The Cell MembraneThe Cell Membrane•All cells are separated from the All cells are separated from the external environment by a external environment by a Plasma Membrane Plasma Membrane •  Membrane is pliable and Membrane is pliable and semipermiable only allowing semipermiable only allowing certain materials in/out of cells. certain materials in/out of cells. •It is composed of many It is composed of many components. components.

–Phospholipids Phospholipids –Cholsterol Cholsterol –Proteins Proteins

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to triglycerides Very similar to triglycerides however, one of the fatty acid however, one of the fatty acid chains has been replaced by chains has been replaced by a very polar Phosphate group a very polar Phosphate group

Phospholipids & WaterPhospholipids & Water• This poses a problem for the molecule since the molecule is This poses a problem for the molecule since the molecule is

both polar and non-polar simultaneously. both polar and non-polar simultaneously. • It is therefore very unstable in water, polar end is attracted It is therefore very unstable in water, polar end is attracted

to it, non polar tails are repelled by it. to it, non polar tails are repelled by it. • This forces the phospholipids to take an arrangement such This forces the phospholipids to take an arrangement such

that their tails face each other. This accomplishes 2 things.. that their tails face each other. This accomplishes 2 things.. – The polar heads can be adjacent to polar water molecules The polar heads can be adjacent to polar water molecules – The non-polar tails can be adjacent to each other. The non-polar tails can be adjacent to each other.

The Lipid BilayerThe Lipid Bilayer

• As the layer As the layer enlarges it forms a enlarges it forms a bubble shape bubble shape formed of a formed of a lipid lipid bilayerbilayer which is which is the foundation of the foundation of the plasma the plasma membrane membrane

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More Membrane More Membrane ComponentsComponents

• The membrane requires a few more components The membrane requires a few more components to make it complete: to make it complete:

• Cholesterol Cholesterol • Functions to hold the membrane together like Functions to hold the membrane together like

glue. Adds strength to the membrane. glue. Adds strength to the membrane. • Proteins (2 types) Proteins (2 types) • Intrinsic Proteins Intrinsic Proteins

– Extend into or through lipid bilayer Extend into or through lipid bilayer – Often are channel proteins allowing large molecules Often are channel proteins allowing large molecules

in/out in/out • Extrinsic Proteins Extrinsic Proteins

– Exist only on surfaces of the bilayer Exist only on surfaces of the bilayer – Often serve as receptor molecules Often serve as receptor molecules – Glycoproteins serve as cellular markers (nametags) Glycoproteins serve as cellular markers (nametags)

The Fluid Mosaic ModelThe Fluid Mosaic Model

• Entire model is referred to as the Entire model is referred to as the Fluid MosaicFluid Mosaic model of the plasma membranemodel of the plasma membrane