Plasma Membrane  Outer boundary of a cell  Gatekeeper - controls what enters and leaves the cell

Selective Permeability  Only certain molecules pass through the membrane  Factors affecting permeability:  size  solubility  charge (ions or uncharged molecules)

Phospholipid Molecule Cell membrane is made of phospholipids PO 4 3- Glycerol Fatty acid

Phospholipid Molecule

Model of Plasma Membrane  Lipid Bilayer  Two layers of phospholipids Polar Phosphate group Nonpolar Fatty acids

Lipid Bilayer  Polar phosphate group dissolves in water  Point toward the outside and inside of the cell (toward water)  Nonpolar fatty acids DO NOT dissolve in water  Point toward the middle of membrane

Dynamic structure  Fatty acid tails wave back and forth  Phospholipids glide through bilayer  Cholesterol  Embedded in membrane  Rigid  Restrict phospholipid movement

Membrane Proteins  Embedded throughout bilayer  Lie on inner or outer surface  Polar and nonpolar portions  Migrate

Fluid Mosaic Model  Fluid-Components of membrane are constantly moving  Mosaic- random pattern of proteins Cell organelles have membranes with the “fluid mosaic” structure.