Two structurally distinct types of cells have evolved that vary greatly in their internal complexity. Prokaryote cells are the simplest type and are evolutionary precursors to eukaryote cell types. What is thought to be the earliest known fossilized cells were discovered by paleontologists working near the Great Lakes in North America. They discovered microfossil evidence with enough detail to classify the cells as prokaryote. How did they know they were prokaryote?
Although both prokaryote and eukaryote cells can have a cell wall and a cell membrane to enclose the cellular cytoplasm, the structural similarities end there. Inside a typical prokaryote cell, such as a bacteria cell, there are no membrane-bound organelles. An organelle is a subcellular structure that has a specific function. Even the genetic material, although often contained and cornered inside the cell, is not bound by a membrane. Eukaryotic cells, which basically include every cell type except bacteria, are characterized by internal organelles surrounded by a membrane, which helps to increase their organization and efficiency. In contrast to prokaryotes, in eukaryotes the chromosomes are made of distinct lengths of DNA and are stored within a nuclear membrane. Because prokaryotes are simpler, lacking membrane-bound organelles, they are also much smaller (1 to 10 micrometers) than eukaryotes, which range from 10 to 100 micrometers in size.
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.