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The Cell Membrane
Key words:
Cell Membrane
Phospholipids
Hydrophobic
Hydrophilic
Lipid Bilayer
Semipermeable
As you now know, all living things consist of one or more
cells. Although cell structure may vary from one organism to
another, the cells of all organisms are surrounded by a cell
membrane.
The cell membrane is a thin layer of lipid and protein that separates
the cell’s contents from its environment. The cell membrane functions
like a fence with gates, controlling what enters and leaves the cell.
Question One: What is the function of the cell membrane?
Membranes are made mostly of phospholipid molecules. A
phospholipid molecule is a type of lipid made from glycerol, two fatty
acids, and a phosphate group. The picture below illustrates a
phospholipid.
Question Two: What are the components of a phospholipid?
The two ends of the phospholipid molecule have different properties
in water. The phosphate head of the phospholipid is hydrophilic,
meaning “water-loving.” The phosphate head dissolves easily in
water. The lipid tails of the molecule are hydrophobic, meaning
“water-fearing.” The lipid tails do not dissolve in water.
Question Three: Label the hydrophilic and hydrophobic ends of the
phospholipid. Draw a water molecule near the side that like water .
When dropped in water, phospholipids will form two layers, called a
lipid bilayer. The phosphate heads face the watery fluids inside the
cell and outside the cell. Lipid tails are sandwiched inside the bilayer.
The phospholipids in the cell membrane are constantly being formed
and broken down by chemical reactions in living cells. The picture
below illustrates the lipid bilayer.
Protein molecules are embedded in the lipid bilayer. Most of the
proteins extend all the way through the bilayer, and are similar to
channels. There are many different kinds of proteins in the
membrane. Some of these proteins help transport materials in and
out of the cell, while others help with cell recognition.
Question Four: What is the function of proteins in cell membranes?
In order to live all cells must take in nutrients and eliminate wastes. In
both cases these materials must pass through the cell membrane. Do
all materials pass through the cell membrane the same way? The
answer is no. The cell membrane is considered semipermeable,
meaning that the membrane lets certain molecules pass through and
prevent others molecules from crossing. Small molecules, such as
water, can enter and leave the cell freely. Large molecules, such as
proteins and carbohydrates, cannot. And even thought they are
small, particles with strong electrical charges, such as ions, cannot
pass easily through the membrane. The electrical charge prevents
the ion from moving through the lipid bilayer.
Question Five: What does semipermeable mean?
Question Six: What two factors effect what crosses the membrane?