosphere. Cell types: (1) Prokaryotic — bacteria and archaea; no membrane-bound nucleus; smaller, simpler; circular DNA in nucleoid; (2) Eukaryotic — plants, animals, fungi, protists; membrane-bound nucleus containing DNA in chromosomes; membrane-bound organelles (mitochondria, ER, Golgi, lysosomes, chloroplasts in plants). Major cell structures: cell membrane (boundary, selectively permeable), cytoplasm (interior), nucleus (control center, DNA), mitochondria (powerhouse, ATP production), ribosomes (protein synthesis). Plant cells additionally have: cell wall (rigid, cellulose), chloroplasts (photosynthesis), large central vacuole.