Microbiology, 2021

112 3 • The Cell
Figure 3.43
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
The endoplasmic reticulum (ER) is an interconnected array of tubules and cisternae (flattened sacs) with a
single lipid bilayer (Figure 3.44). The spaces inside of the cisternae are called lumen of the ER. There are two
types of ER, rough endoplasmic reticulum (RER) and smooth 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 3.44). 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 vesicles and
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. 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 3.44
appearance).
The rough endoplasmic reticulum is studded with ribosomes for the synthesis of membrane proteins (which give it its rough
Golgi Apparatus
The Golgi apparatus was discovered within the endomembrane system in 1898 by Italian scientist Camillo
Golgi (1843–1926), who developed a novel staining technique that showed stacked membrane structures
within the cells of Plasmodium, the causative agent of malaria. The Golgi apparatus is composed of a series of
membranous disks called dictyosomes, each having a single lipid bilayer, that are stacked together (Figure
3.45).
Enzymes in the Golgi apparatus modify lipids and proteins transported from the ER to the Golgi, often adding
carbohydrate components to them, producing glycolipids, glycoproteins, or proteoglycans. Glycolipids and
glycoproteins are often inserted into the plasma membrane and are important for signal recognition by other
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