Download Name: Assignment: Cell #4: Structure of Cell Membranes Let`s take

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Assignment: Cell #4: Structure of Cell Membranes
Let's take a look at the structure of the cell membrane. Surprisingly, this barrier that separates
the inside of the cell from the outside is not thick, nor is it particularly sturdy. It is composed
mostly of layers of special lipids called phospholipids.
Phospholipids are a lot like triglycerides. They have a glycerol. However, they have only two
fatty acids. Instead of a third fatty acid, the phospholipids have a group of atoms called the
phosphate group. The phosphate group contains a phosphorous atom
surrounded by four oxygen atoms. If you look at the shape of the phospholipid, you could say
that the phosphate group forms the head and the fatty acids form two tails.
The phosphate group gives this lipid a special property. Lipids do not dissolve in water. In
other words, they are nonpolar. Nonpolar substances are said to be insoluble. That is, they do not
form hydrogen bonds with water. (Remember that water is a polar molecule, and only polar
molecules dissolve in it.) This is only partially true for phospholipids. Whereas the fatty acid
tails are nonpolar, the phosphate head has a negative charge and is, therefore, polar. Polar
substances can hydrogen bond with water and are, therefore, described as soluble. So, if you
drop a phospholipid into a glass of water, the phosphate head will go into the water; the fatty
acid tails will float up toward the air.
(1) What are the three parts of a phospholipid?
1. ____________________________________________
2. ____________________________________________
3. ____________________________________________
(2) What part of the phospholipid forms its:
a. head? _________________________________________________________
b. tail? ___________________________________________________________
(3) Why are nonpolar molecules insoluble in water?
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(4) Describe what happens it you drop a phospholipid molecule in water.
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(5) What special property of a phospholipid causes it to do this?
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Cell membranes contain two layers of phospholipids. There is water on the inside and the
outside of the cell. The water attracts the phospholipid heads but repels the fatty acid tails. As a
result, the fatty acids in each layer line up inside. The phosphate heads in the outer layer face
out into the water. The phosphate heads of the inner layer face the inside of the cell. The fatty
acid tails in both layers face each other as they try to get as far away from the water as possible.
(6) Cell membranes are made up of how many layers of phospholipids molecules?
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(7) Why do the fatty acid tails line up on the inside?
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Cell membranes contain large molecules other than phospholipids. They contain cholesterol,
which increases the membrane's fluidity (the ability for molecules making up the membrane to
move among one another). They also contain various types of proteins. Some proteins are
attached to the surface of the membrane. Others extend
completely through the membrane.
Some of these proteins are called glycoproteins. They have branched sugar chains sticking out
of their tops. (Some of the phospholipids also have sugar chains sticking out of the phosphate
groups.) These sugar chains are like name tags. They enable cells to recognize and communicate
with each other. This ability is especially important when it comes to fighting diseases. Your
immune system uses these markers to differentiate between enemy cells, such as invading
bacteria, and your own cells. As a result, your immune system does not attack the wrong cells.
(8) What is the role of cholesterol in cell membranes?
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(9) How do glycoproteins differ from other proteins?
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(10) What is the function of glycoproteins?
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(11) Why is it important that cells be able to recognize one another?
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Other proteins in the membrane act as gatekeepers. They transport, or carry, molecules back
and forth across the membrane. Cell membranes are permeable to water and gases such as
oxygen and carbon dioxide. These molecules are free to diffuse back and forth depending on the
level of concentration. Other molecules need help from proteins in the membrane. These
proteins bind to the molecules on one side of the membrane and release them on the other side.
Like enzymes, transport proteins are specialized; each type can transport only one type of
molecule.
There are two kinds of transport.
PASSIVE TRANSPORT:
In passive transport, the protein moves molecules from an area of high
concentration to an area of low concentration. Passive transport does not require the cell to
expend, or use, energy.
ACTIVE TRANSPORT:
Active transport moves molecules against the level of concentration. In
other words, it moves molecules from an area of low concentration to an area of high
concentration. Active transport is equivalent to pushing a rock uphill. The cell must expend
energy. That is, ATP must be broken down to release the energy required by active transport.
(12) What molecules are able to pass through the lipid bilayer without the help of
transport proteins? _________________________________________________________
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(13) Tell whether the following are true of
(A) passive transport
(B) active transport
(C) both active and passive
(D) neither
________ - needs energy
________ - particles move from an area of high to low concentration
________ - particles move from an area of low to high concentration
________ - need to pass through transport protein molecules
________ - molecules pass directly through the phospholipid bilayer
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