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The plasma membrane is a selectively permeable membrane that regulates the steady traffic that enters and leaves the cell. It is essential to the life of a cell because the plasma membrane protects the cell from its surroundings. But before we discuss why the plasma membrane is selectively permeable, we need to understand what it means to be selectively permeable. 

What does Selectively Permeable Mean?
Essentially, selectively permeable means that only ¬†certain molecules can enter or exit the cell. You can imagine a selectively permeable membrane as the police ūüöď. It regulates what goes in and out of the cell.¬†

What are the parts of the Plasma Membrane?
The plasma membrane is selectively permeable because it consists of a phospholipid bilayer. The bilayer of eukaryotic cells looks like this:

As you can see in the picture, the membrane consists of phospholipids in addition to many other parts: 

  • Hydrophillic heads are attracted to water
  • Hydrophobic tails tend to repel water
  • Integral Proteins have non polar regions and span the hydrophobic interior of the membrane
  • Peripheral proteins are loosely bound to the surface of the membrane¬†
  • Cholesterol molecules are embedded in the interior of the bilayer to stabilize the membrane¬†
  • Glycoproteins are carbohydrates covalently bonded to proteins¬†

How is the Plasma Membrane Selectively Permeable?
Now that we know the parts of the plasma membrane, we can understand how it can be selectively permeable. The key to the plasma membrane is the lipid in the phospholipid bilayer. Because of this, small lipid molecules are able to pass through the membrane. In addition, the molecules must be uncharged because the phospholipid heads and tails. The heads are polar -- they carry a partial electric charge -- but the tails are electrically neutral. Because the phospholipids are charged, no charged molecules (like ions) can travel through the membrane. 

To recap, here are the criteria for molecules that can pass through the plasma membrane: 

  • Small
  • Lipids¬†
  • Uncharged¬†

Lastly, it is important to know that while small, uncharged lipids can get through the plasma membrane, other molecules that are larger or charged can get through by other methods such as passive transport, active transport, and through channel proteins. 

By understanding the movement of molecules across the plasma membrane you'll be able to learn concepts such as transport and cell communication easier.  If you are ever confused, consider talking to your teacher or chatting with us here at Fiveable. Now that you know about selective permeability and the plasma membrane, you'll be ready to take on any AP Biology topic that comes your way. 

ūüďĆ Here are some additional resources about the plasma membrane:¬†

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