cannot be seen under a bght 
microscope. Despite its thin- 
ness, the surface membrane is 
exceedingly sturdy, controlling 
everything that goes into and 
out of the cell and relaying 
vital messages. Similar 
membranes enclose or make 
up a large number of the cell’s 
organelles. 
and cyanobacteria (blue-green 
algae), do not have a mem- 
brane around their nuclear 
region. Eukaryotic cells, in 
contrast, have two membranes 
separating the nucleus from 
the cytoplasm, as well as many 
other internal membranes to 
segregate their organelles. 
The cells of all animals and 
plants are eukaryotic. 
higher eukaryotic organisms 
are capable of differentiation 
into many kinds of cells. This 
gives eukaryotes certain 
obvious advantages. However, 
prokaryotes have advantages 
of them own: simpler nutritional 
requirements, resistance to 
adverse conditions, and much 
more rapid growth and division. 
There is a fundamental 
distinction between two of the 
major categories of cells. 
Prokaryotic cells, which 
include bacteria, mycoplasma, 
Only eukaryotic cells are able 
to form large, multicellular 
systems — an important step up 
the evolutionary ladder. And 
while, in general, prokaryotic 
organisms produce only exact 
duplicates of themselves, 
The main job of most cells is 
to manufacture proteins. In 
eukaryotes, protein produc- 
tion begins in the most 
prominent organelle — the 
nucleus. 
19 
