Cellular membranes are fluid mosaics of lipids and proteins

• Phospholipids are amphipathic molecules containing hydrophobic and hydrophilic regions

• Fluid mosaic model = membrane is a fluid structure with a “mosaic” of various proteins embedded in it
• Proteins are not randomly distributed in the membrane
• Phospholipids in the membrane can move within the bilayer
• Most of the lipids and some proteins drift laterally

Membrane Proteins and Their Functions

• Proteins determine most of the membrane’s specific functions
• Peripheral proteins = bound to the surface of the membrane
• Integral proteins = penetrate the hydrophobic core
• Integral proteins that span the membrane = transmembrane proteins
• Hydrophobic regions of an integral protein = 1+ stretches of nonpolar amino acids, often coiled into alpha helices

Membrane structure results in selective permeability

• A cell must exchange materials with its surroundings
• Plasma membranes are selectively permeable, regulating the cell’s molecular traffic
• Hydrophobic portion = determines if a molecule will pass through without assistance
  • Hydrophobic (nonpolar) molecules, i.e., hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly
• Hydrophilic molecules, including ions and polar molecules, do not cross the membrane easily

Transport Proteins