Essential Cell Biology 5th edition

16 CHAPTER 1 Cells: The Fundamental Units of Life
the airless depths of marine sediments, the sludge of sewage treatment
plants, pools beneath the frozen surface of Antarctica, as well as in the
acidic, oxygen-free environment of a cow’s stomach, where they break
down ingested cellulose and generate methane gas. Many of these
extreme environments resemble the harsh conditions that must have
existed on the primitive Earth, where living things first evolved before the
atmosphere became rich in oxygen.
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Figure 1–14 Yeasts are simple, freeliving
eukaryotes. The cells shown in this
micrograph belong to the species of yeast,
Saccharomyces cerevisiae, used to make
dough rise and turn malted barley juice
into beer. As can ECB5 be e1.13/1.14 seen in this image, the
cells reproduce by growing a bud and then
dividing asymmetrically into a large mother
cell and a small daughter cell; for this
reason, they are called budding yeast.
THE EUKARYOTIC CELL
Eukaryotic cells, in general, are bigger and more elaborate than bacteria
and archaea. Some live independent lives as single-celled organisms,
such as amoebae and yeasts (Figure 1–14); others live in multicellular
assemblies. All of the more complex multicellular organisms—including
plants, animals, and fungi—are formed from eukaryotic cells.
By definition, all eukaryotic cells have a nucleus. But possession of a
nucleus goes hand-in-hand with possession of a variety of other organelles,
most of which are membrane-enclosed and common to all
eukaryotic organisms. In this section, we take a look at the main organelles
found in eukaryotic cells from the point of view of their functions,
and we consider how they came to serve the roles they have in the life of
the eukaryotic cell.
The Nucleus Is the Information Store of the Cell
The nucleus is usually the most prominent organelle in a eukaryotic cell
(Figure 1–15). It is enclosed within two concentric membranes that form
Figure 1–15 The nucleus contains most
of the DNA in a eukaryotic cell. (A) This
drawing of a typical animal cell shows its
extensive system of membrane-enclosed
organelles. The nucleus is colored brown,
the nuclear envelope is green, and the
cytoplasm (the interior of the cell outside
the nucleus) is white. (B) An electron
micrograph of the nucleus in a mammalian
cell. Individual chromosomes are not visible
because at this stage of the cell-division
cycle the DNA molecules are dispersed as
fine threads throughout the nucleus.
(B, by permission of E.L. Bearer and
Daniel S. Friend.)
cytoplasm
mitochondrion
nuclear
envelope
nucleus
(A)
(B)
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