81
CHAPTER THREE
Cells
called an
integrin
contacts an adhesion receptor protein pro-
truding into the capillary space near the splinter and pushes
up through the capillary cell membrane, grabbing the passing
slowed white blood cell and directing it between the tilelike
cells of the capillary wall. White blood cells collecting at an
injury site produce infl ammation and, with the dying bacteria,
form pus. (The role of white blood cells in body defense is dis-
cussed further in chapter 14, pp. 531–532.)
Cellular adhesion is critical to many functions. CAMs
guide cells surrounding an embryo to grow toward mater-
nal cells and form the placenta, the supportive organ linking
a pregnant woman to the fetus (see F g. 23.18). Sequences
of CAMs help establish the connections between nerve cells
that underlie learning and memory.
Abnormal cellular adhesion affects health. Lack of cel-
lular adhesion, for example, eases the journey of cancer
cells as they spread from one part of the body to another.
Arthritis may occur when white blood cells are reined in by
the wrong adhesion molecules and infl
ame a joint where
there isn’t an injury.
PRACTICE
4
What is a selectively permeable membrane?
5
Describe the chemical structure of a cell membrane.
6
What are some functions of cell membrane proteins?
7
What are some of the events of cellular adhesion?
Cytoplasm
When viewed through a light microscope, cytoplasm usually
appears clear with scattered specks. However, a transmission
electron microscope (see F g. 3.4) reveals networks of mem-
branes and organelles suspended in the cytosol. Cytoplasm also
types. The immune system can distinguish between “self” cell
surfaces and “nonself” cell surfaces that may indicate a poten-
tial threat, such as the presence of infectious bacteria. Blood
and tissue typing for transfusions or transplants consider the
cell surface’s protein and glycoprotein topography.
Cellular Adhesion Molecules
Often cells must interact dynamically and transiently, rather
than form permanent attachments. Proteins called cellular
adhesion molecules, or CAMs for short, guide cells on the
move. Consider a white blood cell moving in the bloodstream
to the site of an injury, where it is required to F
ght infection.
Imagine that such a cell must reach a woody splinter embed-
ded in a person’s palm
(f
g. 3.8)
. Once near the splinter, the
white blood cell must slow down in the turbulence of the
bloodstream. A type of CAM called a
selectin
does this by
coating the white blood cell and providing traction. The white
blood cell slows to a roll and binds to carbohydrates on the
inner capillary surface. Clotting blood, bacteria, and decaying
tissue at the injury site release biochemicals (chemoattrac-
tants) that attract the white blood cell. ±inally, a type of CAM
Double
layer of
phospholipid
molecules
Fibrous protein
Extracellular side
of membrane
Cytoplasmic side
of membrane
Carbohydrate
Hydrophobic
fatty acid
“tail”
Hydrophilic
phosphate
“head”
Cholesterol
molecules
Globular
protein
Glycolipid
Glycoprotein
FIGURE 3.7
The cell membrane is composed primarily
of phospholipids (and some cholesterol), with proteins
scattered throughout the lipid bilayer and associated with
its surfaces.
TABLE
3.1
|
Types of Membrane Proteins
Protein Type
Function
Receptor proteins
Receive and transmit messages into a cell
Integral proteins
Form pores, channels, and carriers in cell
membrane, transduce signals
Enzymes
Catalyze chemical reactions
Cellular adhesion molecules
Enable cells to stick to each other
Cell surface proteins
Establish self
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