Cell Membrane Structure &
Permeability
• Plasma Membrane
– The plasma membrane that surrounds the cell is a fluid mosaic of
phospholipids and proteins that separates the living cell from its
nonliving surroundings. The membrane is selectively permeable
consisting of a amphipathic (both hydrophilic and hydrophobic) bilayer
of phosphate molecules. The layers are held together by hydrophobic
interactions of the fatty acid tails.
~8 nm
Membrane Components
• Both the phospholipids and the proteins present in the
plasma membrane are amphipathic
– Having both a hydrophilic and hydrophobic region
– All parts float freely.
• Often referred to as a “SEA” of phospholipids
• Components move laterally but do change orientation (exo- to endo-)
– Movement is constant (~2nm/s for phospholipids)
– Movement can be directional for components such as proteins
Cholesterol & Unsaturated Fatty Acids
• Cholesterol
– make the membrane more rigid at higher temperatures
– lowers the temperature at which the membrane solidifies
• Unsaturated fatty acids
– Make the membrane more fluid (resists tight making)
Membrane Proteins
• Membrane proteins
– float freely with their hydrophilic regions in the center of the
molecules and their hydrophilic regions extending outwards
– some directed or held rigid by internal cytoskeleton fibers
– 2 classes
• Integral Proteins
– has imbedded regions (can be transmembrane or partially imbedded)
• Peripheral Proteins
– loosely held only at the surface of the membrane
• Integrins
– Attach to the ECM & to the cytoskeleton to give greater strength & structure
Protein Types
Protein Type
Transport
Enzyme
Signal
Function:
Hydrophilic channels and gates
Active sites are exposed to speed up metabolic pathways
Extracellular receptor sites cause a conformation change sending
messages to the internal environment
Cell-Cell Recognition Have glycoproteins that act as identification tags
Adhesion (joining)
Attachment to the
ECM
Act as gap and tight junctions
Maintains cell shape and stabilizes location