68 ■ CHAPTER 04 Life Is Cellular
CELLS
within eukaryotic cells. Depending on the
fundamental structure of their cells, all living
organisms can be sorted into one of two groups:
prokaryotes or eukaryotes (Figure 4.8).
Mycoplasma mycoides and all other bacteria are
prokaryotes, but virtually all the organisms you
see every day, including all plants and animals,
are eukaryotes.
Eukaryotic cells are larger and more complex
than prokaryotic cells: they are roughly ten
times wider, with a cell volume about a thou-
sand times greater. Unlike prokaryotic cells,
eukaryotic cells have a membrane-enclosed
nucleus (plural “nuclei”) that contains the
organism’s DNA, and they have a variety of
membrane-enclosed subcellular compartments
called organelles. Through specialization and
division of labor, these organelles act like cubi-
cles in a large office, allowing the cell to local-
ize different processes in different places. In
contrast, prokaryotic cells are like an open floor
plan: they lack a cell nucleus or any membrane-
encased organelles.
Though the first fully artificial cell will most
likely be a simple prokaryotic cell, synthetic biol-
ogists aspire to build a eukaryotic cell. “We’re
not quite there yet, but it’s interesting to think
about making complex structures like organ-
elles,” says Devaraj. Having achieved a self-
assembling and growing artificial membrane,
his team is working to create membranes inside
preexisting vesicles, mimicking organelles like
mitochondria that are made up of membranes
within membranes. It is the first step toward
creating the variety of organelles inside eukary-
otic cells.What’s in a Cell?
The nucleus is the control center of the cell. It
contains most of the cell’s DNA and may occupy
up to 10 percent of the space inside the cell
(Figure 4.9, top left). Inside the nucleus, long
strands of DNA are packaged with proteins into
a remarkably small space. The boundary of the
nucleus, called the nuclear envelope, is made
up of two concentric phospholipid bilayers. The
nuclear envelope is speckled with thousands
of small openings called nuclear pores. These
pores allow chemical messages to enter and exit
the nucleus.molecules, or particles. The pocket deepens until
the opening in the membrane pinches off and
the membrane breaks free as a closed vesicle,
now wholly contained within the cell. Endocyto-
sis can be nonspecific or specific. In nonspecific
endocytosis, all of the material in the immedi-
ate area is surrounded and enclosed; in specific
endocytosis, one particular type of molecule is
enveloped and imported.
There are three types of endocytosis.
Receptor-mediated endocytosis (Figure 4.7,
bottom left) is a form of endocytosis in which
receptor proteins embedded in the membrane
recognize specific surface characteristics of
substances to be incorporated into the cell.
For example, our cells use receptor-mediated
endocytosis to take up cholesterol-containing
packages called low-density lipoprotein (LDL)
particles. Phagocytosis (Figure 4.7, top right),
or “cellular eating,” is a large-scale version of
endocytosis in which particles considerably
larger than biomolecules are ingested. Specific
cells in the immune system use phagocytosis
to ingest an entire bacterium or virus. Pino-
cytosis (Figure 4.7, bottom right) is a form of
endocytosis that is often described as “cellu-
lar drinking” because it involves the capture
of fluids. However, the cell does not attempt
to collect particular solutions. Pinocytosis is
nonspecific: the vesicle budding into the cell
contains whatever happened to be dissolved in
the fluid when the cell “drank.”
There is a long way to go before an artificial
plasma membrane will perform processes like
endocytosis and exocytosis, says Neal Devaraj.
“But just because a research problem is difficult
doesn’t mean we shouldn’t tackle it,” he adds.Prokaryotes versus Eukaryotes
Devaraj’s team continues to pursue that ideal.
Prior synthetic membranes, their own included,
did not have the ability to grow by adding new
phospholipids, but in 2015 the team succeeded
in designing and synthesizing an artificial
membrane that sustained continual growth, just
like a living cell. Membranes are important not
only because they form the structure of a cell,
but because they compartmentalize processes