Chapter 3
Cells: The Living Units
63
3
passive envelope. As you will see, its unique structure allows it to
play a dynamic role in cellular activities.
The Fluid Mosaic Model
Te
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
(Figure 3.3)
. Te proteins, many
of which float in the fluid
lipid bilayer
, form a constantly changing
mosaic pattern. Te model is named for this characteristic.
Membrane Lipids
Te lipid bilayer forms the basic “fabric” of the membrane. It is
constructed largely of
phospholipids
, with smaller amounts of
glycolipids
,
cholesterol
, and areas called
lipid rafs.
The Plasma Membrane:
Structure
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.
Te flexible
plasma membrane
defines the extent of a cell,
thereby separating two of the body’s major fluid compartments—
the
intracellular
fluid within cells and the
extracellular
fluid (ECF)
outside cells. Te term
cell membrane
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
Peroxisome
Ribosomes
Rough
endoplasmic
reticulum
Nucleus
Nuclear envelope
Chromatin
Golgi apparatus
Nucleolus
Smooth endoplasmic
reticulum
Cytosol
Lysosome
Mitochondrion
Centrioles
Centrosome
matrix
• Micr
otubule
Cytoskeletal
elements
• Int
ermediate
filaments
Plasma
membrane
Figure 3.2
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.
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