Membrane Proteins
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: November 19, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Membrane Proteins
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Membrane Proteins
• Distinguish between integral and peripheral membrane proteins.
• Describe the structure and function of the membrane proteins.
• Summarize the Fluid Mosaic Model.
Can anything or everything move in or out of the cell?
No. It is the semipermeable plasma membrane that determines what can enter and leave the cell. So, if not everything
can cross the membrane, how do certain things get across?
Membrane Proteins
The plasma membrane contains molecules other than phospholipids, primarily other lipids and proteins. The green
molecules in Figure 1.1, for example, are the lipid cholesterol. Molecules of cholesterol help the plasma membrane
keep its shape. Many of the proteins in the plasma membrane assist other substances in crossing the membrane.
The plasma membranes also contain certain types of proteins. A membrane protein is a protein molecule that is
attached to, or associated with, the membrane of a cell or an organelle. Membrane proteins can be put into two
groups based on how the protein is associated with the membrane.
Integral membrane proteins are permanently embedded within the plasma membrane. They have a range of
important functions. Such functions include channeling or transporting molecules across the membrane. Other
integral proteins act as cell receptors. Integral membrane proteins can be classified according to their relationship
with the bilayer:
• Transmembrane proteins span the entire plasma membrane. Transmembrane proteins are found in all types of
biological membranes.
• Integral monotopic proteins are permanently attached to the membrane from only one side.
Some integral membrane proteins are responsible for cell adhesion (sticking of a cell to another cell or surface). On
the outside of cell membranes and attached to some of the proteins are carbohydrate chains that act as labels that
identify the cell type. Shown in Figure 1.1 are two different types of membrane proteins and associated molecules.
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Peripheral membrane proteins are proteins that are only temporarily associated with the membrane. They can
be easily removed, which allows them to be involved in cell signaling. Peripheral proteins can also be attached to
integral membrane proteins, or they can stick into a small portion of the lipid bilayer by themselves. Peripheral
membrane proteins are often associated with ion channels and transmembrane receptors. Most peripheral membrane
proteins are hydrophilic.
FIGURE 1.1
Some of the membrane proteins make up a major transport system that moves molecules and ions through the
polar phospholipid bilayer.
The Fluid Mosaic Model
In 1972 S.J. Singer and G.L. Nicolson proposed the now widely accepted Fluid Mosaic Model of the structure of
cell membranes. The model proposes that integral membrane proteins are embedded in the phospholipid bilayer, as
seen in Figure 1.1. Some of these proteins extend all the way through the bilayer, and some only partially across it.
These membrane proteins act as transport proteins and receptors proteins.
Their model also proposed that the membrane behaves like a fluid, rather than a solid. The proteins and lipids of the
membrane move around the membrane, much like buoys in water. Such movement causes a constant change in the
"mosaic pattern" of the plasma membrane.
A further description of the Fluid Mosaic Model can be viewed at http://www.youtube.com/watch?v=Qqsf_UJcfBc
(1:27).
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Chapter 1. Membrane Proteins
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/94255
Extensions of the Plasma Membrane
The plasma membrane may have extensions, such as whip-like flagella or brush-like cilia. In single-celled organisms, like those shown in Figure 1.2, the membrane extensions may help the organisms move. In multicellular
organisms, the extensions have other functions. For example, the cilia on human lung cells sweep foreign particles
and mucus toward the mouth and nose.
FIGURE 1.2
Flagella and Cilia. Cilia and flagella are extensions of the plasma membrane of many cells.
Summary
• The plasma membrane has many proteins that assist other substances in crossing the membrane.
• The Fluid Mosaic Model depicts the biological nature of the plasma membrane.
• Cilia and flagella are extensions of the plasma membrane.
Explore More
Use these resources to answer the questions that follow.
Explore More I
• Cell Membranes at http://johnkyrk.com/cellmembrane.html .
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1.
2.
3.
4.
5.
What is the major role of many membrane proteins?
How much of a cell’s genetic material may code for membrane proteins?
What are transmembrane proteins, and what is their main function?
How can a protein "tunnel" form through the membrane?
How can a protein "channel" form through the membrane?
Explore More II
• Construction of the Cell Membrane at http://www.wisc-online.com/Objects/ViewObject.aspx?ID=AP1101
.
1.
2.
3.
4.
How may water molecules enter the cell?
How may ions enter the cell?
What type(s) of protein(s) identify the cell?
What molecule is found in the membrane of animal cells but not plant cells?
Review
1.
2.
3.
4.
What is the main difference between the two main types of proteins associated with the plasma membrane?
What are two functions of integral membrane proteins?
Discuss the Fluid Mosaic Model.
What are flagella and cilia?
References
1. Mariana Ruiz Villarreal (User:LadyofHats/Wikimedia Commons). Types of proteins contained in the plasma
membrane . Public Domain
2. Flagella: Courtesy of Dr. Stan Erlandsen/Centers for Disease Control and Prevention (Image #11643); Cilia:
Courtesy of Charles Daghlian. Flagella and cilia in the plasma membrane . Public Domain
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