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Phospholipids Cholsterol Proteins Phospholipids Very similar to triglycerides however, one of the fatty acid chains has been replaced by a very polar Phosphate group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