Types of Organisms

A few representatives of life’s diversity.
Bacteria are the most numerous organisms on Earth. Clockwise from upper left, a bacterium with a row of iron crystals that acts like a tiny compass; a common resident of cat and dog stomachs; spiral cyanobacteria; types found in dental plaque.

Living things differ in their observable characteristics
Biodiversity
Scope of variation among living organisms
Various classification schemes help organize the variation
One broad classification is based on whether organism has a nucleus
Nucleus: sac with two membranes that encloses and protects DNA

Bacteria and archaea
Organism types with DNA not contained within a nucleus
Single-celled
Prokaryotes
Older designation for organisms without a nucleus
Eukaryotes
DNA contained within a nucleus