Biology Now, 2e

74 ■ CHAPTER 04 Life Is Cellular

CELLS

functions, says Gibson. “There’s not a single

organism in the world where we understand

what every gene does,” he notes. If they can

reach such understanding with those 473 genes

in the M. mycoides genome, “this would be the

first example of that.”

know about a third of essential life, and we’re

trying to sort that out now,” Venter told Nature

magazine in 2016.

Knowing both the identities and function of

a core set of genes necessary for life will make it

easier to build other synthetic cells with specific

Figure 4.10

Scientist in Action

Laboratory technician

Javier Quinones is setting

up the beginning of the

sequencing procedure

used to sequence the

M. mycoides genome in the

sequencing laboratory

at the J. Craig Venter

Institute in Rockville,

Maryland.

● (^) Cells are the basic units of all living organisms. Cell
theory states that all living things are composed of
one or more cells, and that all cells living today came
from a preexisting cell.
● (^) Every cell is surrounded by a plasma membrane
that separates the chemical reactions inside the
cell from the surrounding environment. The plasma
membrane is selectively permeable and formed by
a phospholipid bilayer embedded with proteins that
perform a variety of functions.
● (^) In passive transport, substances move across the
plasma membrane without the direct expenditure
of energy. Active transport by cells requires energy.
Diffusion is the passive transport of a substance
from a region where it is at a higher concentration to
a region where it is at a lower concentration.
● (^) Osmosis is the diffusion of water across a selectively
permeable membrane. In a hypotonic solution, a
cell gains water. In a hypertonic solution, a cell
loses water. In an isotonic solution, there is no net
movement of water. Cells can actively balance their
water content through osmosis.
● (^) Cells export materials by exocytosis and import
materials by endocytosis.

REVIEWING THE SCIENCE

● (^) In receptor-mediated endocytosis, receptor proteins
in the plasma membrane recognize and bind the
substance to be brought into the cell.
● (^) Prokaryotes are single-celled organisms lacking
a nucleus and complex internal compartments.
Eukaryotes may be single-celled or multicellular,
and their cells typically possess many membrane-
enclosed compartments.
● (^) By volume, eukaryotic cells can be a thousand times
larger than prokaryotic cells. They require internal
compartments, or organelles, that concentrate and
organize cellular chemical reactions.
● (^) The nucleus of a eukaryotic cell contains DNA. It is
bounded by the nuclear envelope, which has nuclear
pores that allow communication between the nucleus
and the cell interior.
● (^) Lipids are made in the smooth endoplasmic
reticulum. Some proteins are manufactured in the
rough endoplasmic reticulum.
● (^) The Golgi apparatus receives proteins and lipids, sorts
them, and directs them to their final destinations.
● (^) In animals, lysosomes break down biomolecules
such as proteins into simpler compounds that can