Cracking the AP Biology Exam

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Cells

WHAT ARE THE DIFFERENT TYPES OF CELLS?

For centuries, scientists have known about cells. However, it wasn’t until the development of the electron microscope that scientists were able to figure out what cells do. We now know that there are two distinct types of cells: eukaryotic cells and prokaryotic cells. A eukaryotic cell contains a membrane-bound structure called a nucleus and cytoplasm, filled with tiny structures called organelles (literally “little organs”). Examples of eukaryotic cells are fungi, protists, plant cells, and animal cells.

A prokaryotic cell, which is a lot smaller than a eukaryotic cell, lacks both a nucleus and membrane-bound organelles. Examples of prokaryotic cells are bacteria and Rickettsiae. The genetic material in a prokaryote is one continuous, circular DNA molecule that lies free in the cell in an area called the nucleoid. In addition to a plasma membrane, most prokaryotes have a cell wall composed of peptidoglycan. Prokaryotes may also have ribosomes (although smaller than those found in eukaryotic cells) as well as a flagellum, a long fiber that helps them move.