Cells: The Living Units
passive envelope. As you will see, its unique structure allows it to
play a dynamic role in cellular activities.
The Fluid Mosaic Model
ﬂuid mosaic model
of membrane structure depicts the plasma
membrane as an exceedingly thin (7–10 nm) structure composed of
a double layer, or bilayer, of lipid molecules with protein molecules
“plugged into” or dispersed in it
. Te proteins, many
of which ﬂoat in the ﬂuid
, form a constantly changing
mosaic pattern. Te model is named for this characteristic.
Te lipid bilayer forms the basic “fabric” of the membrane. It is
constructed largely of
, with smaller amounts of
, and areas called
The Plasma Membrane:
Describe the chemical composition of the plasma
membrane and relate it to membrane functions.
Compare the structure and function of tight junctions,
desmosomes, and gap junctions.
deﬁnes the extent of a cell,
thereby separating two of the body’s major ﬂuid compartments—
ﬂuid within cells and the
outside cells. Te term
is commonly used as a
synonym for plasma membrane, but because nearly all cellular
organelles are enclosed in a membrane, in this book we will al-
ways refer to the cell’s surface, or outer limiting membrane, as the
plasma membrane. Te plasma membrane is much more than a
Secretion being released
from cell by exocytosis
Structure of the generalized cell.
No cell is exactly like this one, but this composite
illustrates features common to many human cells. Note that not all of the organelles are drawn to
the same scale in this illustration.