Essential Cell Biology 5th edition

26 CHAPTER 1 Cells: The Fundamental Units of Life
Figure 1–29 Where did eukaryotes
come from? The eukaryotic, bacterial,
and archaean lineages diverged from one
another more than 3 billion years ago—
very early in the evolution of life on Earth.
Some time later, eukaryotes are thought
to have acquired mitochondria; later still, a
subset of eukaryotes acquired chloroplasts.
Mitochondria are essentially the same in
plants, animals, and fungi, and therefore
were presumably acquired before these
lines diverged about 1.5 billion years ago.
nonphotosynthetic
bacteria
TIME
photosynthetic
bacteria
chloroplasts
mitochondria
plants
single-celled eukaryote
animals fungi archaea
bacteria
archaea
ancestral prokaryote
hurly-burly, so as to allow more delicate and complex control of the way
the cell reads out its genetic information.
Such a primitive eukaryotic cell, with a nucleus and cytoskeleton, was
most likely the sort of cell that engulfed the free-living, oxygen-consuming
bacteria that were the likely ancestors of the mitochondria (see Figure
1–19). This partnership is ECB5 thought e1.28/1.29 to have been established 1.5 billion
years ago, when the Earth’s atmosphere first became rich in oxygen. A
subset of these cells later acquired chloroplasts by engulfing photosynthetic
bacteria (see Figure 1–21). The likely history of these endosymbiotic
events is illustrated in Figure 1–29.
That single-celled eukaryotes can prey upon and swallow other cells
is borne out by the behavior of many present-day protozoans: a class
of free-living, motile, unicellular organisms. Didinium, for example, is a
large, carnivorous protozoan with a diameter of about 150 μm—roughly
10 times that of the average human cell. It has a globular body encircled
by two fringes of cilia, and its front end is flattened except for a single
protrusion rather like a snout (Figure 1–30A). Didinium swims at high
speed by means of its beating cilia. When it encounters a suitable prey,
usually another type of protozoan, it releases numerous small, paralyzing
darts from its snout region. Didinium then attaches to and devours
Figure 1–30 One protozoan eats another.
(A) The scanning electron micrograph shows
Didinium on its own, with its circumferential
rings of beating cilia and its “snout” at the
top. (B) Didinium is seen ingesting another
ciliated protozoan, a Paramecium, artificially
colored yellow. (Courtesy of D. Barlow.)
(A)
100 µm
(B)