Within the cell, the two most obvious structures are ribosomes and DNA . The DNA is not bounded by a membrane. Many prokaryote cells also have a flagellum which is used for movement. The cells of plants and animals are more complex than those of bacteria, archaea, and cyanobacteria. We call these type of cells eukaryotes .
Eukaryotic cells have a membrane enclosed nucleous which contains the organism's DNA .
Plant and animal cells also contain a variety of membrane-bound structures known as organelles . Eukaryotic cells also show great variation in terms of size. White blood cells of mammals are some of the smallest eukaryotic cells with a diameter between 3 to 4 micrometers. Ostrich ovum are very large cells with a diameter of about 100 micrometers. Internally, cells contain specialized structures known as organelles that carry out distinct cellular functions.