72 ■ CHAPTER 04 Life Is Cellular
CELLS
(adenosine triphosphate), the universal cellular
fuel, in a process called cellular respiration.
Mitochondria provide ATP to all eukaryotic cells
(both plant and animal), but the cells of plants
and some protists have additional organelles
called chloroplasts that capture energy from
sunlight and use it to manufacture food mole-
cules via photosynthesis (Figure 4.9, middle
right). We will explore cellular respiration and
photosynthesis in Chapter 5.
A network of protein cylinders and filaments
collectively known as the cytoskeleton forms
the framework of a cell (Figure 4.9, middle
right). The cytoskeleton organizes the interior
of a eukaryotic cell, supports the intracellular
movement of organelles such as transport vesi-
cles, and enables whole-cell movement in some
cell types. It also gives shape to wall-less cells.
Fungi and plants separately evolved cell walls to
maintain cell structure, but animals rely only on
the cell’s cytoskeleton.Life Goes On
From Venter’s complex synthetic genome to
Devaraj’s simple self-assembling membrane,
scientists debate whether the top-down or
bottom-up approach will be more successful
in the effort to build an artificial cell. But the
two can be complementary, says James Collins.
“I’m not sure one will win out. I think they
bring different things to the table,” he notes.
Gibson agrees: “I would like to one day be able
to combine all of a cell’s parts from nonliving
components, including the genome, and incu-
bate them, and see if we can get life out of those
nonliving components,” he says. “It would help
us better understand how cells work.”
Today, the work on both ends continues.
While Devaraj makes his membranes more
complex, Gibson and his team (Figure 4.10)
recently simplified the Mycoplasma mycoides
genome. With the goal of determining the
smallest set of genes needed to maintain life,
they broke the M. mycoides genome into eight
DNA segments and mixed and matched them to
see which combinations would produce viable
cells. Eventually, they narrowed the genome
down to just 473 genes capable of sustaining
life. Amazingly, the team could not identify
the function of 149 of the 473 genes. “We don’tThe endoplasmic reticulum (ER) is an
extensive and interconnected network of sacs
made of a single membrane that is continuous
with the outer membrane of the nuclear enve-
lope (Figure 4.9, middle left). The membranes
of the ER are classified into two types based on
their appearance: smooth and rough. Enzymes
associated with the surface of the smooth ER
manufacture lipids and hormones. In some
cell types, smooth-ER membranes also break
down toxic compounds. Ribosomes embed-
ded in the rough ER g i ve it t he k nobby a pp e a r-
ance from which it gets its name. Ribosomes
on the rough ER assemble proteins that will
be inserted into the cell’s plasma membrane or
its organelles.
Resembling a pile of f lattened balloons,
the Golgi apparatus is like a post off ice,
packaging and directing proteins and lipids
produced by the ER to their f inal destinations
either inside or outside of the cell (Figure 4.9,
bottom left). Each molecule is first packaged
into a transport vesicle. The transport vesicle
buds off from the ER membrane and delivers
the cargo to its destination by fusing with the
membrane of the target compartment. The
Golgi apparatus targets these destinations by
adding a specific chemical tag to each mole-
cule it receives, like attaching a shipping label
to a package.
In animal cells, transport vesicles bring large
molecules that will be discarded to lysosomes,
organelles that act as garbage and recycling
centers (Figure 4.9, bottom far left). Lyso-
somes contain a variety of enzymes that degrade
biomolecules and release the breakdown prod-
ucts into the cell interior to be discarded or
reused. In plant cells, vacuoles perform func-
tions similar to those of lysosomes, plus a few
additional functions, such as water storage
(Figure 4.9, top right). Some plant vacuoles
stockpile noxious compounds that can deter
herbivores from eating plants.
In most eukaryotic cells, the main source
of energy is the mitochondrion (plural “mito-
chondria”), a tiny power plant that fuels cellular
activities (Figure 4.9, bottom right). Mitochon-
dria are made up of double membranes—a
smooth external membrane and a folded inter-
nal membrane—that form a mazelike interior.
Mitochondria use chemical reactions to trans-
form the energy of food molecules into ATP