EIN PROCESSOR 
28 
n 1945, as the electron 
microscope was becom- 
ing a useful research 
tool, Albert Claude of 
Belgium and Keith 
Porter, who was then at The 
Rockefeller Institute, used it 
to discover a vast network of 
channels bounded by mem- 
branes in the cytoplasm of 
chick embryo cells. At times, 
this network looked like the 
concentric circles of a slice of 
onion. Porter called this net- 
work the endoplasmic 
reticulum (ER) because it was 
more concentrated in the 
inner (endoplasmic) region of 
the cell than in the peripheral 
(ectoplasmic) region. Similar 
networks were later found in 
all eukaryotic cells, except 
mammalian red blood cells. 
It was discovered that the 
membranes of the endoplasmic 
reticulum all interconnect, 
forming a system of tubes and 
flattened sacs. Some parts of 
the endoplasmic reticulum 
look smooth, while others 
appear rough because they 
are dotted with ribosomes 
that form granules on their 
outer surfaces. 
The smooth endoplasmic 
reticulum is involved in the 
synthesis of fatty acids and 
membrane components. This 
ER also contains enzymes that 
help detoxify and process 
chemicals. It is especially 
prevalent in liver cells. 
Many of the proteins the 
ribosomes of the rough endo- 
plasmic reticulum synthesize 
are intended to be exported 
outside the cell. These proteins 
carry specific amino acid 
sequences, or “addresses,” 
that allow them to enter the 
inner space of the rough 
endoplasmic reticulum, where 
they undergo additional bio- 
chemical modifications. 
In the mid- 1950’s, George 
Palade, then of The 
Rockefeller Institute, con- 
cluded that the amount of 
rough endoplasmic reticulum 
in a cell corresponds closely 
to the quantity of protein the 
cell exports. For example, 
white blood cells that produce 
infection-fighting immune 
system proteins called anti- 
bodies have highly developed 
rough endoplasmic reticula. 
Most of the proteins leaving 
the endoplasmic reticulum are 
still not mature; they must 
undergo further processing in 
another organelle, the Golgi 
apparatus, before they are 
ready to perform their 
functions within or outside 
the cell. 
