Bio H Name____________________________________
Introduction to Cell Membranes: Linking Macromolecules to Cell structure
Cell membranes have a wide variety of functions. The functions are dictated by the structure of the membranes. This structure is
made up of lipids, carbohydrates, and proteins. Some functions happen due to simple physics principles (molecules move from an
area of high concentration to low concentration), some due to hydrophobicity and hydrophilicity and some require not only the
structures but energy as well.
The cell is composed of two distinctive environments: the hydrophilic aqueous environment (both within the cell (cytoplasm) and
outside the cell (extracellular space) and the hydrophobic lipid membranes. The lipid environment within the membrane is created
by phospholipids. In order to understand the structure of phospholipids, let’s first revisit the lipids we have already discussed,
triglycerides and relook at their structure in order to understand the structure of a phospholipid. We will then discuss the important
role of cholesterol in all cell membranes.
The Structure of Lipids (review)
http://www.wisc-online.com/Objects/ViewObject.aspx?ID=AP13204
Lipids serve many functions in organisms. They are the major components of waxes, pigments, steroid hormones, and cell
membranes. Phospholipids and steroids are very important to the functioning of membranes in cells. Before we discuss
phospholipids, let’s review the structure of a triglyceride. Lipid molecules are slightly soluble to insoluble in water. Triglycerides are
hydrophobic because a large part molecules consist of long, 16-22 carbon fatty acids which have only a small amount of oxygen on
one end of the molecule.
Shorthand structure (yes we left out the oxygens as well!)
This triglyceride is SATURATED (no double bonds)
Triglycerides (Fats and Oils or Neutral lipids)
Fats are synthesized from two different classes of molecules: fatty acids and glycerol. The fatty acids are generally, 16-22 carbons
long, unbranched hydrocarbons that terminate with a single carboxyl functional group (this is the end of the fatty acid that contains
the oxygen). The long unbranched hydrocarbon ‘tails” are what create the hydrophobicity. Fatty acids can be saturated (no double
bonds) or unsaturated (having one or more double bonds). These double bonds introduce "kinks" in the carbon chain which have
important consequences for the cell membranes that they will form. Unsaturated fatty acids have lower melting points than
saturated fatty acids. To construct a triglyceride, three fatty acid molecules are attached to the glycerol through a condensation
reaction (dehydration synthesis) that results in formation of a triglyceride and the release of a water molecule.
An unsaturated fat has at least one unsaturated fatty acid whereas a saturated fat has none. Because
the double bonds of the unsaturated fatty acids introduce kinks in the hydrocarbon backbone,
unsaturated fats will not pack into a regular structure and thus remain fluid at lower temperatures. A
saturated fat will pack well and be a solid a low temperatures.
Fats are mainly energy storage and insulating molecules. Per gram, fats contain twice as much
energy as carbohydrates. Layers of fat also surround the vital organs of animals to cushion them,
and layers of fat under the skin of animals provide insulation.
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Bio H Name____________________________________
The Role of Phospholipids in the Cell Membrane
Phospholipids contain only two fatty acids attached to a glycerol head. The phosphate group, along with the glycerol group, form the
head of the phospholipid hydrophilic area, whereas the fatty acid tail is hydrophobic. Thus phospholipids are molecules with a polar
end (hydrophilic) and a hydrophobic end. When phospholipids are in an aqueous (water) solution they will self assemble into
micelles or bilayers, structures that exclude water molecules from the hydrophobic tails while keeping the hydrophilic head in
contact with the aqueous solution.
View the animation that demonstrates the formation of micelles and bilayers. Be sure to scroll down to the animation labeled
Micelle and Liposome formation
Phospholipid structure. Two fatty acids with a third group that contains phosphorus
http://telstar.ote.cmu.edu/biology/MembranePage/index2.html
1.
Draw a picture of the formation of a single phospholipid layer, that might form where lipids and water interface. Indicate
which side of the phospholipid is immersed in water (in this picture), and the phosphate group and the fatty acids.
2.
Sonication is “vibration”. As you sonicate the phospholipid layer, what does it form? Draw a picture of micelle. Is the
inside of the micelle hydrophobic or hydrophilic?
3.
Now draw a straight bilayer of phospholipids,and then a picture of the liposome. Identify where the hydrophilic and
hydrophobic areas are in this diagram
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Bio H Name____________________________________
4.
How is a micelle different from a liposome?
Phospholipids serve a major function in the cells of all organisms: they form the phospholipid membranes that surround the cell
and also form around intracellular organelles such as the mitochondria. The cell membrane is a fluid, semi-permeable bilayer that
separates the cell's contents from the environment. The animation below the micelle formation is an illustration of how the micelle
layer forms the cell membrane. Remember that the cell is a sphere (3D), so all components are entirely surrounded by the cell
membrane.
The membrane is fluid at physiological temperatures and allows cells to change shape due to physical constraints or changing
cellular volumes. The phospholipid membrane allows free diffusion of some small molecules such as oxygen, carbon dioxide, and
small hydrocarbons, but not charged ions, polar molecules or other larger molecules such as glucose. This semi-permeable nature of
the membrane allows the cell to maintain the composition of the cytoplasm independent of the external environment.
