Ribosomes and the
Endoplasmic Reticulum
Unlike prokaryotic cells, eukaryotic cells have a system of internal
membranes that play an essential role in the processing of proteins.
Cells make proteins on ribosomes. Each ribosome is made of dozens
of different proteins as well as RNA. Some of the ribosomes in a
eukaryotic cell are suspended in the cytosol, as they are in prokary-
otic cells. These “free” ribosomes make proteins that remain inside
the cell, such as proteins used to build new organelles.
Production of Proteins
Proteins that are exported from the cell, such as some signal
molecules, are made on the ribosomes that lie on the surface
of the endoplasmic reticulum, shown in Figure 14. The
(ehn doh PLAZ mihk rih TIHK yuh luhm), or
ER, is an extensive system of internal membranes that move pro-
teins and other substances through the cell. Like the cell mem-
brane, the membranes of the ER are made of a lipid bilayer with
embedded proteins.
The part of the ER with attached ribosomes is called rough ER
because it has a rough appearance when viewed in the electron
microscope. The rough ER helps transport the proteins that are
made by its attached ribosomes. As each protein is made, it crosses
the ER membrane and enters the ER. The portion of the ER that
contains the completed protein then pinches off to form a vesicle. A
is a small, membrane-bound sac that transports substances
in cells. Because certain proteins are enclosed inside vesicles, these
proteins are kept separate from proteins that are produced by free
ribosomes in the cytoplasm.
The rest of the ER is called smooth ER because it lacks ribosomes
and thus appears smooth when viewed in the electron microscope.
The smooth ER performs various functions, such as making lipids
and breaking down toxic substances.
vesicleendoplasmic reticulumSECTION 3 Cell Organelles 63
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Topic:Proteins
Keyword:HX4151Smooth ERRibosomesRough ERThe ER moves proteins and other substances within eukaryotic cells.
Figure 14 Endoplasmic reticulum