Figure 3.41 The endomembrane system is composed of a series of membranous intracellular structures that
facilitate movement of materials throughout the cell and to the cell membrane.
Endoplasmic Reticulum
Theendoplasmic reticulum (ER)is an interconnected array of tubules andcisternae(flattened sacs) with a single
lipid bilayer (Figure 3.42). The spaces inside of the cisternae are calledlumenof the ER.
There are two types of ER,rough endoplasmic reticulum (RER)andsmooth endoplasmic reticulum (SER).
These two different types of ER are sites for the synthesis of distinctly different types of molecules. RER is studded
with ribosomes bound on the cytoplasmic side of the membrane. These ribosomes make proteins destined for the
plasma membrane (Figure 3.39,Figure 3.40, andFigure 3.42). Following synthesis, these proteins are inserted
into the membrane of the RER. Small sacs of the RER containing these newly synthesized proteins then bud off as
transport vesiclesand move either to the Golgi apparatus for further processing, directly to the plasma membrane,
to the membrane of another organelle, or out of the cell (Figure 3.41). Transport vesicles are single-lipid, bilayer,
membranous spheres with hollow interiors that carry molecules. SER does not have ribosomes and, therefore, appears
“smooth.” It is involved in biosynthesis of lipids, carbohydrate metabolism, and detoxification of toxic compounds
within the cell.
Figure 3.42 The rough endoplasmic reticulum is studded with ribosomes for the synthesis of membrane proteins
(which give it its rough appearance).
Chapter 3 | The Cell 119