5.
Read both paragraphs above. Then write 2-3 sentences explaining the purpose of the phospholipid membrane.
Steroids
The steroids are a family of lipids based on a molecule with four fused carbon rings. This family includes many hormones and
cholesterol. Cholesterol is a component of the cell membrane in animals and functions to moderate membrane fluidity because it
restricts the motion of the fatty acid tails.
6.
Draw a picture of a cell membrane that includes phospholipids, cholesterol. What is the purpose of cholesterol in the cell
membrane?
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Bio H Name____________________________________
Review of Lipids
Why does the lipid bilayer need a variety of lipids. Lets look at what happens to cell membranes as temperature increases. Using
the phase transition animation. Remember, the temperature is in C NOT F. This animation will only show you one concept at a
time, so you need to “click” the button after each part of the animation (but ALL parts are important).
7.
What does this animation tell you about the composition of the lipid bilayer? (read the following paragraph as well
before you answer) What can you say about the “best composition” of the bilayer as the temperature changes?
Fluid Quality of Membranes
The cell membrane must be a dynamic structure if the cell is to grow and respond to environmental changes. To keep the membrane
fluid at physiological temperatures the cell alters the composition of the phospholipids. The right ratio of saturated to unsaturated
fatty acids keeps the membrane fluid at any temperature conducive to life. For example winter wheat responds to decreasing
temperatures by increasing the amount of unsaturated fatty acids in cell membranes. In animal cells cholesterol helps to prevent the
packing of fatty acid tails and thus lowers the requirement of unsaturated fatty acids. This helps maintain the fluid nature of the cell
membrane without it becoming too liquid at body temperature. The fluidity of the membrane is demonstrated in the following
animation. The lipids in the membrane are in random bulk flow moving about 22 µm (micrometers) per second. Phospholipids freely
move in the same layer of the membrane and rarely flip to the other layer. Flipping of phospholipids from one layer to the other
rarely occurs because flipping requires the hydrophilic head to pass through the hydrophobic region of the bilayer.
8.
Watch the membrane bilayer animation (same website). Compare the relative ratio of saturated to unsaturated fats in
the cell membranes of cold water fish (like cod) as compared to the ratio of fats in the membranes of bacteria living in hot
springs that would be necessary for each organism to survive.
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Bio H Name____________________________________
The Mosaic Quality of Membranes
Proteins
Because the cell membrane is only semi-permeable, the cell needs a way to communicate with other cells and exchange nutrients
with the extracellular space (the fluid outside the cell). These roles are primarily filled by proteins. Membrane proteins are classified
into two major categories, integral proteins and peripheral proteins.
Many integral proteins span the entire lipid bilayer and can serve as pores that selectively allow ions or nutrients into the cell (these
are molecules that cannot pass through the phospholipid bilayer. Some also transmit signals into and out of the cell (hormones or
other chemicals).
Unlike integral proteins that span the membrane, peripheral proteins reside on only one side of the membrane and are often
attached to integral proteins. Some peripheral proteins serve as anchor points for the cytoskeleton or extracellular fibers. Proteins
are much larger than lipids and move more slowly. Some move in seemingly directed manner while others drift.
Carbohydrates
The extracellular surface of the cell membrane is decorated with carbohydrate groups attached to lipids and proteins. Carbohydrates
are added to lipids and proteins by a process called glycosylation, and are called glycolipids or glycoproteins. These short
carbohydrates, or oligosaccharides, are usually chains of 15 or fewer sugar molecules. Oligosaccharides give a cell identity (i.e.,
distinguishing self from non-self) and are the distinguishing factor in human blood types and transplant rejection.
Membranes are Asymmetric
As discussed above and seen in the picture, the cell
membrane is asymmetric. The extracellular face
(external side) of the membrane is in contact with
the extracellular matrix (environment outside the
cell). The extracellular side of the membrane
contains oligosaccharides that distinguish the cell
as self. It also contains the end of integral proteins
that interact with signals from other cells and
sense the extracellular environment. The inner
membrane is in contact the contents of the cell.
This side of the membrane anchors to the
cytoskeleton and contains the end of integral
proteins that relay signals received on the external
side.
Summary: Membranes as Mosaics of Structure and Function
The biological membrane is a collage of many different proteins embedded in the fluid matrix of the lipid bilayer. The lipid bilayer is
the main fabric of the membrane, and its structure creates a semi-permeable membrane. The hydrophobic core impedes the
diffusion of hydrophilic structures, such as ions and polar molecules but allows hydrophobic molecules, which can dissolve in the
membrane, to cross it with ease. Proteins determine most of the membrane's specific functions. The plasma membrane and the
membranes of the various organelles each have unique collections of proteins. For example, to date more than 50 kinds of proteins
have been found in the plasma membrane of red blood cells.
9.
Use the diagram above to identify integral proteins, peripheral proteins, carbohydrate molecules, phospholipid bilayer.
Draw in cholesterol. Identify the role of each of these organic molecules in the cell membrane on a separate page.
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