Microbiology, 2021

3.4 • Unique Characteristics of Eukaryotic Cells 111
ribosome assembly is completed (Figure 3.42).
Figure 3.42
(a) The nucleolus is the dark, dense area within the nucleus. It is the site of rRNA synthesis and preribosomal assembly. (b)
Electron micrograph showing the nucleolus.
Ribosomes
Ribosomes found in eukaryotic organelles such as mitochondria or chloroplasts have 70S ribosomes—the
same size as prokaryotic ribosomes. However, nonorganelle-associated ribosomes in eukaryotic cells are 80S
ribosomes, composed of a 40S small subunit and a 60S large subunit. In terms of size and composition, this
makes them distinct from the ribosomes of prokaryotic cells.
The two types of nonorganelle-associated eukaryotic ribosomes are defined by their location in the cell: free
ribosomes and membrane-bound ribosomes. Free ribosomes are found in the cytoplasm and serve to
synthesize water-soluble proteins; membrane-bound ribosomes are found attached to the rough endoplasmic
reticulum and make proteins for insertion into the cell membrane or proteins destined for export from the
cell.
The differences between eukaryotic and prokaryotic ribosomes are clinically relevant because certain
antibiotic drugs are designed to target one or the other. For example, cycloheximide targets eukaryotic action,
whereas chloramphenicol targets prokaryotic ribosomes. 24 Since human cells are eukaryotic, they generally
are not harmed by antibiotics that destroy the prokaryotic ribosomes in bacteria. However, sometimes
negative side effects may occur because mitochondria in human cells contain prokaryotic ribosomes.
Endomembrane System
The endomembrane system, unique to eukaryotic cells, is a series of membranous tubules, sacs, and flattened
disks that synthesize many cell components and move materials around within the cell (Figure 3.43). Because
of their larger cell size, eukaryotic cells require this system to transport materials that cannot be dispersed by
diffusion alone. The endomembrane system comprises several organelles and connections between them,
including the endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles.
24 A.E. Barnhill, M.T. Brewer, S.A. Carlson. “Adverse Effects of Antimicrobials via Predictable or Idiosyncratic Inhibition of Host
Mitochondrial Components.” Antimicrobial Agents and Chemotherapy 56 no. 8 (2012):4046–4051.