Name__________________________________ Date_________ Block_________
Cell Membrane Information Worksheet
Composition of the Cell Membrane & Functions
The cell membrane is also called the plasma membrane. It is made of a
phospholipid bilayer. This double layer of lipids is also known as a fluid mosaic
because the phospholipids that make up the membrane lie next to each other but
are not connected. This gives the cell membrane a fluid motion. The cell
membrane is embedded with many different proteins and other substances that
allow some selective materials to enter and leave the cell. The many different
substances that are embedded in the cell membrane give it a look similar to a
mosaic in the art world which uses pieces of tile to form a picture or design.
The phospholipids have a polar, hydrophilic (water attracting) head and two
nonpolar, hydrophobic (water repelling) tails. Phospholipids can move laterally
and allow water and other small molecules to pass through into or out of the cell.
Since the cell membrane is mostly made of lipids, only lipid soluble substances
such as oxygen and carbon dioxide can pass through in this way. Other
substances that are water soluble need to pass through openings created by
different kinds of proteins which are embedded in the membrane. Water soluble
substances include molecules like glucose, amino acids, ions and water.
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A concentration gradient is a difference in concentration between the fluid or
solution outside a cell and the concentration within the cell. When the movement
of molecules does not require energy the molecules are moving down the
concentration gradient (from a higher concentration to a lower concentration).
This is known as simple diffusion. The special term for the diffusion of water is
osmosis.
Membrane proteins that make up the mosaic of the membrane are very
important. Some proteins are attached to the surfaces of the cell membrane.
These peripheral proteins are located on both the interior surface and the exterior
surface of the cell membrane. The proteins that are embedded in the lipid bilayer
are called integral proteins. Because the cell membrane is selectively permeable,
cells must have mechanisms for transporting molecules through the lipid bilayer.
Membrane proteins play an important role in this process. For example, some
integral proteins form channels or pores through which certain substances can
pass. Other proteins bind to a substance on one side of the membrane and carry
it to the other side of the membrane. These proteins are called transport proteins.
Proteins form pores or channels in the cell membrane which allows water,
which is not lipid soluble, to move freely across the cell membrane from an area of
greater concentration to an area of lesser concentration.
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Large molecules like glucose or other carbohydrates us transport proteins
with carbohydrate chains to help move across cell membranes. The carbohydrate
chains act like ID cards to help cell recognize certain molecules. Because the large
molecules are moving down the concentration gradient and do not require energy,
it is still a type of diffusion, but with the help of a protein. This process is called
facilitated diffusion.
Refer to page 182 in the textbook. Color the diagram of the phospholipid
bilayer as follows.
Heads of the lipid molecules – red
Tails of the lipid molecules – blue
Proteins (B, C, and H) - yellow
Carbohydrate Chains - orange
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Questions:
1. Explain why the cell membrane is called a fluid mosaic.
2. The phospholipids have a polar, ___________ head and two nonpolar
_____________ tails.
3. Define hydrophilic - _______________________________________________
4. Define hydrophobic - ______________________________________________
5. Since the cell membrane is mostly lipids, only lipid soluble substances such as
_______________ and _______________ can pass through.
6. Water soluble substances need to pass through openings created by different
kinds of _________________ which are embedded in the membrane.
7. Give examples of water soluble substances: ________________, _______
_________, ___________, and water.
8. What is a concentration gradient?
9. When molecules move from a higher concentration to a lower concentration, it
is known as simple __________________.
10.
The diffusion of water is called ___________________.
11.
Describe the function of a transport protein.
12.
What type of molecules use transport proteins?
13. Carbohydrate chains act like ______ cards to help cells recognize certain
_____________________.
14.
What is facilitated diffusion?
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