Section 2: The Plasma Membrane

The plasma membrane or cell membrane is the flexible boundary of a cell that separates the cell from the rest of the world.  It regulates what enters and leaves a cell and provides protection.  This regulatory ability is called selective permeability.  Some molecules can pass through the plasma membrane at any time, others are only allowed at certain times, and some are not allowed to pass through at all.

The plasma membrane is made of phospholipids, which are made up of glycerol, two fatty acids, and a phosphate group.  The phospholipid bilayer is a double-layered sheet that makes up nearly all of the plasma membrane.  It’s arranged this way to give it a flexible structure, and it keeps the cell separate from its environment.

Scientists use the fluid mosaic model to describe the structure of a plasma membrane. The plasma membrane contains protein molecules that are embedded in the phospholipid bilayer.  Carbohydrate molecules are attached to many of the proteins.


  1. What is selective permeability?
  2. What are phospholipids made up of?
  3. What is the fluid mosaic model?

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