Figure 3.40 (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 are80S 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
ribosomesandmembrane-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.
The Endomembrane System
Theendomembrane 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 3.41). 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.
- 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 Chemotherapy56 no. 8 (2012):4046–4051.
118 Chapter 3 | The Cell
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