Essential Cell Biology 5th edition

CHAPTER ELEVEN
11
Membrane Structure
A living cell is a self-reproducing system of molecules held inside a container.
That container is the plasma membrane—a protein-studded, fatty
film so thin that it cannot be seen directly in the light microscope. Every
cell on Earth uses such a membrane to separate and protect its chemical
components from the outside environment. Without membranes, there
would be no cells, and thus no life.
THE LIPID BILAYER
MEMBRANE PROTEINS
The structure of the plasma membrane is simple: it consists of a two-ply
sheet of lipid molecules about 5 nm—or 50 atoms—thick, into which proteins
have been inserted. Its properties, however, are unlike those of any
sheet of material we are familiar with in the everyday world. Although
it serves as a barrier to prevent the contents of the cell from escaping
and mixing with molecules in the surrounding environment (Figure
11−1), the plasma membrane does much more than that. If a cell is to
survive and grow, nutrients must pass inward across the plasma membrane,
and waste products must make their way out. To facilitate this
plasma
membrane
(A) BACTERIAL CELL
(B) EUKARYOTIC CELL
internal
membrane
Figure 11–1 Cell membranes act as
selective barriers. The plasma membrane
separates a cell from its surroundings,
enabling the molecular composition of a
cell to differ from that of its environment.
(A) In some bacteria, the plasma membrane
is the only membrane. (B) In addition to a
plasma membrane, eukaryotic cells also
have internal membranes that enclose
individual organelles. All cell membranes
prevent molecules on one side from freely
mixing with those on the other, as indicated
schematically by the colored dots.