The Cell Membrane
•All cells are separated from the
external environment by a Plasma
Membrane
• Membrane is pliable and
semipermiable only allowing certain
materials in/out of cells.
•It is composed of many
components.
–Phospholipids
–Cholsterol
–Proteins
• Phospholipids
–Very similar to triglycerides
however, one of the fatty acid
chains has been replaced by a
very polar Phosphate group
Phospholipids & Water
•
•
•
This poses a problem for the molecule since the molecule is both
polar and non-polar simultaneously.
It is therefore very unstable in water, polar end is attracted to
it, non polar tails are repelled by it.
This forces the phospholipids to take an arrangement such that
their tails face each other. This accomplishes 2 things..
– The polar heads can be adjacent to polar water molecules
– The non-polar tails can be adjacent to each other.
The Lipid Bilayer
• As the layer
enlarges it forms a
bubble shape formed
of a lipid bilayer
which is the
foundation of the
plasma membrane
•
More Membrane Components
• The membrane requires a few more components to
make it complete:
• Cholesterol
• Functions to hold the membrane together like glue.
Adds strength to the membrane.
• Proteins (2 types)
• Intrinsic Proteins
– Extend into or through lipid bilayer
– Often are channel proteins allowing large molecules in/out
• Extrinsic Proteins
– Exist only on surfaces of the bilayer
– Often serve as receptor molecules
– Glycoproteins serve as cellular markers (nametags)
The Fluid Mosaic Model
• Entire model is referred to as the Fluid Mosaic model
of the plasma membrane