Types of Cells
There are two major types of cells: prokaryotes
and eukaryotes. Eukaryotic cells, whose name derives from the Greek eu,
meaning “good,” and karyon, “kernel” or “nucleus,”
have a nucleus and membrane-bound organelles. Prokaryotic cells,
whose name derives from the Greek pro, meaning
“before,” contain neither nucleus nor organelles. As the names imply,
prokaryotic cells are less evolutionarily advanced than eukaryotic
Prokaryotes include some of the most primitive
forms of life: bacteria and blue-green algae (also known as cyanobacteria).
Prokaryotic organisms are generally single-celled.
Prokaryotes have a cell membrane, and they are made up
of generally undifferentiated fluid, called the cytoplasm,
in which floats a circular ring of DNA that controls the functioning
of the cell. Prokaryotes maintain their shape through a cytoskeleton
and have ribosomes that float in the cytoplasm. In addition, some
prokaryotes have a special type of cell wall made of a
protein-sugar combination called peptidoglycan. A few prokaryotes
possess whiplike tails called flagella that help propel
the cells through water.
Though less complex and less efficient than eukaryotes,
prokaryotes are hardy because of their simplicity. They are able
to survive environmental extremes that would kill higher life forms.
All living things besides bacteria and cyanobacteria consist
of eukaryotic cells, which are larger and structurally more complex
than prokaryotic cells. Like prokaryotes, eukaryotes are surrounded
by a lipid bilayer cell membrane and have cytoplasm and ribosomes.
However, unlike prokaryotes, eukaryotes also contain organelles and
a defined nucleus containing DNA.
Eukaryotes benefit enormously from the presence of membrane-bound
organelles. Each organelle creates an additional compartment in
the cell that can specialize in particular activities or processes,
increasing productivity as a result. The structure of eukaryotic cells
and the specific functions of the various organelles are often tested
by the SAT II Biology.