Microbiology, 2021

3.4 • Unique Characteristics of Eukaryotic Cells 119
mitochondrion in which kinetoplast DNA, organized as multiple circular pieces of DNA, is found concentrated
at one pole of the cell.
MICRO CONNECTIONS
Mitochondria-Related Organelles in Protozoan Parasites
Many protozoans, including several protozoan parasites that cause infections in humans, can be identified by
their unusual appearance. Distinguishing features may include complex cell morphologies, the presence of
unique organelles, or the absence of common organelles. The protozoan parasites Giardia lamblia and
Trichomonas vaginalis are two examples.
G. lamblia, a frequent cause of diarrhea in humans and many other animals, is an anaerobic parasite that
possesses two nuclei and several flagella. Its Golgi apparatus and endoplasmic reticulum are greatly reduced,
and it lacks mitochondria completely. However, it does have organelles known as mitosomes, doublemembrane-bound
organelles that appear to be severely reduced mitochondria. This has led scientists to
believe that G. lamblia’s ancestors once possessed mitochondria that evolved to become mitosomes. T.
vaginalis, which causes the sexually transmitted infection vaginitis, is another protozoan parasite that lacks
conventional mitochondria. Instead, it possesses hydrogenosomes, mitochondrial-related, double-membranebound
organelles that produce molecular hydrogen used in cellular metabolism. Scientists believe that
hydrogenosomes, like mitosomes, also evolved from mitochondria. 28
Plasma Membrane
The plasma membrane of eukaryotic cells is similar in structure to the prokaryotic plasma membrane in that
it is composed mainly of phospholipids forming a bilayer with embedded peripheral and integral proteins
(Figure 3.54). These membrane components move within the plane of the membrane according to the fluid
mosaic model. However, unlike the prokaryotic membrane, eukaryotic membranes contain sterols, including
cholesterol, that alter membrane fluidity. Additionally, many eukaryotic cells contain some specialized lipids,
including sphingolipids, which are thought to play a role in maintaining membrane stability as well as being
involved in signal transduction pathways and cell-to-cell communication.
Figure 3.54
The eukaryotic plasma membrane is composed of a lipid bilayer with many embedded or associated proteins. It contains
cholesterol for the maintenance of membrane, as well as glycoproteins and glycolipids that are important in the recognition other cells or
28 N. Yarlett, J.H.P. Hackstein. “Hydrogenosomes: One Organelle, Multiple Origins.” BioScience 55 no. 8 (2005):657–658.